Thursday, November 27, 2008

Making metals useful-gcse chemistry

Making metals useful

The reactivity of aluminium and anodisingAluminium is high in the reactivity series but does not corrode in air or water.


The reason is that aluminium covers itself with a thin layer of aluminium oxide which protects it from further corrosion.

Anodising is the industrial process of coating aluminium objects with a thicker layer of aluminium oxide.

The aluminium object forms the anode (positive electrode) in a sulfuric acid electrolyte during electrolysis.

Oxygen atoms form at the anode and join with the aluminium. This is oxidation.





Draw a labelled diagram for apparatus suitable for anodising an aluminium rod.

Show the electrodes, the electrolyte, and the cell.


Explain duralumin use in aircraft instead of aluminium.

Explain magnalium use in window frames instead of aluminium.





Important uses of aluminium and its alloys
metal or alloy



aluminium -overhead power cables

good electrical conductor, low density

aluminium -drinks cans -Does not react with water

aluminium -cooking pots -good heat conductor

duralumin -aircraft and bicycle parts -high strength, low density and hard

magnalium -aircraft parts -high strength, low density and corrosionresistance





Chemical reactions in different parts of the blast furnace (high tier)Iron is made in the blast furnace.





Write equations for each of the reactions in the blast furnace.


The limited uses of pure iron and impure iron from the blast furnaceImpure iron from the blast furnace (only 93% pure) is called cast iron.

Pure or wrought iron is not now mass produced but is still available.


cast iron

brittle, high compression strength
car engine blocks, man hole covers, gas stoves.


wrought iron
soft, bends easily, easily worked, low corrosion
nails, bolts, chains, garden gates, decorative ironwork







The production of mild steelIron from the blast furnace contains impurities like carbon, sulfur, silicon and phosphorus.

These are removed in the basic oxygen process. In this process a water cooled lance is put into impure molten iron and pure oxygen is blown through it.

The impurities are changed to oxides which come out as gases such carbon dioxide, and sulfur dioxide.

Solid oxides formed react with added calcium oxide to form a slag which floats on top of the iron.

When the amount of carbon drops to about 1 or 2% the process is stopped and the result is called mild steel.


The uses of mild steelMild steel containing a small amount of carbon eg 0.5% is has the hardness and strength for making machines, rails, ship's plates and girders for bridges and buildings.

The uses of alloy steelsThe properties of steels can be controlled by carefully changing the amounts of carbon removed and amounts of other metals put into them.

Quantitative chemistry( chemical calculation)-IGCSE/GCSE

Avogadro’s Law and use it to

calculate volumes of gases in reactionsAvogadro's law states that equal volumes of gases measured at the same temperature and pressure contain equal numbers of molecules.

The numbers of molecules shown in a chemical equation give the ratio of volumes of reacting gases. e.g. if steam is made the equation is:2H2(g) +O2(g) ---> 2H2O(g)

This means that 2 molecules of hydrogen react with 1 molecule of oxygen to form 2 molecules of steam.So 2dm3 of hydrogen reacts with 1 dm3 of oxygen and form 2dm3 of steam.

Or 4dm3 of hydrogen reacts with 2dm3 of oxygen and form 4dm3 of steam etc.volume of H2(g)/ volume of O2(g)= molecules of H2(g)/ molecules of O2(g)Write two other equations linking volumes and molecules in the above equation.

Calculate the volume of steam formed if 10cm3 of hydrogen is burned in oxygen.volume of steam/volume of hydrogen = molecules of steam/molecules of hydrogenvolume of steam= volume of hydrogen * molecules of steam/molecules of hydrogenvolume of steam = 10cm3 * 2/2 = 10cm3

Calculate the volume of steam formed if 10cm3 of oxygen is used to burn hydrogen.Calculate the volume of HCl gas formed if 2dm3 of hydrogen is burned in chlorine.

What volume of hydrogen and nitrogen is needed to make 30dm3 of ammonia by the equationN2 (g) + 3H2 (g)= 2NH3(g)

When ammonia is oxidised by oxygen what volume of NO and steam is formed by 25cm3 of ammonia? 4NH3(g) + 5O2(g) = 4NO(g) + 6H2O(g)

The chemical amount represents a number of particles.


One mole is a special very large number called the Avogadro number. It is 6*1023. We cannot count particles but can weigh substances. The molar mass is the mass of one mole of particles e.g. atoms, of a substance.
The molar mass of an element made of atoms is the relative atomic mass of the element in grams.
The molar mass of a compound of element made of molecules is the relative molecular mass or relative formula mass of that substance in grams.


These 3 quantities are connected:amount = mass/molar masse.g

What amount of methane molecule CH4 is there is 4g of methane?C=12 H=1 so molar mass of methane = 12 + (4*1) = 16g/molAmount = mass/molar mass = 4g/16g/mol = 0.25mol

What are the molar masses of the following; He, Na, Cl, Cl2, O2, N, FeS, MgO, KF, HCl, H2O, NH3, NaOH, HNO3, H2SO4, Ca(OH)2.

What amounts are the following masses; 2g of H, 2g of He, 7g of Cl2, 11.2g of FeS, 73g of HCl, 8g of NaOH, 25g of CaCO3.

What is the mass of: 1 mol of Li, 2 mol of C, 3 mol of S, 1 mol of O2, 1 mol of O3, 1 mol of NaCl, 0.1 mol of NH3, 0.5 mol of H2O, 0.2 mol of CaCO3.C7.22

Calculating the volume of a given mass of gas and vice versaThe volume of one mole of any gas is a constant known as the molar volume.At room temperature and pressure it is 24dm3/mol.amount of gas molecules = volume of gas/molar volumee.g.

What is the mass of 6dm3 of hydrogen H2 at room temperature and pressure if the molar volume under these conditions is 24dm3/mol?

H=1 so Molar mass of H2 = 1*2 = 2g/molamount of H2 = volume of H2/molar volume =6dm3/24dm3/mol =0.25molmass of H2 =amount of H2 * molar mass of H2 = 0.25mol*2g/mol = 0.5g


What is the mass of: 24dm3 of He, 12dm3 of N2, 6dm3 of CO2, 4dm3 of O2, 3dm3 of F2, 48dm3 of SO2, 2dm3 of H2, 1dm3 of H2S.What is the volume of 1g of He, 2.8g of N2, 22g of CO2, 64g of O2, 19g of F2, 64g SO2, 0.5g H2, 0.34g of H2S. (assume that the molar volume is 24dm3 at room temp)


Calculating reacting masses of substances or volumes of gasesA balanced chemical equation shows the amounts which react so masses or volumes of gases can be worked out from an equation. E.g. What mass of aluminium oxide can be made from 216g of aluminium and what volume of oxygen is needed?

Method 1

4Al(s) + 3O2(g) ---> 2Al2O3(s)Al = 27, so relative formula mass of 4Al = 4*27 =108O = 16 so relative formula mass of 2Al2O3 = 2(27*2 + 16*3) = 204so 108g of aluminium forms 204g of aluminium oxideso 1g of aluminium forms 204/108g of aluminium oxideso 216g of aluminium forms 216*204/108g of aluminium oxideso 216g of aluminium forms 408g of aluminium oxide

Method 2

4Al(s) + 3O2(g) ---> 2Al2O3(s)mass of Al = 216gamount of Al = mass/molar mass = 216g/27g/mol = 8molfrom equation: amount Al2O3/amount Al =2/4amount Al2O3 =amount of Al * 2/4= 8 mol *2/4 =4molmass Al2O3 = amount*molar mass =4mol*204gmol =408gFrom equation amount of O2/amount of Al =3/4amount of O2 =amount of Al*3/4 = 8mol *3/4mol = 6molmolar mass of O2 = 32g/molmass of O2 = amount of O2 * molar mass of O2mass of O2 = 6mol *32g/mol = 192gFor the reaction N2(g) +3H2(g) ---> 2NH3(g)


Calculate the masses and volumes of nitrogen and hydrogen needed to make 17g of ammonia NH3.


Converting mass-concentration into mol dm-3 and vice versaconcentration = mass/volume OR concentration = amount/volume

To convert them just change masses into amounts or vice versa.e.g. What is the concentration in mol/dm3 of a solution of sodium hydroxide NaOH of concentration 4g/dm3?

amount of NaOH in 1 dm3 = mass of NaOH/molar mass of NaOHamount of NaOH in 1 dm3 = 4g/40g/mol = 0.1mol so concentration of NaOH is 0.1mol/dm3 = 0.1MNB a concentration of 1M = 1mol/dm3.



Some dilute sulphuric acid, H2SO4, had a concentration of 4.90gdm-3. What is its concentration in mol dm-3?2. What is the concentration in gdm-3 of some potassium hydroxide, KOH, solution with a concentration of 0.200 mol dm-3?3.
What mass of sodium carbonate, Na2CO3, would be dissolved in 100cm3 of solution in order to get a concentration of 0.100 mol dm-3?

Simple calculations from the results of titrationse.g.

What is the concentration of a solution of sodium hydroxide NaOH if 10cm3 of the NaOH require 20cm3 of a 0.5M solution of sulphuric acid H2SO4 for neutralisation in a titration.


amount of H2SO4 = concentration of H2SO4 * volume of H2SO4amount of H2SO4 = 0.5mol/dm3 *20/1000dm3 = 0.01molH2SO4 + 2NaOH ---> Na2SO4(aq) + 2H2O(l)so amount of NaOH/amount of H2SO4=2/1so amount of NaOH = amount of H2SO4*2/1 = 0.01mol *2/1 = 0.02molconcentration of NaOH = amount of NaOH/volume of NaOH concentration of NaOH = 0.02mol/20/1000dm3 = 1mol/dm3NB 1cm3 = 1/1000dm3



What is the concentration of hydrochloric acid, 25.0cm3 of which neutralise 20.0cm3 of sodium hydroxide solution of concentration 0.15moldm-3. (ans = 0.15M)2.

What is the concentration of sulphuric acid, 20.0cm3 of which neutralise 30.0cm3 of a potassium hydroxide solution of concentration 0.1moldm-3 (ans=0.1M)3.

What is the concentration of sodium hydroxide, 10.0cm3 of which neutralise 15.0cm3 of hydrochloric acid solution of concentration 2.5moldm-3? (ans=2.5M)4.

What is the concentration of nitric acid, 10.0cm3 of which react with 25.0cm3 of a solution of sodium carbonate of concentration 0.2moldm-3? (ans = 0.2M)


Hard water- GCSE CHEMISTRY

Hard water
definition of hard water

Soft water e.g. distilled water, easily forms a lather with soap.
Hard water e.g. London tap water, does not easily form a lather and forms a scum.
Soaps and soapless detergentsDetergents are chemicals with large molecules which help clean.
They have one end which dissolves in oil and one end which is ionic and dissolves in water.
oil soluble end C17H35COO-Na+ water soluble ionic end Hard water contains soluble calcium and/or magnesium salts. It contains calcium ions Ca2+(aq), which can be detected by a flame test or by testing with sodium hydroxide and/or magnesium ions Mg2+(aq).A soap has ions which react with ions in hard water to form a precipitate (a scum).
e.g. sodium stearate C17H35COO-Na+ is a soap.
stearate ion + calcium ion ---> calcium stearate
Calcium stearate is insoluble so forms a precipitate (a scum) in hard water.

2C17H35COO-(aq) + Ca2+(aq) ---> (C17H35COO)2Ca(s)
A soapless detergent has ions which do not react with the ions in hard water.e.g. sodium 3-dodecylbenzene sulfonate
C18H29SO3-Na+ is a soapless detergent.
C18H29SO3-Na+ and (C18H29SO3)2Ca are both soluble so no scum forms in hard water.
Draw labelled diagrams to show beakers with the ions and molecules that they contain for
(a) soft water, hard water,
soft water + soap,
soft water + soapless detergent, hard water + soap,
hard water + soapless detergent.
Limestone, chalk and gypsum and hard water
Hard water forms when calcium or magnesium salts in rock dissolve in rain water as it flows through the rock.
Gypsum is a rock containing calcium sulfate.
Calcium sulfate is insoluble but a little does dissolve to leave some calcium ions in the water where is has past over gypsum.Limestone and chalk are rocks containing calcium carbonate.
Calcium carbonate is very insoluble and none of it dissolves as water passes over limestone or chalk.
Calcium ions from limestone and chalk do dissolve to make water hard in a reaction with carbonic acid.
water + carbon dioxide ---> carbonic acid
H2O(l) + CO2(g) ---> H2CO3(aq)
calcium carbonate + carbonic acid ---> calcium hydrogencarbonate
CaCO3(s) + H2CO3(aq) ---> Ca(HCO3)2(aq)does not exist!

Industrial and domestic problems caused by scaleHard water normally contains dissolved calcium hydrogencarbonate. This can slowly change back into insoluble calcium carbonate but the change is speeded up by heating.
The solid calcium carbonate formed is called scale.
calcium hydrogencarbonate ---> calcium carbonate + carbon dioxide +water

Ca(HCO3)2(aq)) ---> CaCO3(s) + H2O(l) + CO2(g)
scale
Scale forms on a kettle's heating element insulating it and wasting energy when it boils.Scale forms inside hot water pipes which can even block a pipe.
Stalactites and stalagmites are made of calcium carbonate and form in caves in hard water areas. Explain how this might happen.
Benefits of hardness in water
The formation of thin layers of scale on the inside of pipes can be useful.Corrosion is reduced because the water in the pipes is not in contact with the metal.Poisonous metal salts of lead or copper from the metal surface of pipes cannot enter drinking water if the metal surface is covered in scale.Calcium is needed in the diet for healthy bones and teeth.
The treatment of water to make it suitable for domestic use







Collection of gases-IGCSE /GCSE CHEMISTRY

Collection and identification of gases
Methods for collecting gasesdownward deliverysuitable for heavy dense gas solubility not important e.g. chlorine, upward deliverysuitable for light low density gas, solubility not important e.g hydrogen








collection over waterSuitable for any gas with a low solubility e.g. nitrogencollection using a gas syringeSuitable for any gas especially if volume is to be measured.Learning activity - use the data in the table and the rules above to select a collection method and give a reason.



Hazards with common gasesHydrogen - flammable, explosive when mixed with air. Use in small amounts.Hydrogen chloride and sulphur dioxide- poisonous as they are very acidic. Use small amounts and only in the fume cupboard.

Titration-IGCSE /GCSE-CHEMISTRY

Titration
Reasons for using titrationWhen an acid and an alkali react it is not possible to see the end of the reaction unless an indicator is used. If an indicator is used the salt made will be impure. Titration is used to find out exactly what volume of acid and alkali should be mixed.








Methods for carrying out titration

1.Fill a 25cm3 pipette up to the line with the alkali solution of known concentration.
2. Transfer exactly 25.0cm3 of alkali to a conical flask and add an indicator.
3. Rinse a burette with acid then fill it with the acid using a funnel.Burette
4. Carefully run acid from the burette int the conical flask until the indicator changes colour.


method for salt preparation-IGCSE CHEMISTRY

Choosing a method for salt

preparationSalts can be made by the following methods:

direct combination not normally used in school labs

e.g. iron + sulphur ----> iron sulphide

adding a carbonate to an

acidcarbonate + acid ---> salt + water + carbon dioxide

calcium carbonate + hydrochloric acid ---> callcium chloride + water + carbon dioxide

normally used to make soluble salts from insoluble carbonate, the reaction is complete (the acid is neutralised) when the effervescence (fizzing) finishes and some undissolved solid remains, filter, crystallise, filter again, wash and dry to obtain saltadding a metal to an

acidmetal + acid ---> salt + hydrogeniron + sulfuric acid ---> iron II sulfate + hydrogen

normally used to make soluble salts, the reaction is complete (the acid is neutralised) when the effervescence (fizzing) finishes and some undissolved solid remains, filter and crystallise, filter again, wash and dry to obtain saltadding an base to an acidacid + base ---> salt + water

normally used to make soluble salts from an insoluble base (most oxides and hydroxides),nitric acid + copper oxide ---> copper nitrate + waterthe reaction is complete (the acid is neutralised) when some undissolved solid remains, filter and crystallise, filter again, wash and dry to obtain saltIf a solution of a soluble base (an alkali e.g. soluble hydroxide) is used titration is carried out first to find out how alkali to add, crystallise, filter, wash and dry to obtain saltlearning activity -
Forming precipitatesprecipitationsoluble salt1 + soluble salt2 ----> insoluble salt + soluble salt3
sodium chloride + lead nitrate ----> lead chloride + sodium nitrateused to make insoluble salts from two solutions of soluble saltslearning activity - choose suitable pairs of solutions to make the following insoluble salts:calcium carbonate, barium sulfate, silver chloride, zinc carbonate, and lead chloride.

Purification of insoluble saltsWhen made an insoluble salt made is filtered, washed with distilled water and dried.

The Earth and its atmosphere-GCSE CHEMISTRY

The Earth and its atmosphere
The composition of the atmosphereThe atmosphere is made up of the following gases, Nitrogen 78%, Oxygen 21%, Argon 1%, Carbon dioxide 0.03%, water - variable
Earth's early atmosphere and volcanoesThe primary atmosphere of the Earth was hydrogen and helium. These light gases were slowly lost. They were replaced by a secondary atmosphere produced by the action of volcanoes.
Composition of the Earth's early atmosphereThe Earth's secondary atmosphere was made up of some left over hydrogen, carbon dioxide, water vapour, nitrogen, carbon monoxide, sulphur dioxide ammonia and methane.Task C6.10 Match these formulae to secondary atmosphere gases: CH4, SO2, CO2, H2, NH3, CO, N2, H2O.
Origin of the oceansAs the Earth cooled to below 100oC oceans were formed when water vapour condensed and formed liquid water. Oceans are reservoirs for carbon dioxide because they can store the gas when it dissolves in them. The new oceans dissolved a great deal of the carbon dioxide in the atmosphere.

The oceans still play a part in keeping carbon dioxide levels constant. If there is a lot of carbon dioxide in the air then more can dissolve. If there is less carbon dioxide in the air then some comes out of solution back into the air.

The release of oxygen into the atmosphereAs the temperature of the Earth cooled simple green plants evolved in the oceans to use the carbon dioxide in the environment. These green plants steadily removed carbon dioxide and produced oxygen by photosynthesis. Oxygen levels in the atmosphere slowly increased.

The carbon cycleThe carbon cycle helps to keep the atmospheric composition constant by adding carbon dioxide to the atmosphere and also taking carbon dioxide away from the atmosphere. Carbon dioxide is taken away from the atmosphere or out of the cycle by photosynthesis, dissolving in water and by chemical reactions, for example with rock. It is brought into the atmosphere or into the cycle by respiration, combustion, volcanic activity and decay.

Formation of igneous rocksIgneous rocks are formed when magma pushes up into the crust and cools. It is made up of crystals It does not contain any fossils. Any living thing falling into the molten rock, from which it is made would be burnt and leave no trace. Igneous rock forms as magma cools slowly under the surface e.g. Granite. Magma reaching the surface through a volcano cools quickly e.g. basalt.

Crystal size and igneous rockIgneous rocks which cool slowly have large crystals e.g. granite but rock forming quickly has smaller crystals e.g. balsalt.

The formation of sedimentary rockThis rock is formed in shallow seas. After long periods of time sediment layers pile up and the lower ones come under great pressure.
This pressure pushes the water out of the layers or sediments and salt crystallizes and sticks the particles together to form sedimentary rocks. This process is called lithification. Living things falling into the sediments leave an impression as the rock forms (a fossil). Fossils show that a rock was made from sediments.

Manufacture of ammonia and fertilisers-GCSE /IGCSE CHEMISTRY

Manufacture of ammonia and fertilisers

The chemical reaction used to make ammonia


Conditions used in the Haber processThe optimum (best) conditions for the Haber process that turns nitrogen and hydrogen into ammonia are:

350 atmospheres; high pressure increases yield

about 450ºC ; high temperature cuts yield but increases rate

and the use of a catalyst, which is usually iron; increases rate

ammonia is:nitrogen + hydrogen --> ammoniaN2 + 3H2 ---> 2NH3Ammonia can also decompose to form nitrogen and hydrogen:2NH3 ---> N2 +3H2

This reaction is reversible. The reaction is never complete but does reach a state when no more change can be seen.
This state is called equilibrium.Although no change is seen at equilibrium the reaction still carries on with some ammonia molecules being made and some decomposed. This is called dynamic equilibrium.


Temperature, pressure and the position of dynamic equilibriumTemperatureThe reaction below is exothermic, energy is given out when ammonia forms, but energy is taken in if ammonia
breaks up.
N2(g) + 3H2(g) ---> 2NH3(g) ; DH = - 92kJ/mol

Reactions resist changes.

If the temperature goes up the reaction tries to prevent this by taking in energy.
The reaction can take in energy by breaking up ammonia.
The position of the equilibrium moves to the left.So if the temperature goes up then ammonia breaks up which is not helpful.

If the temperature goes down the reaction tries to prevent this by giving out energy.The reaction can give out energy by forming ammonia. The position of the equilibrium moves to the right.So if the temperature goes down then ammonia form which is helpful.PressureOn the left hand side of the equation there are 4 moleculesOn the right hand side there are only 2 molecules which take up less space than 4 moleculesPressure is reduced if the position of the equilibrium moves to the right, so an increase in pressure causes a shift to the right so more ammonia is formed.

As high pressure favours a big yield of ammonia so 200 atmospheres pressure is used.

State and explain the effect of

(a) increasing the temperature and

(b) decreasing the pressure on the following reactions:2NO(g) + O2(g) = 2NO2(g) ; DH = +57kJ/mol2H2(g) + O2(g) = 2H2O(g) ; DH = - 280kJ/molCH4(g) + H2O(g) = CO(g) + 3H2(g) ; DH = + 40kJ/mol


Reaction rates and equilibriumUsing a high pressure gives a small increase in rate because the gases are more concentrated. Using a low temperature gives a low rate so it takes a long time to reach equilibrium.
A catalyst like iron in the Haber process is needed to speed up the reaction.


Neutralising ammonia with nitric acidWhen nitric acid reacts with ammonia (an alkali) the acid is neutralised and a salt is formed.acid + alkali ---> salt + waternitric acid + ammonium hydroxide ---> ammonium nitrate + waterHNO3(aq) + NH4OH(aq) ---> NH4NO3(aq) + H2O(l)

Ammonium nitrate contains fixed (chemically combined) nitrogen and so is a good fertiliser.


Write equations for the neutralisation of ammonium hydroxide, potassium hydroxide and ammonia by nitric acid HNO3, sulphuric acid H2SO4, phosphoric acid H3PO4.


Nitrogenous fertilisers and plant growthPlants grow well when they can obtain fixed nitrogen from the soil.
Only a few plants like peas and beans can make use of nitrogen in the air. Most plants need fixed nitrogen in compounds like nitrates. Plants need nitrogen to make proteins which gives them strong stems and healthy leaves. Some nitrates find their way into the soil naturally but intensive farming removes a lot when crops are harvested. Fertilisers containing nitrogen are used to replace nitrogen lost from the soil during farming.



The leaching of artificial fertilisers
1. Fertilisers are very soluble.
2. Fertilisers dissolve in rain water.
3. Fertilisers are leached from the soil and washed into rivers.
4. Water plants grow very well in fertilised river water.
5. The over growth of plants like algae at the surface cuts out the light to plants below.
6. Plants without light stop growing and die.
7. Dead plants rot due to bacteria that use a lot of oxygen
8 The amount of oxygen in the water drops.
9 Fish and other animals start to die because of a lack of oxygen.
10 The process is called eutrophication.

energetics- IGCSE / GCSE

Energy changes accompanying reactions

Temperature changes in reactionsMany reactions give off heat such as the burning of wood which causes a temperature increase. Other reactions take in energy and cause a temperature fall.

Exothermic reactionsAn Exothermic reaction is one which gives out energy to the surroundings, usually in the form of heat and usually shown by a rise in temperature. An example of an exothermic process is the burning of fuels.

Endothermic reactionsAn Endothermic reaction is one which takes in energy from the surroundings, usually in the form of heat and usually shown by a fall in temperature. An example of an endothermic is photosynthesis. This is because it takes in energy from the sun in the form of light.

Bond breaking and making in reactionsDuring a chemical reaction, old bonds are broken and new ones formed. Energy must be supplied to break existing bonds and so this is endothermic. Energy is released when new bonds are formed and so the formation of bonds is exothermic.Hydrogen reacts with oxygen to form water.

O=O + H-H + H-H --->O +O + H +H + H +H --->H-O-H + H-O-H Old bonds break single atoms with new bonds form in oxygen and no bonds in water molecules hydrogen molecules exothermicendothermic

Rates of reaction-igcse notes



Rates of reaction

Fast and slow reactionsSome chemical reactions are slow like the rusting of iron.Some chemical reactions are faster like the burning of wood.Some chemical reactions are very fast like the explosion of gunpowder.



State the rate for following reactions; TNT exploding, petrol burning, rock reacting with water, copper roof turning green, dynamite being used in a quarry, bread baking.

Experiments to investigate ratesConcentration-

Carry out reactions between sodium thiosulphate and hydrochloric acid in a conical flask using identical volumes and identical temperatures but at different concentrations. Record the concentrations and the times needed for a cross under the flask to disappear.
Temperature- Carry out the reaction between magnesium and hydrochloric acid in a conical flask for different acid temperatures but for identical volumes of acid, at identical concentrations and for identical masses of magnesium. Record the time for the magnesium to completely react.












Particle size-










Carry out the reaction between calcium carbonate and hydrochloric acid in a conical flask fitted with a stopper and a delivery tube to a measuring cylinder inverted in water. Use the same masses of powder, small chips and large lumps of calcium carbonate with the same volumes, concentrations and temperatures of acid. Record the volume of gas formed every 30 seconds for 10 minutes.

Interpreting experimental resultsTemperature From this graph between sodium thiosulphate and hydrochloric acid you can see that the rate of reaction is fastest at highest temperatures



ConcentrationThe rate increases as the concentration increases.










Particle sizeThe rate gets bigger as the pieces get smaller.The rate gets bigger as the surface area gets bigger.


Explaining the effect of changing temperature, concentration and surface area


(i)Concentration
The concentration of a substance, normally a solution, is the amount in a given volume. concentration = amount {units = mol/dm3 or M} volume In a higher concentration solution there are more particles to react therefore there are more collisions.
As a reaction depends on collisions happening, more collisions lead to a faster reaction rate.

If we were doing a reaction with acid and we double the number of acid particles, we double the number of collisions and therefore are likely to double the reaction rate.

Draw diagrams to show particles in a low concentration HCl solution and particles in a high concentration HCl solution. Show water molecules and HCl and magnesium particles in the diagrams. Show different numbers of collisions in each diagram.

(ii)Temperature


Increasing the temperature increases the speed of the reacting particles and faster particles collide more often than slow ones.

The increase in the number of collisions leads to an increase in the rate of reaction. Increasing the temperature also gives the particles more energy so that they collide with more violence. Energetic particles have a better chance of their collisions leading to a reaction.



Draw diagrams to show water molecules and HCl and magnesium particles in two diagrams at two different temperatures. Show different numbers of collisions and different energies for the collisions.

(iii) Particle size


Particle size is all to do with surface area. Powder has a higher surface area than lumps and therefore powder makes more collisions possible than lumps. This simple diagram explains the idea of surface area clearly:

Collision theoryReactions can only occur when particles collide and if you increase the frequency and/or energy of the collisions, you increase the rate of reaction.
If you increase the temperature you give the particles more energy so that they move quicker and have more collisions
Increasing the concentration means more frequent collisions as there are more particles in a certain volume








If you cut a lump into powder you give each individual particle a larger surface area, this gives an increased area in which the acid, for example, particles can react with.

few collisions in 1 second (small surface area, low concentration, low temperature) = slow reactionmany collisions in 1 second (large surface area, high concentration, high temperature) = fast reactionlow energy collisions (low temperature) = slow reactionhigh energy collisions (high temperature) = fast reaction
Describe what would happen on a motorway if cars were like particles in a chemical reaction and if
(a) there were more cars on the road,
(b) the cars travelled faster. Link the description to collision theory by using the words concentration and temperature in your answer.

The effects of catalysts

A catalyst is a substance that speeds up the rate of a reaction without being used up. Catalysts are usually transition metals or transition metal compounds. An example of a catalyst is iron which catalyses the reaction of nitrogen and hydrogen to produce ammonia. A catalyst usually works either by providing a surface for the reaction to take place or by forming intermediate compounds

Match the (catalysts) with the reactions that they affect.(iron, manganese dioxide, nickel, platinum, vanadium pentoxide)hydrogen peroxide ---> water + oxygenCarbon monoxide + oxygen ----> carbon dioxidesulfur dioxide + oxygen ---> sulfur trioxidenitrogen + hydrogen ----> ammoniavegetable oil + hydrogen ---> margarine

EnzymesEnzymes are biological catalysts. They speed up reactions. They work best around body heat (37oC). They break up (denature) and stop working if the temperature rises much above 40oC. Enzymes are found in biological washing powder. Enzymes in yeast are used to help fermentation in making bread and beer.

Useful products from crude oil- igcse

Useful products from crude oil

HydrocarbonsHydrocarbons are compounds which contain only the elements carbon and hydrogen. Crude oil is a mixture of different sized hydrocarbon molecules. These hydrocarbons are basically fuels such as petrol.Task C4.11 Pick out the hydrocarbons in the following list CH4, CH3OH, C6H6, C2H6, H2O, C8H18, C4H8, C2H3N, C5H12, C6H14.



Fractional distillation of crude oilThe fractional distillation of crude oil is the process which gives us the different substances (fractions) made from crude oil. The crude oil is pumped in to the fractional column from the bottom. The heat is applied at the bottom of the fractionating column. The different fractions are obtained in different positions in the column.

Top 70oC, small molecules, light colour, runny, easy to light

Gases
Petrol
Naptha
Kerosene
Diesel
Lubricating oil
Fuel oil
BitumenBottom 360oC,

large molecules, dark colour, viscous, hard to light

Draw diagrams to show the relative sizes of molecules of gas, diesel and bitumen.

The size of molecules and boiling pointAs the size of a hydrocarbon molecule increases the boiling point increases. If it has a low boiling point it is very volatile (forms a vapour easily). If it has a high boiling point it is not volatile.

size of hydrocarbon molecule (carbon atoms)

Boiling point/oC

4-12 carbon

40

11-15carbon

180

15-19 carbon

260

Over 50 carbon

Over 340


Uses of fractions from crude oil

Fraction

Use

gases

Bottled gas for gas cookers, boilers, camping gas

Petrol

Cars, electricity generators

Kerosine

Jet fuel

Diesel oil

Trucks, and some cars

oil

lubrication

Fuel oil

Boilers in ships or buildings

Bitumen


Covering road surfaces

Butane gas for camping

Complete and incomplete combustionComplete combustionHappens with plenty of oxygen. All of the carbon and hydrogen in a hydrocarbon turns to carbon dioxide and water.

Hydrocarbon + oxygen carbon dioxide + water

e.g. CH4 + 2O2 --> CO2 + 2H2O

carbon dioxide

Not poisonous but build up in atmosphere. carbon dioxide molecules trap energy from the sun. This leads to global warming.

water
harmless product
Sulphur Dioxide


Produced by sulphur impurities in burning fossil fuels. Can be dangerous if inhaled. Also it can dissolve into clouds to form acid rain. When this precipitates it is harmful to the environment by killing fish in lakes, damaging forests and can corrode buildings and metals structures.
Nitrogen Dioxide


This is formed in car engines when oxygen and nitrogen combine. This is an acid gas and turns into nitric acid when it dissolves. Acid rain results.


Complete the following equations for complete combustion of hydrocarbonsethene + oxygen ---> C2H4 + 3O2 --->ethyne + oxygen --->2C2H2 + 5O2 ---> propane +oxygen --->C3H8 +5O2 --->Incomplete combustionHappens if there is not enough oxygen.

The hydrocarbon turns into soot (carbon) and poisonous carbon monoxide as well as carbon dioxide.

Hydrocarbon + oxygen ---> carbon monoxide + water 2CH4 + 3O2 ---> 2CO + 4H2O
carbon


Formed by incomplete combustion as soot. It is bad for the lungs and disfigures buildings.
Carbon Monoxide


This is produced by traffic and some gas fires. When inhaled by people it replaces oxygen in the haemoglobin, however, it does not give it up, this eventually leads to suffocation.

Complete the following equations for incomplete combustion of hydrocarbonsethyne +oxygen --->

Match these combustion products to their formulae: carbon, water, C, sulfur dioxide, CO2, nitrogen dioxide, NO2, carbon dioxide, SO2, carbon monoxide, H2O, CO.


Chemical tests for carbon dioxide and waterIf carbon dioxide is bubbled through limewater it turns milky. We can test for the presence of water using anhydrous copper sulphate which is white. The copper sulphate turns blue when water is added to it. Cobalt chloride can also be used. This turns from blue to pink when water is added.Task C4.16 Show the above information in a suitable table.


CrackingCracking is the splitting up of long chain hydrocarbons in to smaller chains. Cracking is a form of thermal decomposition. A lot of longer molecules produced from fractional distillation are cracked into smaller ones because there's more demand for products like petrol and Kerosine than for diesel oil. The products include compounds with double bonds such as ethene. For example kerosine could be broken down to octane and ethene.

C-C-C-C-C-C-C-C-C-C- ----cracking-----> C=C + C=C-C + C-C=C + C-Cbig molecules small moleculealkanes mostly alkenessingle bonds double bondssaturated unsaturatedonly good for fuels can be made into polymers


Industrial conditions for crackingIndustrial conditions for cracking vaporised hydrocarbons are to use a powdered catalyst at about 400ºC to 700ºC. The catalyst could be Aluminium oxide.


The products of crackingCracking turns big molecules into small molecules which are mostly alkenes. Some alkanes are also made.


Saturated and unsaturated hydrocarbonsAlkanes are saturated hydrocarbons. They have single C-C bonds only.Alkenes are unsaturated hydrocarbons. They have double C=C bonds.Saturated hydrocarbons are so called because their C atoms have no spare bonds left to join with any more hydrogen atoms. They contain C-C single bonds and each C atom is joined to 4 other atoms. Unsaturated hydrocarbons are so called because they have some spare bonds which could be used to add on some more hydrogen atoms.

Natural gasNatural gas is mostly made up of an alkane called methane CH4.



The formulae and structures of alkanesMethane CH4 H H-C-H H
Ethane C2H6 H H H-C-C-H H H
Propane C3H8 H H H H-C-C-C-H H H H
Butane C4H10 H H H H H-C-C-C-C-H H H H H



Formulae and Structures of alkenes
Ethene C2H4 H H C=C H H
Propene C3H6 H H H H-C-C=C-H H C4.24 Testing alkanes and alkenes with bromine water



Making polymers from small moleculesPolymers like polythene, polypropene and polystyrene are large molecules, which can be formed by combinations of many smaller molecules called monomers.


Addition polymerslots of monomers add together ---> 1 addition polymerEach has C=C bonds only single bondsunsaturated saturatedlots of ethene molecule ---> 1 polyethene moleculeH H H H H H H H H H H H H H C=C + C=C + C=C + .... ---> -(C- C- C- C-C- C- C- C)-n H H H H H H H H H H H H H H


Draw structures and equations as above for the formation of polypropene and polychloroethene.


Uses and properties of polymers
Material
Use
Properties
Poly(ethene)
Plastic bags, bottles, buckets and bowls
Softens when warm so easily formed and moulded
Poly(propene)
Crates, rope, carpets, car bumpers, fishing nets
tough and strong
poly(chloroethene)polyvinyl chloridePVC
gutter, drain pipes, window frames, covering for electrical wiring.
electrical and heat insulator, tough, not easliy decomposed by sunlight

Extraction and uses of metals-igcse








Extraction and uses of metals





Oxidation and reduction
Oxidation is a reaction where oxygen is gained. eg iron + oxygen ---> iron oxide
Reduction is a reaction where oxygen is lost. Eg iron oxide is reduced to form ir












Classify the following reactions as oxidation or reduction: burning magnesium, making aluminium from aluminium oxide, burning carbon in air,
copper oxide + carbon --> copper + carbon dioxide
carbon monoxide forming carbon dioxide.





Electron loss and gain




Oxidation is also a loss of electrons eg Fe --> Fe2+ + 2e-
Reduction also the gain of electrons eg (when Fe ions becomes Fe atoms)




Identify all of the elements below as being oxidised or reduced in the following reactions:
Cu2+ + 2e- ---> Cu
lead ions forming lead atoms
Na ---> Na+ + e-
Ca + Cl2 --> CaCl2
Fe2+ ---> Fe3+ + e-




Reduction of metal ores




An ore is a material found in the ground which contains a metal. An ore is often a metal oxide mixed with rock . When a metal is extracted its ore is reduced. Metal loses oxygen from its oxide.









Extraction and uses of metals




Oxidation and reduction







Oxidation is a reaction where oxygen is gained. eg iron + oxygen ---> iron oxideReduction is a reaction where oxygen is lost.







Eg iron oxide is reduced to form iron.


Classify the following reactions as oxidation or reduction: burning magnesium, making aluminium from aluminium oxide, burning carbon in air,



copper oxide + carbon --> copper + carbon dioxide



carbon monoxide forming carbon dioxide.








Electron loss and gainOxidation is also a loss of electrons eg Fe --> Fe2+ + 2e- Reduction also the gain of electrons eg (when Fe ions becomes Fe atoms)











Identify all of the elements below as being oxidised or reduced in the following reactions:







Cu2+ + 2e- ---> Cu lead ions forming lead atoms







Na ---> Na+ + e-Ca + Cl2 --> CaCl2Fe2+ ---> Fe3+ + e-




Reduction of metal oresAn ore is a material found in the ground which contains a metal. An ore is often a metal oxide mixed with rock . When a metal is extracted its ore is reduced. Metal loses oxygen from its oxide.











Examples include:
haematite which is mostly iron III oxide Fe2O3

bauxite which is mostly aluminium oxide Al2O3

malachite which is mostly copper carbonate CuCO3








Extraction and position in reactivity seriesThe method of extraction of a metal depends on the reactivity of the metal. Unreactive metals like silver can be found uncombined. More reactive metals like iron can be reduced with carbon because carbon is more reactive than iron. Very reactive metals like aluminium can only be extracted by electrolysis. Carbon is not used as it is less reactive than aluminium.







The cost of electricity for extracting metalsElectrolysis is an extraction method that uses a lot of electrical energy. Electrical energy is more expensive than energy from burning carbon.











The extraction of aluminiumAluminium is found in the ground in an ore called bauxite. Bauxite is aluminium oxide (Al2O3) with iron oxide impurities. After purification aluminium oxide is mixed with cryolite to lower the melting point from 2000º to 1000º, which saves money. This mixture is heated and the molten liquid used as the electrolyte.







Both electrodes are made of graphite (carbon). The anode (+ve) is graphite and the cathode (-ve) is a graphite lining to a steel case.







The carbon anodes react with oxygen so have to be replaced.C + O2 --> CO2

At cathode -




positive aluminium ions attracted, gain electrons and become atoms.







Al3+ + 3e- ---> Al








At anode - negative oxide ions attracted, lose electrons and become atoms.







2O2- ----> O2 + 4e-












Uses and properties of aluminium





overhead power cables
good electrical conductor, low density
drinks cans
Does not react with water
aircraft parts
high strength and low density
















Carbon and carbon monoxide for reducing oxidesCarbon and carbon monoxide can both remove oxygen from other compounds so are good for reduction. They are used to reduce the ores of metals below carbon in the reactivity series. E.g. zinc, iron, tin and lead.





Iron extraction using the blast furnace


A blast furnace is used in the process of extracting iron.

The raw materials iron ore, coke and limestone are put in at the top. Hot air is blasted into this furnace at the bottom making the coke (carbon) burn faster and the temperature rises to about 1500º.





When the coke burns, carbon dioxide is produced.

C + O2 ---> CO2

CO2 reacts with the unburnt coke to form carbon monoxide


CO CO2 + C ---> 2CO

Iron oxide Fe2O3 in the ore is reduced to iron by the reaction with the carbon monoxide.

3CO + Fe2O3 ---> 3CO2 + 2Fe


Molten iron is a dense liquid, so runs to the bottom of the furnace and is tapped off.







Limestone CaCO3 helps remove impurities during the extraction by forming calcium oxide CaO.


CaCO3 ---> CaO + CO2 The rock impurities silicon dioxide SiO2 are then removed by the following reaction.


CaO + SiO2 ---> CaSiO3 CaSiO3 is known as slag and can be used in making cement and road building.











The purification of copper

Very pure copper is needed for copper wires. Electrolysis is needed to purify copper. The anode is a mass of impure copper and the cathode is pure copper. The electrolyte is sulphuric acid. The impurities drop at the anode as sludge during electrolysis.

At anode Cu ---> Cu2+ + 2e-


At cathode Cu2+ + 2e- ---> Cu









Chemical from calcium carbonate- gcse only

Chemical from calcium carbonate

Thermal decomposition of calcium carbonatecalcium carbonate ---heat---> calcium oxide + carbon dioxideCaCO3(s) ---heat---> CaO(s) +CO2(g)(limestone) (quicklime)

Water and calcium oxidecalcium oxide + water ---> calcium hydroxideCaO(s) + H2O(l) ---> Ca(OH)2(s) (slaked lime)

looks flaky and gets hotWith more water the calcium hydroxide dissolves to make limewater Ca(OH)2(aq)

Neutralising soil acidityCalcium oxide and calcium hydroxide are bases. (like all metal oxides and hydroxides)acid + base ---> salt + waterso acid soil is neutralised by calcium oxide or calcium hydroxide

e.g. nitric acid + calcium oxide --> calcium nitrate + water 2HNO3(aq) + CaO(s) ---> Ca(NO3)2(aq) +H2O(l)

Farmers can add calcium oxide or calcium hydroxide to their acid soil to neutralise it.The neutral soil is better for growing plants.Task C3.25 Write word equations and balanced chemical equations for neutralising the following acids using calcium oxide and calcium hydroxide: nitric acid, sulphuric acid, hydrochloric acid, phosphoric acid (H3PO4)


Uses of calcium carbonateCement is made from calcium carbonate (limestone).

limestone --heat--> calcium oxide + carbon dioxideCalcium oxide + clay ----> cementGlass is made using calcium carbonate.

Calcium carbonate + sand + sodium carbonate ---heat--> glassIron is manufactured using calcium carbonate in the blast furnace.calcium carbonate --heat--> calcium oxide + carbon dioxideCalcium oxide + silicon dioxide ----> calcium silicate (mostly sand) (slag) (floats on top of molten iron)

Chemicals from salt- gcse/igcse

The electrolysis of aqueous sodium chloride solution

When the electrolysis of aqueous sodium chloride takes place, hydrogen and chlorine are given off as gases and sodium hydroxide is left. Aqueous sodium chloride contains hydrogen ions H+ and hydroxide ions OH- (from the water) and sodium ions Na+ and chloride ions Cl-.

The positive sodium and hydrogen ions go to the cathode and the negative chloride and hydroxide ions go to the anode. Hydrogen is formed at the cathode and chlorine is formed at the anode.



substance test result

hydrogen

lighted splint squeaky pop

chlorine

damp blue litmus turns red then white

sodium hydroxide

damp red litmus turns blue/purple

Write a word and a balanced chemical equation for the reaction in which hydrogen burns.
Complete: Chlorine is (acidic/alkaline) because
it turns litmus red.
Chlorine is a (dye/bleach) because it turns litmus white.
This type of reaction is (oxidation/reduction).

Uses of sodium chloride, hydrogen, chlorine and sodium hydroxide


sodium chloride preserve food, flavouring, stops ice on roads because salt lowers freezing point of water to -5oC,

manufacture of sodium, chlorine and sodium hydroxide

chlorine kills bacteria in swimming pools and drinking water, bleach, making hydrochloric acid and solvents.

hydrogen rocket fuel, making margarine, making ammonia
sodium hydroxide detergents, bleach, paper, fibres, purifying bauxite

Transition metals-igcse

Transition metals

The colours of transition metal compoundsAll compounds of transition metals are coloured. Some examples are:copper sulphate - blue, iron III oxide - brown, cobalt chloride pink.



Transition metals as catalysts
Transition metal or compound can be used as Catalyst



manganese (IV) oxide


decomposing hydrogen peroxide

platinum

catalytic converter in car exhaust

nickel

making margarine from vegetable oil and hydrogen

vanadium (V) oxide

making sulphuric acid

platinum with rhodium

making nitric acid

iron

making ammonia

Uses and properties of titanium, iron and copper



iron


Bridge Construction/High Strength


iron

permanent magnets /Magnetic


copper


water pipes/does not react with water


copper


Electrical wires/Good conductor of electricity


titanium

propeller shafts on ships, hip joints/does not react with water


titanium

aircraft engines/high strength, low density, high mp

Halogens -igcse


Physical properties of halogens

The colour of the Halogens changes from a lighter colour to a dark colour as we go down the group and the melting and boiling points increase as you go down the group.
Fluorine at the top of the group is a yellow gas

This changes to a yellow-green gas for chlorine, a red liquid for bromine and finally black solid for iodine at the bottom of the group.






Reactions of halogens with metals

Metal + Halogen --> Metal Halide Iron reacts slowly with iodine to form iron II iodide. Iron reacts faster with bromine to form iron III bromide.



Iron reacts with chlorine reacts even faster to form iron III chloride.



Other metals like sodium also react.sodium + chlorine ---> sodium chloride




The reaction of halogens with hydrogenHydrogen reacts with halogens to form hydrogen



halides. E.g.Hydrogen + chlorine ---> hydrogen chlorideH2(g) +Cl2(g) --> 2HCl

(g) (a hydrogen halide)



Hydrogen halides are very soluble in water.



The gas hydrogen chloride forms hydrochloric acid when it dissolves in water.

Other hydrogen halides like hydrogen bromide, HBr and hydrogen iodide, HI also dissolve to form acid solutions. An acid solution has a pH less than 7 e.g. 1. An acid turns universal indicator red.







Displacement reactionsHalogens like chlorine are very reactive and displace less reactive halogens like bromine from halides like bromides.

Potassium bromide + chlorine ---> potassium chloride + bromine

2KBr(aq) + Cl2(aq) --> 2KCl(aq) + Br2(aq)

2Br-(aq) + Cl2(aq) --> 2Cl-(aq) + Br2(aq)

Chlorine and bromine also react with potassium iodide showing that the order of reactivity is: Chlorine > bromine > iodine



Uses of halogens and halidesFluorine compounds (fluorides) are put into toothpaste and some drinking water supplies.

Fluorides join with tooth enamel and make teeth resist attack by acid which prevents tooth decay.

Chlorine is used to in swimming pools and drinking water to kill bacteria.

Iodine is used as an antiseptic because it will kill the germs on the skin without damaging it.

Noble gases uses and properties-igcse

Argon /Light bulbs/Doesn’t react with the metal filament
Helium/Used with O2 for deep sea dives/Low solubility of helium in the blood.
Helium/To inflate the tyres of large aircraft/Non-flammable


Helium/To fill airships and weather balloons/Low density, does not burn
Neon/In advertising signs because it glows red when electricity passes/Conductor of electricity/at high voltage


Krypton/Xenon/In lamps used in photographic flash units, in stroboscopic lamps used in lighthouses/Gives out a lot of light when electricity passes through

Representing reactions- igcse notes





If a reaction occurs between magnesium and oxygen, magnesium oxide is produced, here is the word equation for this reaction: -

magnesium + oxygen --> magnesium oxide

Some other examples are: hydrochloric acid + calcium carbonate --> calcium chloride+ carbon dioxide+ water

sodium + water --> sodium hydroxide + hydrogen

hydrochloric acid + sodium hydroxide --> sodium chloride + water


Write word equations for the reactions in which the following compounds form from a halogen and another suitable element: hydrogen fluoride, hydrogen chloride, iron III chloride, iron III bromide, sodium chloride, copper chloride.


Formulae

The formula of an element or compound is simply the symbol of each element present and numbers to show how many atoms are present.
Carbon dioxide has the formula CO2.
This means that it has one carbon atom and two oxygen atoms in each molecule.

substance-
methane-formula -CH4

substance -bromine -formula -Br2

ethane -C2H6

hydrogen -H2

propane -C3H8

ethanol -C2H5OH

butane -C4H10


State the name and the number of atoms of each element in the formulae above. Formulae can be worked out from valency.

Atom or ion with that valency 1

hydrogen,
group 1 e.g. sodium and potassium, group 7
e.g. fluorine and chlorine, ammonium NH4+, hydroxide OH-, nitrate NO3-

Atom or ion with that valency2

group 2

e.g. magnesium and calcium, group 6, sulphate SO42-, carbonate CO32-, copper , II Cu2+ iron II Fe2+

Atom or ion with that valency3

group 3

e.g. aluminium, group 5, phosphate PO43-, iron III Fe3+

Atom or ion with that valency4

group 4

e.g. carbon



Use valency to work out the formulae of the following compounds: sodium chloride, potassium bromide, magnesium oxide, calcium sulphide, aluminium nitride, calcium iodide, lithium oxide, aluminium chloride, aluminium sulphide, magnesium nitride.


Calculating relative formula massAdd up the relative atomic mass (found in periodic table) of each atom in the compound.e.g. Al203 relative atomic masses of Al = 27, O = 16 (found in periodic table).


The formula shows 2 atoms of aluminium and 3 atoms of oxygen so:
formula mass of = (2*27) + (3*16) =54 + 48 = 102


Work out the relative formula masses of the following: MgO, FeS, O2, H2O, CaBr2, Na2S, CaCO3, NaOH, HCl, (NH4)2SO4.

Relative atomic masses Mg=24, O=16, Fe=56, S=32, Ca=40, Br=80, C=12, Na=23, H=1, Cl=35.5.


Simple balanced equations

It is possible to write balanced equations for reactions.

For example substances such as hydrogen and magnesium combine with oxygen.


One method to write them is:Write a word equation first.Magnesium + oxygen --> magnesium oxideWrite in the formulae of the substances used.Mg + O2 --> MgO


Balance the equation so that each element has the same number of atoms on each side.
2Mg + O2 --> 2MgO

sodium + oxygen --> sodium oxide

4Na{s} + O2{g} --> 2Na2O{s}

(word equation) hydrogen + oxygen ---> water

(formulae) H2 + O2 -----> H2O

(balance) 2H2 + O2 -----> 2H2O


State symbols


The state symbols are put in a balanced equation to show whether something is a solid, liquid, gas or dissolved in water (aqueous solution).

Magnesium + oxygen --> magnesium oxide


2Mg{s} + O2{g} --> 2MgO{s}



hydrochloric acid + calcium carbonate --> calcium chloride + carbon dioxide + water


2HCl (aq) + CaCO3 ---> CaCl2(aq) + CO2(aq) + H20(l)


Balanced equations and ionic equations

Ionic equations only show ions which change in a reaction and ignore those which do not change.

E.g.word equationhydrochloric acid + sodium hydroxide --> sodium chloride + water

balanced chemical equationHCl(aq) + NaOH(aq) ---> NaCl(aq) + H2O(l)ionic equationH+(aq) + OH-(aq) ---> H2O(l)

E.g. in the electrolysis of sodium chlorideCl-(aq) --> Cl(g) + e-2Cl(g) --> Cl2(g)

Working out formulae from reacting masses



elements reacting magnesium chlorine

symbols of elements Mg Cl

masses reacting (from experiment) 2.4g 7.1g

amounts (amount = mass/molar mass) 2.4g/24g/mol 7.1g/35.5g/mol = 0.1mol 0.2mol

ratio of atoms (divide by smallest) 1 : 2



formula MgCl2

Work out formulae of compounds formed when the following react:56g of iron and 32g of sulphur (Fe =56, S =32)2g of hydrogen and 16g of oxygen (H=1, O=16)14g of lithium and 16g of oxygen (Li=7)32g of copper and 8g of oxygen (Cu=64)6.4g of copper and 0.8g of oxygen


Calculating reacting masses using equations


You can work out ratio of the masses of products and reactants by simply multiplying the number of moles shown in the equation by the formula mass of each substance.


Example 1: What mass of magnesium oxide can be made from 12g of magnesium? Relative atomic masses are Mg =24, O = 16.


equation 2Mg(s) + O2(g) --> 2MgO(s) formula 2*24 1(16*2) 2(24+16)
masses =48 =32 =80

reacting 48g of Mg forms 80g of MgOmasses 1g of Mg forms 80/48 g of MgO 12g of Mg forms 12*80/48 g of MgO = 20g

Example 2: What mass of magnesium oxide can be made from 12g of magnesium?

Equation 2Mg(s) + O2(g) --> 2MgO(s)

amounts 2 moles 1 mole 2 moles masses 2*24 1{16*2} 2{24+16} =48g =32g =80g so 48g Mg forms 80g MgO 1g Mg forms 80/48 g MgO 12g Mg forms 12*80/48 g MgO = 20g

Also note that the ratio of amounts of reactants and products in the equation above can be written as:

Amount of Mg/amount of O2 =2/1 Or Amount of O2/amount of MgO = 1/2



Thursday, November 20, 2008

chemicalbonding-igcse notes

Elements forming compounds with chemical bonds


Electron transfer and ionic bondsAtoms have no charge.
A charged particle is called an ion.
If an atom loses an electron, it becomes a positively charged (+) ion.
An ion that is positively charged is known as a cation. If an atom gains an electron, it becomes a negatively charged (-) ion.
An ion that is negatively is known as an anion. The negative and positive ions attract each other to form an ionic bond.


Complete the gaps in the text below:_____ have no charge. A charged particle is called an ___. If an atom loses an ________, it becomes a positively charged (+) ion. An ion that is positively charged is known as a ______. If an atom gains an electron, it becomes a negatively charged (-) ion. An ___ that is negatively is known as an anion. The negative and positive ions attract each other to form an _____ bond.


The formation of sodium and chloride ion

















Draw atoms and ions for lithium, potassium, fluorine, magnesium, oxygen, sulfur and aluminiuM

Draw diagrams of ionic bonding in LiF, KF, LiCl, NaF, MgCl2, AlF3, MgO, MgS, Na2O and Al2O3.


Physical properties of giant ionic structuresIonic bonds form when metal and non-metal atoms join. A substance with ionic bonding has an ionic structure. Each ion is firmly held in place by strong ionic bonds so they have high melting and boiling points. If melted, charged ions become free to carry an electric current. The ions also become free if dissolved in water so solutions are also electrolytes. The solids are insulators because the ions are not free to move and cannot carry a current. Sodium chloride NaCl, and magnesium oxide MgO are good examples.








Pick out the substances which are (a) ionic (b) have covalent bonds (c) have high melting points (d) conduct electricity when molten: sodium chloride, sulfur dioxide, magnesium oxide, iron fluoride, carbon dioxide, NaBr, H2O, NH3, Al2O3, KCl.


Covalent bonds and electron sharingNon-metal atoms join using covalent bonds. When a covalent bond is formed, atoms share their electrons. The atoms then have full shells. One covalent bond needs one shared electron from each atom. Each atom involved has to make enough covalent bonds to fill up its outer shell. Sharing electrons is called covalent bonding. Below is a diagram to show hydrogen gas (H2).


Dot and cross diagrams

Draw atoms of F, H, O, N and C.Draw a dot and cross diagram for fluorine F2, hydrogen fluoride HF, water H2O, ammonia NH3, methane CH4, oxygen O2, nitrogen N2, ethene C2H4.






Physical properties of simple molecular substancesSimple Molecular Substances have low melting and boiling points and most are gases or liquids at room temperature. This is because of weak forces between the molecules. Molecular substances do not conduct electricity, because there are no ions. E.g. water.Draw diagrams to show how the molecular structures for the following might look: fluorine F2, hydrogen fluoride HF, water H2O, ammonia NH3, methane CH4, oxygen O2, nitrogen N2





Giant structures with covalent bondsIn giant covalent structures all the atoms are bonded to each other by strong covalent bonds so they have very high melting and boiling points. They do not usually conduct electricity even if in the liquid state. Diamond and graphite are two examples, which are made from carbon atoms. These two different types of the same element are called allotropes.



Diamond: Each carbon atom forms four covalent bonds in a very rigid giant covalent structure.
Diamond:


Physical properties of giant covalent structuresGiant molecular structures have very high melting points because all atoms are held firmly in place by strong covalent bonds. In graphite each carbon atom is held in place by three strong covalent bonds which gives graphite a high melting point. In diamond 4 strong covalent bonds holds each atom in place. This also gives diamond a very high melting point. The four bonds make diamond very hard. Graphite has weak bonds between layers so the layers slip over each other making graphite soft.
They do not usually conduct electricity even when molten because there are no charged partic
les to carry the current. There are free electrons between layers in graphite so it conducts electricity.




Explaining differences between propertiesSimple molecular substances like water have weak bonds between molecules so melt at low temperatures because little energy is needed to separate the molecules. Giant covalent structures like diamond have strong covalent bonds holding each atom in place. They melt at high temperatures because a lot of energy is needed to break these strong bonds.

Thursday, November 6, 2008

The Extraction of Aluminium-igcse


Aluminium is obtained from mining the mineral bauxite.
The purified bauxite ore of aluminium oxide is continuously fed in. Cryolite is added to lower the melting point and dissolve the ore.

Ions must be free to move to the electrode connections called the cathode (-, negative), attracting positive ions e.g. Al3+, and the anode (+, positive) which attracts negative ions e.g. O2-.

When the d.c. current is passed through aluminium forms at the negative cathode (metal*) and sinks to the bottom of the tank.

At the positive anode, oxygen gas is formed (non-metal*). This is quite a problem. At the high temperature of the electrolysis cell it burns and oxidises away the carbon electrodes to form toxic carbon monoxide or carbon dioxide. So the electrode is regularly replaced and the waste gases dealt with!
It is a costly process (6x more than Fe!) due to the large quantities of expensive electrical energy needed for the process.

* Two general rules: for electrolysis

Metals and hydrogen (from positive ions), form at the negative cathode electrode.
Non-metals (from negative ions), form at the positive anode electrode.

Raw materials for the electrolysis process:

Bauxite ore of impure aluminium oxide [Al2O3 made up of Al3+ and O2- ions]

Carbon (graphite) for the electrodes.

Cryolite reduces the melting point of the ore and saves energy, because the ions must be free to move to carry the current

Electrolysis means using d.c. electrical energy to bring about chemical changes e.g. decomposition of a compound to form metal deposits or release gases. The electrical energy splits the compound!

At the electrolyte connections called the anode electrode (+, attracts - ions) and the cathode electrode (-, attracts + ions). An electrolyte is a conducting melt or solution of freely moving ions which carry the charge of the electric current.

The redox details of the electrode processes:
At the negative (-) cathode, reduction occurs (electron gain) when the positive aluminium ions are attracted to it. They gain three electrons to change to neutral Al atoms.

Al3+ + 3e- ==> Al
At the positive (+) anode, oxidation takes place (electron loss) when the negative oxide ions are attracted to it. They lose two electrons forming neutral oxygen molecules.

2O2- ==> O2 + 4e-
or 2O2- - 4e- ==> O2


Note: Reduction and Oxidation always go together!
The overall electrolytic decomposition is ...
aluminium oxide => aluminium + oxygen
2Al2O3 ==> 4Al + 3O2

and is a very endothermic process, lots of electrical energy input!

anode graphite react with oxigen to form carbodioxide. so the it should be replaced time to time

Wednesday, November 5, 2008

chemical calculation - igcse practice

Introducing the connection between moles, mass and formula mass

The mole is most simply expressed as the 'formula mass in g' of the defined chemical 'species', and that is how it is used in most chemical calculations.

Every mole of any substance contains the same number of the defined species.

The actual particle number is known and is called the Avogadro Constant and is equal to 6.023 x 1023 'defined species' per mole. This means there are that many atoms in 12g of carbon (C = 12) or that many molecules of water in 18g* (H2O = 1+1+16 = 18, H = 1; O = 16) * this is about 18cm3, so picture this number of molecules in a nearly full 20cm3 measuring cylinder or a 100ml beaker less than 1/5th full!

However, the real importance of the mole is that it allows you to compare ratios of the relative amounts of reactants and products, or the element composition of a compound, at the atomic and molecular level. If you have a mole ratio for A:B of 1:3

it means 1 particle of A to 3 particles of B irrespective of the atomic or formula masses of A and B. (see also section 6. for reacting masses not using moles)


Important Note. Relative is just a number based on the carbon-12 relative atomic mass scale. Molar mass is a term used to describe the mass of one mole i.e. the relative atomic/formula/molecular mass in grams (g).




Examples:

Example 7.1.1: 1 mole of ammonia, NH3,

consists of 1 mole of nitrogen atoms combined with 3 moles of hydrogen atoms.

Or you could say 2 moles of ammonia is formed from 1 mole of nitrogen molecules (N2) and 3 moles of hydrogen molecules (H2).

Example 7.1.2: 1 mole of aluminium oxide,

Al2O3, consists of 2 moles of aluminium atoms combined with 3 moles of oxygen atoms

(or 1.5 moles of O2 molecules).

For calculation purposes learn the following formula for 'Z' and use a triangle if necessary.

(1) mole of Z = g of Z / atomic or formula mass of Z,

(2) or g of Z = mole of Z x atomic or formula mass of Z

(3) or atomic or formula mass of Z = g of Z / mole of Z

where Z represents atoms, molecules or formula of the particular element or compound defined in the question.


Example 7.2.1: How many moles of potassium ions and bromide ions in 0.25 moles of potassium bromide?

1 mole of KBr contains 1 mole of potassium ions (K+) and 1 mole of bromide ions (Br-).

So there will be 0.25 moles of each ion.

Example 7.2.2: How many moles of calcium ions and chloride ions in 2.5 moles of calcium chloride?

1 mole of CaCl2 consists of 1 mole of calcium ions (Ca2+) and 2 moles of chloride ion (Cl-).

So there will be 2.5 x 1 = 2.5 moles of calcium ions and 2.5 x 2 = 5 moles chloride ions.

Example 7.2.3: How many moles of lead and oxygen atoms are needed to make 5 moles of lead dioxide?

1 mole of PbO2 contains 1 mole of lead combined with 2 moles of oxygen atoms (or 1 mole of oxygen molecules O2).

So 1 x 5 = 5 mol of lead atoms and 2 x 5 = 10 mol of oxygen atoms (or 5 mol oxygen molecules) are needed.

Example 7.2.4: How many moles of aluminium ions and sulphate ions in 2 moles of aluminium sulphate?

1 mole of Al2(SO4)3 contains 2 moles of aluminium ions (Al3+) and 3 moles of sulphate ion (SO42-).

So there will be 2 x 2 = 4 mol aluminium ions and 2 x 3 = 6 mol of sulphate ion.

Example 7.2.5: How many moles of chlorine gas in 6.5g? Ar(Cl) = 35.5)

chlorine consists of Cl2 molecules, so Mr = 2 x 35.5 = 71

moles chlorine = mass / Mr = 6.5 / 71 = 0.0944 mol

Example 7.2.6: How many moles of iron in 20g? (Fe = 56)

iron consists of Fe atoms, so moles iron = mass/Ar = 20/56 = 0.357 mol Fe

Example 7.2.7: How many grams of propane C3H8 are there in 0.21 moles of it? (C = 12, H = 1)

Mr of propane = (3 x 12) + (1 x 8) = 44, so g propane = moles x Mr = 0.21 x 44 = 9.24g

Example 7.2.8: 0.25 moles of molecule X was found to have a mass of 28g. Calculate its molecular mass.

Mr = mass X / moles of X = 28 / 0.25 = 112

Example 7.2.9: What mass and moles of magnesium chloride is formed when 5g of magnesium oxide is dissolved in excess hydrochloric acid?

reaction equation: MgO + 2HCl ==> MgCl2 + H2O

means 1 mole magnesium oxide forms 1 mole of magnesium chloride (1 : 1 molar ratio)

formula mass MgCl2 = 24+(2x35.5) = 95,

MgO = 24+16 = 40, 1 mole MgO = 40g, so 5g MgO = 5/40 = 0.125 mol

which means 0.125 mol MgO forms 0.125 mol MgCl2,

Mass = moles x formula mass = 0.125 x 95 = 11.9g MgCl2

Example 7.2.10: What mass and moles of sodium chloride is formed when 21.2g of sodium carbonate is reacted with excess dilute hydrochloric acid?

reaction equation: Na2CO3 + 2HCl ==> 2NaCl + H2O + CO2

means 1 mole sodium carbonate gives 2 moles of sodium chloride (1:2 ratio in equation)

Formula mass of Na2CO3 = (2x23) + 12 + (3x16) = 106

Formula mass of NaCl = 23 + 35.5 = 58.5

moles Na2CO3 = 21.2/106 = 0.2 mole

therefore 2 x 0.2 = 0.4 mol of NaCl formed.

mass of NaCl formed = moles x formula mass = 0.4 x 58.5 = 23.4g NaCl

Using the Avogadro Constant, you can actually calculate the number of particles in known quantity of material.

Example 7.3.1: How many water molecules are there in 1g of water, H2O ?

formula mass of water = (2 x 1) + 16 = 18

every mole of a substance contains 6 x 1023 particles of 'it' (the Avogadro Constant).

moles water = 1 / 18 = 0.0556

molecules of water = 0.0556 x 6 x 1023 = 3.34 x 1022

Since water has a density of 1g/cm3, it means in every cm3 or ml there are

33 400 000 000 000 000 000 000 individual H2O molecules or particles.

Example 7.3.2: How many atoms of iron (Fe = 56) are there in an iron filing of mass 0.001g ?

0.001g of iron = 0.001 / 56 = 0.00001786 mol

atoms of iron in the nail = 0.00001786 x 6 x 1023 = 1.07 x 1019 actual Fe atoms

(10.7 million million million atoms!)

Example 7.3.2: (a) How many particles of 'Al2O3' in 51g of aluminium oxide?

Atomic masses: Al =27, O = 16, f. mass Al2O3 = (2x27) + (3x16) = 102

moles 'Al2O3' = 51/102 = 0.5 mol

Number of 'Al2O3' particles = 0.5 x 6 x 1023 = 3 x 1023

(b) Aluminium oxide is an ionic compound. Calculate the number of individual aluminium ions (Al3+) and oxide ions (O2-) in the same 51g of the substance.

For every Al2O3 there are two Al3+ and three O2- ions.

So in 51g of Al2O3 there are ...

0.5 x 2 x 6 x 1023 = 6 x 1023 Al3+ ions, and

0.5 x 3 x 6 x 1023 = 9 x 1023 O2- ions.

More advanced use of the mole and Avogadro Number concepts (for advanced level students only)

You can have a mole of whatever you want in terms of chemical species e.g.

In terms of electric charge, 1 Faraday = 96500 C (coulombs) = 6 x 1023 electrons

If you have 2.5 moles of the ionic aluminium oxide (Al2O3) you have ...

2 x 2.5 = 5 moles of aluminium ions (Al3+) and 3 x 2.5 = 7.5 mol of oxide ions (O2-)

When you write ANY balanced chemical equation, the balancing numbers, including the un-written 1, are the reacting molar ratio of reactants and products.

Extra Advanced Questions - more suitable for Advanced AS-A2 students which can be completely tackled after ALSO studying section 9 on the molar volume of gases and ANSWERS to QA7.1

QA7.1 This question involves using the mole concept and the Avogadro Constant in a variety of situations.

The Avogadro Constant = 6.02 x 1023 mol-1. The molar volume for gases is 24dm3 at 298K/101.3kPa.

Atomic masses: Al = 27, O = 16, H = 1, Cl = 35.5, Ne = 20, Na = 23, Mg = 24.3, C = 12

Where appropriate assume the temperature is 298K and the pressure 101.3kPa.

Calculate ....

(a) how many oxide ions in 2g of aluminium oxide?

(b) how many molecules in 3g of hydrogen?

(c) how many molecules in 1.2 cm3 of oxygen?

(d) how many molecules of chlorine in 3g?

(e) how many individual particles in 10g of neon?

(f) the volume of hydrogen formed when 0.2g of sodium reacts with water.

(g) the volume of hydrogen formed when 2g of magnesium reacts with excess acid.

(h) the volume of carbon dioxide formed when the following react with excess acid

(1) 0.76g of sodium carbonate and (2) 0.76g sodium hydrogencarbonate

(i) the volume of hydrogen formed when excess zinc is added to 50 cm3 of hydrochloric acid, concentration 0.2 mol dm-3.

(j) the volume of carbon dioxide formed when excess calcium carbonate is added to 75 cm3 of 0.05 mol dm-3 hydrochloric acid.

chemistry notes / igcse-gcse- Olevel

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