Thursday, November 27, 2008

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

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.


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.

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