Catalysts and Reaction Rates for the ESAT
Updated July 2026
A catalyst increases the rate of a chemical reaction by providing an alternative reaction mechanism with a lower activation energy. It remains chemically unchanged and is not consumed during the process. For the ESAT, you must be able to identify these properties and interpret their effects on energy level diagrams and chemical equilibria.
A catalyst speeds up a reaction by offering a lower-energy pathway (alternative mechanism) without being used up or altering its chemical identity. It lowers the activation energy () but does not change the enthalpy change () or the position of equilibrium.
The Nature of a Catalyst
A catalyst is defined as a substance that increases the rate of a chemical reaction without undergoing any permanent chemical change itself. This definition leads to two critical observations required for the ESAT. First, catalysts are not used up. While they may participate in intermediate steps of a reaction, they are regenerated by the end of the sequence. Consequently, the mass of the catalyst at the start of a reaction is identical to its mass at the conclusion. Second, catalysts are chemically unchanged. The chemical formula and molecular structure of the catalyst remain the same before and after the reaction process. Because they are not consumed, catalysts are often needed only in very small quantities to facilitate the conversion of a large amount of reactant into product.
Reaction Mechanisms and Activation Energy
To understand how a catalyst works, we must consider collision theory. For a reaction to occur, reactant particles must collide with sufficient energy to break existing bonds and initiate the rearrangement of atoms. This minimum energy required is known as the activation energy (). A catalyst functions by providing an alternative route or reaction mechanism for the reaction. This new pathway involves different intermediate steps that have a lower activation energy than the original uncatalysed route. Because the energy barrier is lower, a higher proportion of reactant particles will possess energy equal to or greater than the required activation energy at any given temperature. This results in a higher frequency of successful collisions, thereby increasing the reaction rate.
Interpreting Energy Level Diagrams
The effect of a catalyst is most clearly seen on an energy level diagram. In these diagrams, the y-axis represents the potential energy of the system and the x-axis represents the progress of the reaction. An uncatalysed reaction is shown as a high curve or hump starting from the reactants and ending at the products. The distance from the reactant energy level to the peak of this curve is the activation energy (). When a catalyst is introduced, a second curve is drawn. This catalysed route has a lower peak, representing the reduced activation energy (). It is vital to note that the energy levels of the reactants and the products themselves do not change. Therefore, the overall enthalpy change (), which is the difference between the reactant and product energy levels, remains exactly the same for both the catalysed and uncatalysed reactions.
Catalysts and Chemical Equilibrium
In reversible reactions, a state of dynamic equilibrium is reached when the rate of the forward reaction equals the rate of the reverse reaction. A catalyst increases the rate of the forward reaction because it provides a lower energy pathway. However, it also lowers the activation energy for the reverse reaction by the exact same amount. Because both the forward and backward rates are increased by the same factor, the position of equilibrium remains unchanged. A catalyst does not increase the yield of a product in a reversible reaction; it only allows the system to reach that equilibrium state more quickly. This is a common point of confusion in exam questions: always remember that catalysts affect the kinetics (speed) of a reaction but not its thermodynamics (the final equilibrium position).
Key takeaways
- Catalysts are not consumed and remain chemically identical at the end of a reaction.
- The primary function of a catalyst is to provide an alternative reaction mechanism with a lower activation energy ().
- On an energy level diagram, a catalyst lowers the height of the reaction peak but does not change the energy levels of reactants or products.
- Catalysts increase the rate of both forward and reverse reactions equally, meaning the equilibrium position is unaffected.
When looking at energy level diagrams, the activation energy is always measured from the energy of the reactants to the peak of the curve. Be careful not to measure from the energy of the products unless you are specifically calculating the for the reverse reaction.
A common mistake is stating that a catalyst 'lowers the activation energy of the original route'. This is incorrect. The original route still exists with its original . The catalyst provides a brand new, separate route with its own lower .
The effect of a catalyst on the rate of reaction is exponential. Even a small decrease in activation energy can lead to a massive increase in the reaction rate because the number of particles with enough energy to react follows a Maxwell-Boltzmann distribution, where many more particles are available at slightly lower energy thresholds.
Frequently asked questions
Does a catalyst change the enthalpy change of a reaction?
No. The enthalpy change () is the difference in energy between the reactants and the products. Since the energy levels of the reactants and products are not altered by a catalyst, the remains constant.
Why is only a small amount of catalyst usually needed?
Because a catalyst is not used up in the reaction, it is regenerated at the end of each catalytic cycle. This allows a single catalyst molecule to facilitate the reaction of many thousands of reactant molecules in succession.
How does a catalyst affect the yield of a product in a reversible reaction?
A catalyst has no effect on the final yield of a product. It only increases the rate at which the reaction reaches equilibrium. To change the yield, one would need to alter conditions like temperature, pressure, or concentration.
Can a catalyst lower the activation energy of the reverse reaction?
Yes. A catalyst lowers the energy barrier for the entire alternative pathway. This means the peak of the curve is lower when approached from either the reactant side (forward) or the product side (reverse).