Understandings:

• The rate of reaction is expressed as the change in concentration of a particular reactant/product per unit time.

• Concentration changes in a reaction can be followed indirectly by monitoring changes in mass, volume and colour.

Applications and skills:

• Analysis of graphical and numerical data from rate experiments.

• Explanation of the effects of temperature, pressure/concentration and particle size on rate of reaction.

• Investigation of rates of reaction experimentally and evaluation of the results.

Guidance:

• Calculation of reaction rates from tangents of graphs of concentration, volume or mass vs time should be covered.

• Students should be familiar with the interpretation of graphs of changes in concentration, volume or mass against time.

Linking Practical Chemistry to Definition

As we have already defined the rate of reaction as the change in concentration of either products or reactants / unit time then if we can plan an experiment that allows us to follow how these concentrations are changing over time then we can calculate Rate of Reaction from experimental data.

As you can see from the above graph - Rate of Reaction is not constant when a reaction proceeds. This is because:

• There are more particles of Reactant at the start of a reaction
• Therefore there is a higher frequency of successful collisions at the start of a reaction
• There is a higher number of particles of product as the reaction proceeds and so particles of reactant can collide with these as they are 'in the way of the reactants'

All this ultimately means a reaction will slow down as a reaction proceeds.

In the graph you can therefore observe that there is a Curve as the relationship changes over time. The reaction will finish when the line is horizontal as there is no observable change in concentration of the products.

Faster rates of Reaction have a steeper gradient and slower rates have a shallow gradient.

If we link this to the ideas we observed in Factors Affecting Rate we can see that Higher Temperatures, Higher Concentrations, Higher Surface Areas, The use of a catalyst and higher Pressures (in gases!) will all cause a steeper gradient such as the gradient observed when comparing small and large marble chips below.

Loss of Mass

A first example of using a science experiment to determine rate of reaction can be from using the Loss of Mass Experiment.

In this experiment a mass of reactant (such as Marble Chips) with a consistent surface area will be placed in a reactant (such as Hydrochloric Acid) whilst on top of a Measuring Balance.

As the Reaction Proceeds the mass should decrease as gas is produced as a product.

If you time this reaction (with a stopwatch) and take the mass of the reaction at regular intervals (i.e. every 15 seconds). Then you should manage to gain experimental data that shows a rate of reaction curve.

You should aim to finish collecting data after you get three masses in a row that have not changed as this shows you have reached constant mass.

Examiner Tip for Practical Exam Questions:

• Only Change one variable (Temperature, surface area, concentration, use of a catalyst) - this is the Independent variable
• Ensure you mention how you are measuring the Mass Change - this is the Dependent Variable
• Keep other factors that can affect rate of reaction the SAME - these are control variables
• Mention one safety feature of the experiment (such as wearing safety goggles because the Acid is CORROSIVE) - you must link the safety feature to the Hazard to get a mark

Measuring Gas Evolved

Another example of using a science experiment to determine rate of reaction can be from using the Gas Evolved Experiment.

In this experiment a mass of reactant (such as Marble Chips) with a consistent surface area will be placed in a reactant (such as Hydrochloric Acid) whilst being attached to a Gas Syringe with a Delivery Tube.

As the Reaction Proceeds gas evolved should be collected in the gas syringe.

If you time this reaction (with a stopwatch) and measure the volume of gas produced at regular intervals (i.e. every 15 seconds). Then you should manage to gain experimental data that shows a rate of reaction curve.

You should aim to finish collecting data after you get three gas volumes in a row that have not changed as this shows you have reached constant mass.

Examiner Tip for Practical Exam Questions:

• Only Change one variable (Temperature, surface area, concentration, use of a catalyst) - this is the Independent variable
• Ensure you mention how you are measuring the Mass Change - this is the Dependent Variable
• Keep other factors that can affect rate of reaction the SAME - these are control variables
• Mention one safety feature of the experiment (such as wearing safety goggles because the Acid is CORROSIVE) - you must link the safety feature to the Hazard to get a mark

Disappearing X

Another example of using a science experiment to determine rate of reaction can be from using the Disappearing X Experiment.

In this experiment an amount of reactant (such as Sodium Thiosulphate) will be placed in a reactant (such as Hydrochloric Acid) whilst being on top of a Cross of constant Size.

As the Reaction Proceeds A Precipitate should be produced - changing the colour of the reaction mixture. Eventually you will no longer be able to see the X.

If you time this reaction (with a stopwatch) and measure the Time taken for the X to no longer be seen then you should manage to gain experimental data that shows a rate of reaction curve.

Examiner Tip for Practical Exam Questions:

• Only Change one variable (Temperature, surface area, concentration, use of a catalyst) - this is the Independent variable
• Ensure you mention how you are measuring the Mass Change - this is the Dependent Variable
• Keep other factors that can affect rate of reaction the SAME - these are control variables
• Mention one safety feature of the experiment (such as wearing safety goggles because the Acid is CORROSIVE) - you must link the safety feature to the Hazard to get a mark

Temperature - a difficult control variable

For many reactions if you assume a constant temperature you may be incorrect. It is best practice to use water baths at a constant temperature in order to control temperature.