Thursday, November 27, 2008

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.


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.

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

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

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chemistry notes / igcse-gcse- Olevel