Which statement best describes and explains the effect of a catalyst on the rate of a chemical reaction?

The minimum energy required for a chemical reaction to 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 (Figure 7.9). Increasing the reaction temperature has the effect of increasing the number of particles with enough energy to take part in the reaction, and increasing the reaction rate.

Figure 7.9: The distribution of particle kinetic energies at a fixed temperature.

Remember that a molecule must have energy greater than the activation energy, as well as the correct orientation, for the reaction to take place.

Increasing the temperature of a reaction mixture raises the average kinetic energy of the particles. As can be seen on the graph (Figure 7.10), a higher proportion of the particles can now react making the reaction faster. With the increased movement of the molecules the chances of a molecule having the correct orientation is also increased.

Figure 7.10: The distribution of particle kinetic energies with an increase in temperature. There is an increased number of particles with sufficient energy due to the higher temperature.

An endothermic reaction can be represented by:

\(\text{Reactants} + \color{red}{\text{Energy}} \to \text{Products}\) i.e. a reaction that absorbs energy

This can be shown in an activation energy diagram (Figure 7.11). These graphs are also sometimes called a reaction profile or a potential energy graph.

Figure 7.11: An activation energy diagram with reactant energy lower than product energy, i.e. endothermic

An exothermic reaction can be represented by:

\(\text{Reactants}\to \text{Products} + \color{red}{\text{Energy}}\) i.e. a reaction that releases energy

This can be shown in an activation energy diagram (Figure 7.12):

Figure 7.12: An activation energy diagram with reactant energy greater than product energy, i.e. exothermic

Revise this section from Grade 11 (Activation energy, Section 12.3).

A catalyst increases reaction rates in a slightly different way from other methods of increasing reaction rate. The function of a catalyst is to lower the activation energy so that a greater proportion of the particles have enough energy to react. A catalyst can lower the activation energy for a reaction by:

• orienting the reacting particles in such a way that successful collisions are more likely

• reacting with the reactants to form an intermediate that requires lower energy to form the product

Some metals e.g. platinum, copper and iron can act as catalysts in certain reactions. In our own bodies, we have enzymes that are catalysts, which 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.

An activated complex occurs in reactions without catalysts as well as those with catalysts.

The following is an example of how a reaction involving a catalyst might proceed. A and B are reactants, \(\color{blue}{\text{C}}\) is the catalyst, and D is the product of the reaction of A and B.

Step 1: \({\text{A}} + \color{blue}{\text{C}} \to \text{A}\color{blue}{\text{C}}\)

Step 2: \(\text{B} + \text{A}\color{blue}{\text{C}} \to \text{A}\color{blue}{\text{C}}\text{B}\)

Step 3: \(\text{A}\color{blue}{\text{C}}\text{B} \to \color{blue}{\text{C}} + \text{D}\)

\(\text{A}\color{blue}{\text{C}}\text{B}\) represents the intermediate chemical. Although the catalyst (\(\color{blue}{\text{C}}\)) is consumed by reaction 1, it is later released again by reaction 3, so that the overall reaction with a catalyst is as follows:

\(\text{A} + \text{B} + \color{blue}{\text{C}} \to \text{D} + \color{blue}{\text{C}}\)

You can see from this that the catalyst is released at the end of the reaction, completely unchanged. Without a catalyst the overall reaction would be:

\(\text{A} + \text{B}\) \(\to\) \(\text{D}\)

The catalyst has provided an alternative set of reaction steps, which we refer to as an alternative pathway. The pathway involving the catalyst requires less activation energy and is therefore faster.

This can be seen in the following diagram (Figure 7.13).

Figure 7.13: The proportion of particles that have enough energy to react is increased in the presence of a catalyst.

A catalyst speeds up a chemical reaction, without being consumed by the reaction. 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 magnitude of the activation energy on the energy diagram in Figure 7.14), and therefore increase the reaction rate. Remember that with a catalyst, the average kinetic energy of the molecules remains the same but the required energy decreases (Figure 7.13).

Figure 7.14: The effect of a catalyst on the activation energy of an endothermic reaction. The catalyst would act in the same way for an exothermic reaction.