Properties and Effects of Catalysts
Catalysts accelerate chemical reactions by providing an alternative, lower-energy pathway, without being consumed in the process. For the ESAT, you must understand how they affect reaction energy profiles and their role in chemical equilibrium.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- A catalyst is chemically unchanged at the end of a reaction; its mass and composition remain constant.
- It provides a new reaction mechanism, which has a lower activation energy (Ea) than the uncatalysed route.
- By lowering Ea, a greater proportion of particle collisions have sufficient energy to be successful, thus increasing the reaction rate.
- Catalysts do not alter the starting energy of reactants or the final energy of products, so the overall enthalpy change (ΔH) is unaffected.
- Catalysts increase the rate of both the forward and reverse reactions equally.
- Consequently, a catalyst does not change the position of a chemical equilibrium; it only reduces the time taken to reach it.
Definitions
- Catalyst
- A substance that increases the rate of a chemical reaction by providing an alternative reaction pathway with a lower activation energy, without being consumed or chemically altered by the end of the reaction.
- Activation Energy (Ea)
- The minimum amount of energy required for reactant particles to form products upon collision.
Worked example
An uncatalysed reversible reaction has a forward activation energy of +80 kJ/mol and an overall enthalpy change (ΔH) of +20 kJ/mol. An effective catalyst is then added. Which statement is correct?
- 1
Identify the initial values:
Forward Ea = +80 kJ/mol, ΔH = +20 kJ/molThe reaction is endothermic.
- 2
Recall the function of a catalyst:
it lowers the activation energy (Ea) but does not affect the overall enthalpy change (ΔH).
- 3
This means the new forward Ea must be less than +80 kJ/mol, and ΔH must remain +20 kJ/mol.
- 4
Calculate the original reverse activation energy.
For an endothermic reaction, Reverse Ea = Forward Ea - ΔH.
So, Reverse Ea = 80 - 20 = +60 kJ/mol.
- 5
A catalyst lowers the energy of the transition state.
This reduces the energy barrier from both the reactant and product sides.
Therefore, both the forward and reverse activation energies will decrease.
- 6
Evaluate the options based on these facts.
The only true statement will be one where both forward and reverse activation energies decrease while the enthalpy change remains constant.
Answer: The forward activation energy will be less than 80 kJ/mol, the reverse activation energy will be less than 60 kJ/mol, and the enthalpy change will remain +20 kJ/mol.
Common mistakes
- ×Thinking a catalyst changes the overall energy released or absorbed (ΔH). It only changes the path taken, not the start and end points.
- ×Forgetting that a catalyst speeds up the reverse reaction as well as the forward one. This is why it doesn't shift equilibrium.
- ×Confusing the effect of a catalyst (lowers Ea) with the effect of increasing temperature (gives more particles enough energy to overcome the original Ea).
- ×Incorrectly assuming a catalyst makes an endothermic reaction become exothermic or vice-versa. The sign of ΔH is unchanged.
No-calculator tips
- ✓When interpreting energy profile diagrams, a catalyst only ever lowers the peak. The energy levels of the reactants and products stay exactly where they are.
- ✓The overall enthalpy change (ΔH) is the vertical distance between products and reactants. Since these levels don't move, ΔH is constant.
- ✓The activation energy is the vertical distance from the reactants to the peak. Since the peak is lower with a catalyst, Ea must be smaller.