A catalyst is a substance that will increase the rate of a reaction without being used up itself.
This is often done by lowering the activation energy required of a reaction by providing an alternate pathway for the reaction to proceed. Sometimes the catalyst is changed but can later be altered back into an active catalyst thereby completing the cycle.
Without a catalyst, the reaction would still be feasible but would often take a long time to occur.
A catalyst does not affect the position of equilibrium of a chemical reaction but does affect the rate at which the equilibrium is reached.
Types of catalyst
There are different categories of catalyst dependant on how they function:
Homogeneous catalysts work in the same phase as the reactants. Biological enzymes fall into this category but are sometimes considered to be heterogeneous catalysts as some enzymes work in a different phase. (often due to transition metal groups being of solid phase).
Advantages of using a homogeneous catalyst in a reaction are:
- Surface area is not a limiting factor where concentration is generally involving far fewer of the catalyst molecules to be present.
- Dispersion of any physical properties (e.g. heat) is significantly faster as the catalyst is incorporated into a solution with the molecules
- Homogeneous catalysts can generally be of higher specificity, this also means there is less chance of catalyst contamination by a not-intended reactant.
- Cost can be higher as they are more specific to the purpose.
- Contamination of the solution as they cannot be easily removed (therefore difficult to recycle). It is often easier for the product of a reaction to be removed by use of a physical property such as boiling point.
Heterogeneous catalysts work in an alternate phase to the reactants. As a molecule passes over its surface it can adsorb (‘attach to’ by some form of weak bond see Adsorption) to the surface providing an optimal orientation for reaction and/or the surface aiding the chemical reaction by simply holding the molecules in place.
Advantages of using a heterogeneous catalyst in a reaction are:
- Can often be used with more extreme conditions (e.g. high temperature/pressure)
- Does not contaminate the solution (the products may still need separating from reactants)
- Easy to use again as it can be easily removed from the solution.
- The catalyst can be “poisoned” by other molecules adsorbing to the surface by chemical adsorption (chemisorption) which are bound with a high affinity to the catalyst. This requires a treatment process or a new catalyst.
- Surface area is a limiting factor and for efficiency, a high surface area comb or beads are required which can make the production of the catalyst difficult.
Lowering the activation energy means the kinetic energy of a molecule to initiate a reaction is lowered which in turn means a lower temperature is required as EK ∝ T (EK = (3/2)(R/NA)T, see ideal gas laws).
To do this the catalyst can break the reaction up into multiple steps, by doing so each step can have a lower activation energy and so for the reaction to occur it does not need as high a temperature. Alternatively the reaction may proceed by a different route using a catalytic substrate (this will be involved in the reaction but is ultimately left unchanged) this will provide a lower activation energy and as Hess’s law suggests the overall ΔH will remain the same.
The presence of a catalyst can also mean more of the reactants present in a system can be used as more molecules are able to overcome the activation energy as can be seen in a Maxwell-Boltzmann distribution curve
Biological catalyst function (enzymes)
There are different models for enzyme function involving ‘lock and key’ and ‘induced fit’.
Enzymes can hold molecules in place, apply strain to bonds, force molecules together or (by reaction sequences) cleave a bond. For further information specifically about enzyme function see Enzymes.
- ↑ Abdelnour M, Quave A, Ma T, Baghoyan H. Enzymes. 2018 [cited 8/12/2018]. Available from: https://chem.libretexts.org/Textbook_Maps/Biological_Chemistry/Catalysts/Enzymes.
- ↑ TutorVista.com. Homogeneous catalyst. 2018 [cited 8/12/2018]. Available from: https://chemistry.tutorvista.com/inorganic-chemistry/homogeneous-catalyst.html