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Effect of Catalyst

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.

In one line: Catalyst lowers the hill, not the destination.

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Understand it

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.

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.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

  1. 1If 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.
  2. 2The lower barrier means more molecules can react at the same temperature, so the rate constant is larger.

Method you can reuse

  1. 1Look for whether the question asks about rate, energy barrier, enthalpy, or equilibrium.
  2. 2Connect rate increase to lower Ea.
  3. 3Keep reactant and product energy levels unchanged.
  4. 4For reversible systems, state that both directions are accelerated.

Visual tip

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.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Does a catalyst change Delta H for a reaction?

Think first, then show answer
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.

Common wrong answer

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

Don't mix this up

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

Exam tip

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

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

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.
  1. 1

    Enough for 1 mark

    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. 2

    Add for 2 marks

    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.

  3. 3

    Add for 3 marks

    A catalyst makes it easier for reactant particles to cross the activation energy barrier.

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