Energy Profiles With & Without Catalysts
How do catalysts increase the rate of reaction?
- A catalyst increases the rate of a reaction by providing the reactants with an alternative reaction pathway which is lower in activation energy than the uncatalysed reaction
- The catalyst remains chemically unaltered by the end of the reaction
How a catalyst increases the rate of reaction
The diagram shows that the catalyst speeds up a reaction that would normally be slow due to the high activation energy. The catalyst is not used up in the chemical reaction and is not taking part in the chemical reaction
- Catalysts are important in reducing the environmental impact of industrial processes by:
- Reducing the energy requirements of processes as they enable reactions to occur at lower temperatures and pressures
- Reducing waste products as they can be reused and are only used in small quantities, increasing atom economy
- Increasing the selectivity of processes, promoting specific reactions and suppressing undesired side reactions
- Catalysts can be divided into two types:
- Homogeneous catalysts
- Heterogeneous catalysts
- Homogeneous means that the catalyst is in the same phase as the reactants
- For example, the reactants and the catalysts are all liquids
- Heterogeneous means that the catalyst is in a different phase to the reactants
- For example, the reactants are gases but the catalyst used is a solid
Energy profiles of reactions with catalysts
- The lower activation energy of the alternative pathway used by a catalyst can be shown on an energy profile
Energy profile with and without a catalyst
The diagram shows that the catalyst allows the reaction to take place through a different mechanism, which has a lower activation energy than the original reaction
Examples of catalysts
- Enzymes are biological catalysts
- Enzymes act as catalysts in biological systems, controlling many biochemical reactions within cells
- As well as being important for controlling reactions in cells, they are also important in industry
- Enzymes allow industrial reactions to happen at lower temperatures and pressures than usually needed, saving money and energy
- Transition metals are often used as catalysts due to their ability to form more than one stable oxidation state
- For more information about the uses of transition metals as catalysts required in Higher Level Chemistry, see our revision note on the Characteristic Properties of Transition Elements
Worked example
The energy profile below shows the energy changes for a reaction with and without a catalyst.
Which symbols represent the enthalpy change, ∆H, and the activation energy, Ea, for the reaction using a catalyst?
| ΔH | Ea (with catalyst) | |
| A. | a | c |
| B. | b | c |
| C. | c | a |
| D. | b - a | c |
Answer:
- The correct option is C.
- By definition, the enthalpy change is the difference in energy content between reactants and products, in this case, arrow c
- The catalyst lowers the activation energy, which corresponds to arrow a