Mechanism of reaction and catalysis
Earlier it was mentioned that it is the collision of particles that causes reactions to occur and that only some of these collisions are 'successful'. This is because the reactant particles have a wide range of kinetic energy, and only a small fraction of the particles will have enough energy to actually break bonds so that a chemical reaction can take place. The minimum energy that is needed for a reaction to take place is called the activation energy. For more information on the energy of reactions, refer to Grade 11.
- Definition 1: Activation energy
The energy that is needed to break the bonds in reactant molecules so that a chemical reaction can proceed.
Even at a fixed temperature, the energy of the particles varies, meaning that only some of them will have enough energy to be part of the chemical reaction, depending on the activation energy for that reaction. This is shown in Figure 1. Increasing the reaction temperature has the effect of increasing the number of particles with enough energy to take part in the reaction, and so the reaction rate increases.
A catalyst functions slightly differently. The function of a catalyst is to lower the activation energy so that more particles now have enough energy to react. The catalyst itself is not changed during the reaction, but simply provides an alternative pathway for the reaction, so that it needs less energy. Some metals e.g. platinum, copper and iron can act as catalysts in certain reactions. In our own human bodies, enzymes are catalysts that help to speed up biological reactions. Catalysts generally react with one or more of the reactants to form a chemical intermediate which then reacts to form the final product. The chemical intermediate is sometimes called the activated complex.
The following is an example of how a reaction that involves a catalyst might proceed. C represents the catalyst, A and B are reactants and D is the product of the reaction of A and B.
Step 1: A + C AC
Step 2: B + AC ABC
Step 3: ABC CD
Step 4: CD C + D
In the above, ABC represents the intermediate chemical. Although the catalyst (C) is consumed by reaction 1, it is later produced again by reaction 4, so that the overall reaction is as follows:
A + B + C D + C
You can see from this that the catalyst is released at the end of the reaction, completely unchanged.
- Definition 2: Catalyst
A catalyst speeds up a chemical reaction, without being altered in any way. It increases the reaction rate by lowering the activation energy for a reaction.
Energy diagrams are useful to illustrate the effect of a catalyst on reaction rates. Catalysts decrease the activation energy required for a reaction to proceed (shown by the smaller 'hump' on the energy diagram in Figure 2), and therefore increase the reaction rate.
General experiment 1: Catalysts and reaction rates
To determine the effect of a catalyst on the rate of a reaction
Zinc granules, 0,1 M hydrochloric acid, copper pieces, one test tube and a glass beaker.
Place a few of the zinc granules in the test tube.
Measure the mass of a few pieces of copper and keep them separate from the rest of the copper.
Add about 20 cm3 of to the test tube. You will see that a gas is released. Take note of how quickly or slowly this gas is released. Write a balanced equation for the chemical reaction that takes place.
Now add the copper pieces to the same test tube. What happens to the rate at which the gas is produced?
Carefully remove the copper pieces from the test tube (do not get on your hands), rinse them in water and alcohol and then weigh them again. Has the mass of the copper changed since the start of the experiment?
During the reaction, the gas that is released is hydrogen. The rate at which the hydrogen is produced increases when the copper pieces (the catalyst) are added. The mass of the copper does not change during the reaction.
The copper acts as a catalyst during the reaction. It speeds up the rate of the reaction, but is not changed in any way itself.
Exercise 1: Reaction rates
For each of the following, say whether the statement is true or false. If it is false, re-write the statement correctly.
A catalyst increases the energy of reactant molecules so that a chemical reaction can take place.
Increasing the temperature of a reaction has the effect of increasing the number of reactant particles that have more energy that the activation energy.
A catalyst does not become part of the final product in a chemical reaction.
- False. A catalyst lowers the activation energy of a reaction, so that a chemical reaction can take place.
5 g of zinc granules are added to 400 cm3 of 0,5 mol·dm−3 hydrochloric acid. To investigate the rate of the reaction, the change in the mass of the flask containing the zinc and the acid was measured by placing the flask on a direct reading balance. The reading on the balance shows that there is a decrease in mass during the reaction. The reaction which takes place is given by the following equation:
Why is there a decrease in mass during the reaction?
The experiment is repeated, this time using 5 g of powdered zinc instead of granulated zinc. How will this influence the rate of the reaction?
The experiment is repeated once more, this time using 5 g of granulated zinc and 600 cm3 of 0,5 mol·dm−3 hydrochloric acid. How does the rate of this reaction compare to the original reaction rate?
What effect would a catalyst have on the rate of this reaction?
(IEB Paper 2 2003)
- The reaction does not take place in a sealed container and so the hydrogen gas can escape from the reaction vessel.
- The rate will increase since there will be a greater surface area of zinc to react with the acid.
- There is a larger volume of hydrochloric acid. However the concentration is not increased and so the reaction is unchanged.
- The rate of reaction would increase.
Enzymes are catalysts. Conduct your own research to find the names of common enzymes in the human body and which chemical reactions they play a role in.
Some suggestions are:
Amylases break down starch, proteases break down protein. Kinases and phosphatases are used in movement and muscle contraction. Kinases and phosphatases also assist with cell regulation and nerve signalling.
5 g of calcium carbonate powder reacts with 20 cm3 of a 0,1 mol·dm−3 solution of hydrochloric acid. The gas that is produced at a temperature of 25°C is collected in a gas syringe.
Write a balanced chemical equation for this reaction.
The rate of the reaction is determined by measuring the volume of gas that is produced in the first minute of the reaction. How would the rate of the reaction be affected if:
a lump of calcium carbonate of the same mass is used
40 cm3 of 0,1 mol·dm−3 hydrochloric acid is used
2a. The rate of reaction would decrease
b. The rate of reaction depends on the concentration of liquid reactants and not on the volume so the rate will remain the same.