Reactions can only happen if the colliding particles have enough energy.

# Rates of Reaction 3

Many factors affect rates of chemical reactions - pressure of gases, temperature, surface area of solids, concentration and if there is a catalyst. Anything that will change the probability of particles colliding or change the energy of the collisions will affect the rate of a reaction. This is the last of three GCSE Chemistry quizzes looking at these factors.

When investigating rates of reaction, it is necessary to make a series of measurements over a period of time, for example, how much hydrogen is produced during the reaction of an acid with zinc. The experiment should be repeated several times and, after discarding any anomalous results, the readings averaged and plotted on a scatter graph with time along the horizontal axis. The line of best fit will usually be a curve, with the steepest gradient at the start indicating the fastest rate of reaction. Where the curve is horizontal, is shows that the reaction had finished. The conditions of the experiment can then be changed and the whole process repeated. Plotting the results on the same graph, using different colours, gives a quick and easy visual interpretation, from which you can write your conclusion.

When working out rates of reaction mathematically, as higher tier candidates are more likely to do, dividing the amount of reactant used (or product formed) by the time taken gives a valid rate. This is effectively the same as working out the gradient of a graph. If 15 cm3 of carbon dioxide were released in the first 20 seconds of a reaction, the rate would be 0.75 cm3/s. Later in the reaction, it may take 45 seconds to produce 10 cm3 of carbon dioxide, in which case the rate would have dropped to 0.22 cm3/s. This slowing down of the rate occurs because the concentration of one or both of the reactants changes during the reaction. Professional scientists usually refer to the initial rate of reaction, in other words the rate at the very start.

How would a decrease in temperature affect the rate of a reaction? What about surface area? Have a go at this quiz and test your knowledge of the factors which affect rates of reaction, such as temperature, surface area or pressure.

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1. What units do we use to measure concentration?
Moles per cubic decimetre
2. On an energy level diagram, what effect would adding a catalyst have to the reaction on the line showing the energy for a reaction?
A catalyst lowers the activation energy or energy required for the reaction to take place. For the GCSE, make sure that you can label energy level diagrams
3. What apparatus is used to measure the volume of gas given off in a reaction?
You can also use apparatus such as a measuring cylinder, a graduated test tube or even a burette
4. Which of the following does NOT affect the rate of a chemical reaction between a solid and a liquid?
Carrying out the reaction at a different altitude would affect the pressure. Changes in pressure affect only reactions involving gases
5. Calcium carbonate (marble) reacts with hydrochloric acid to produce water, carbon dioxide and calcium chloride. Which of the following combinations is likely to result in the fastest rate of reaction?
Increasing the surface area and the temperature will increase the rate of reaction greatly
6. Which of the following is NOT a disadvantage of using a catalyst in a chemical reaction?
Catalysts are not used up in chemical reactions
7. Which of the following statements about collisions is correct?
If there is sufficient energy involved in a collision between particles, the bonds will break
8. What is activation energy?
Reactions involve bonds being broken followed by new, different bonds being formed
9. Which of the following statements about the effect the size of the pieces of a solid on the rate of a chemical reaction is true?
The larger surface area means that there are more particles available to be involved in collisions. It is only ever the surface layer of particles of solids that are involved in reactions
10. Platinum is used as a catalyst in the production of nitric acid. Which of the following statements is correct?
As the platinum does not take part in the reaction itself, it does not gain or lose mass

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