Catalysts and Energy Profiles
Quick Notes
- Catalysts speed up reactions by offering an alternative pathway with lower activation energy (Ea).
- Catalysts do not get used up in the reaction.
- Energy profile diagrams show lower Ea when a catalyst is used, for both endothermic and exothermic reactions.
- On a Maxwell–Boltzmann curve, a lower Ea means more particles have energy ≥ Ea, increasing the rate.
- Enzymes are biological catalysts.
Full Notes
What Is a Catalyst?
A catalyst is a substance that speeds up a chemical reaction by providing an alternative reaction pathway with a lower activation energy.
This means more collisions between reactants are successful, even at the same temperature.
Catalysts are not used up in the reaction (they may change during a reaction, however will be reformed by the end).
Activation Energy and Boltzmann Distribution
A catalyst lowers the activation energy (Ea).
On a Boltzmann distribution graph, this shifts the Eₐ barrier line to the left.
- More particles now have energy ≥ Ea
- So the rate of reaction increases without needing to raise temperature.
- The shape of the Boltzmann curve stays the same — only the Ea threshold moves.
Reaction Pathway Diagram With and Without a Catalyst
In a reaction profile diagram the catalysed pathway has a lower peak (activation energy).
The enthalpy change (ΔH) is the same with or without the catalyst. The catalyst does not affect equilibrium position — only how fast it is reached.
Enzymes as Catalysts
Enzymes are biological catalysts made from proteins. They increase the rate of metabolic reactions by lowering Ea in very specific, biological pathways.
Summary
- Catalysts lower Ea, increasing the rate without being used up.
- Energy profile diagrams show a reduced energy barrier.
- Maxwell–Boltzmann curves show a higher fraction of particles able to react.
- Enzymes are biological catalysts.
What is the relative effect of a catalyst on the rate of the forward and backward reactions?
A catalyst speeds up both the forward and backward reactions equally by providing an alternative reaction pathway with lower activation energy. It does not affect the position of equilibrium — it just helps the system reach equilibrium faster.
What are the features of transition elements that make them useful as catalysts?
Transition elements make effective catalysts due to their ability to:
- Exist in multiple oxidation states, allowing them to gain and lose electrons easily during reactions.
- Form complexes with reactants, providing an alternative pathway with lower activation energy.
- Adsorb reactants onto their surface (in solid-state catalysts), helping to weaken bonds and facilitate reaction.