Module 3  

Lesson 2.3  Catalysis and Reaction Rates



Key Concepts


As presented earlier in this section, activation energy is the minimum amount of energy required to "activate" the entities (atoms, ions, molecules) involved in a chemical reaction. Therefore, activation energy is an energy barrier that must be overcome if a reaction is to proceed. A catalyst is a substance within a chemical system that provides an alternate lower energy pathway for the reaction. In other words, the catalyst allows the reaction to follow a different pathway - namely, a pathway that has a lower activation energy.


Fig. 1  Effects of a catalyst

Consider Fig. 2 and Fig. 3 shown below.  In both graphs, the number of particles that have sufficient energy to react are represented by the green areas under the curves.   Fig 2 shows what happens when a catalyst is added to the system.  Notice that lowering the activation energy increases the number of particles that have enough energy to react.


Fig. 2    Reaction with no catalyst


Fig. 3  Reaction with a catalyst

IMPORTANT: Although a catalyst lowers the activation energy (Ea) of the reaction, the difference in potential energy between the reactants and the products is still the same, regardless of whether a catalyst is used. Refer to the energy pathway diagram below.


Fig. 4



Regarding catalysis, the following points are important:

  • Catalysts in living systems are referred to as enzymes.

  • Catalysts increase the rate of a reaction.

  • Catalysts are not changed or used up in the general chemical process.

  • Although the catalyst provides a lower activation energy for the reaction, the catalyst does not alter the total energy released or absorbed during a reaction. In other words, the enthalpy of a reaction is unaffected by the use of a catalyst.
Because catalysts allow chemical reactions to proceed at lower temperatures, they are extremely important in technology and industry. Following are some examples:

  • Catalysts are used in the refining of petroleum (catalytic reforming or alkylation) to increase production of the more desirable fractions.

  • In the upgrading of bitumen to synthetic crude, catalysts are employed during the hydrocracking and hydrotreating processes.

  • In the brewing industry, the enzyme in yeast is used in the process of fermentation to convert sugar to ethanol.

  • The catalytic converter in a vehicle's exhaust system reduces air pollution by catalyzing the reduction of NO(g) and NO2(g) to harmless N2(g).

Check Your Understanding


Complete Practice Questions 2, 3, and 4 on page 535 in the textbook. Check your answers by clicking on the link below.

Page 535 Practice Question 2

The catalytic converter and catalytic cracking towers in petroleum refining are examples of technological inventions that involve the use of catalysts.


Page 535 Practice Question 3

Photosynthesis, fermentation, and cellular respiration are natural (biological) processes that involve the use of catalysts. Many other examples exist as most biological reactions involve the use of enzymes as catalysts.


Page 535 Practice Question 4

Because enzymes are a type of catalyst, they are not changed by the chemical reaction. Because enzymes catalyze many reactions, they do not have to be present in large numbers to improve the efficiency of a chemical process in the body. If poisoned, the reaction rate is affected dramatically because very few enzyme molecules exist. The resulting rate of reaction is reduced dramatically because of fewer catalyzed reactions.


Virtual Investigation