high importancemedium8 min

Effect of Catalyst

Learn the concept

Student-friendly explanation

How to write this in exams

1
Start with the exact idea
A catalyst increases reaction rate by providing an alternate pathway with lower activation energy. It does not change the overall enthalpy change of the reaction and is regenerated by the end of the reaction.
2
Then show how to use it
Look for whether the question asks about rate, energy barrier, enthalpy, or equilibrium. Connect rate increase to lower Ea. Keep reactant and product energy levels unchanged. For reversible systems, state that both directions are accelerated.
3
Add one concrete example
In catalytic decomposition of hydrogen peroxide, the catalyst provides an easier pathway so oxygen is released faster. The energy difference between reactants and products remains the same; only the activation barrier is reduced.
4
Avoid this incomplete answer
Drawing the product energy lower for the catalysed reaction and claiming the catalyst makes the reaction more exothermic.

Definition

Example

In catalytic decomposition of hydrogen peroxide, the catalyst provides an easier pathway so oxygen is released faster. The energy difference between reactants and products remains the same; only the activation barrier is reduced.

Rule to remember

Catalyst effect in Arrhenius terms: lowering Ea increases k in k = Ae^(-Ea/RT). The reaction pathway changes, but Delta H = energy of products - energy of reactants remains the same.

Memory hook

Catalyst lowers the hill, not the destination.

Examples and method

Worked example

If two pathways have Ea values 80 kJ mol^-1 and 45 kJ mol^-1 for the same reaction, the 45 kJ mol^-1 pathway is the catalysed pathway. The lower barrier means more molecules can react at the same temperature, so the rate constant is larger.

Method to apply

Look for whether the question asks about rate, energy barrier, enthalpy, or equilibrium. Connect rate increase to lower Ea. Keep reactant and product energy levels unchanged. For reversible systems, state that both directions are accelerated.

Diagram support

Energy profile diagram is important: uncatalysed pathway has a higher peak, catalysed pathway has a lower peak, and both start and end at the same energy levels.

How CBSE asks it

Questions often ask students to compare catalysed and uncatalysed energy diagrams, explain why rate increases, or identify statements about activation energy, enthalpy, equilibrium, and mechanism.

Avoid common mistakes

Common confusion

Students often write that a catalyst increases product yield at equilibrium. A catalyst changes the rate of reaching equilibrium, not the equilibrium composition.

Common wrong answer

Drawing the product energy lower for the catalysed reaction and claiming the catalyst makes the reaction more exothermic.

Exam tip

In an energy profile diagram, draw the catalysed path with a lower peak but the same reactant and product energy levels.

Quick check

Does a catalyst change Delta H for a reaction?

No. A catalyst lowers activation energy by giving an alternate pathway, but Delta H remains unchanged because reactant and product energy levels do not change.

Answer writing and exam use

1-mark answer

2-mark answer

A catalyst increases reaction rate by providing an alternate pathway with lower activation energy. It does not change the overall enthalpy change of the reaction and is regenerated by the end of the reaction. Catalyst effect in Arrhenius terms: lowering Ea increases k in k = Ae^(-Ea/RT). The reaction pathway changes, but Delta H = energy of products - energy of reactants remains the same. In catalytic decomposition of hydrogen peroxide, the catalyst provides an easier pathway so oxygen is released faster. The energy difference between reactants and products remains the same; only the activation barrier is reduced.

3-mark answer

A catalyst makes it easier for reactant particles to cross the activation energy barrier. It changes the mechanism but not the initial energy of reactants, final energy of products, or overall enthalpy change. For reversible reactions, a catalyst speeds up both forward and reverse reactions, so equilibrium is reached faster but the equilibrium constant is unchanged. Catalyst effect in Arrhenius terms: lowering Ea increases k in k = Ae^(-Ea/RT). The reaction pathway changes, but Delta H = energy of products - energy of reactants remains the same. If two pathways have Ea values 80 kJ mol^-1 and 45 kJ mol^-1 for the same reaction, the 45 kJ mol^-1 pathway is the catalysed pathway. The lower barrier means more molecules can react at the same temperature, so the rate constant is larger. Questions often ask students to compare catalysed and uncatalysed energy diagrams, explain why rate increases, or identify statements about activation energy, enthalpy, equilibrium, and mechanism. Drawing the product energy lower for the catalysed reaction and claiming the catalyst makes the reaction more exothermic.

MCQ Quiz

Practice this concept with focused MCQs

Open the concept quiz intro first, review the test details, and then start a focused MCQ set from this concept only. Instant score and answer review are live now.

10 MCQs5 MinutesInstant Results

Practice This Concept

Help improve this page

Found something confusing, incorrect, or missing?