Aim for FiveChemistry

Unit 5 · Topic 5.11

5.11 Catalysis

A catalyst speeds up a reaction by giving it a different pathway with a lower activation energy, and it is regenerated, so it isn't used up overall. Catalysts don't change the overall energy change of a reaction. Enzymes, acid-base catalysis and surface catalysis are the three kinds you need to know.

Key terms

  • catalyst
  • enzyme
  • acid-base catalysis
  • surface catalysis
  • activation energy

How catalysts work

A catalyst opens up a different route from reactants to products. That route has a lower energy barrier, so more collisions can get over it, and some catalysts also make more collisions count by lining reactants up in a useful way. Either way, the rate goes up.

A catalyst really does react. Often it's used up in the slow step and given back in a fast step afterward, so the amount present at the end equals the amount at the start. Because it takes part in the slow step, [catalyst] often shows up in the rate law.

A catalyst doesn't change the energy of the reactants or products, so ΔH stays the same. It speeds up the forward and reverse reactions alike, so it doesn't change how far the reaction goes, only how fast it gets there.

Three kinds of catalysis

  • Binding to reactants (enzymes): an enzyme grabs reactant molecules in a pocket called the active site. Held there, the reactants are lined up the right way or have bonds strained, so they react over a lower barrier. The enzyme-reactant complex is a new intermediate in the mechanism.
  • Acid-base catalysis: the catalyst hands a proton (H⁺) to a reactant, or takes one from it. The protonated or deprotonated species is a new intermediate that reacts more easily, so the mechanism gains extra steps. Many reactions of organic molecules are sped up by acid this way.
  • Surface catalysis: reactant molecules stick to the surface of a solid, often a metal, sometimes forming bonds with surface atoms. While attached, their own bonds weaken or break, so they react more easily. Examples include adding H₂ to carbon-carbon double bonds on nickel or platinum, and catalytic converters, where platinum, palladium and rhodium surfaces convert harmful exhaust gases into less harmful ones.

Catalysts on the graphs

On an energy profile, the catalyzed path has a lower peak (or several lower peaks, if the new mechanism has more steps). The reactants and products are at the same energies as before, so ΔH doesn't change.

On a Maxwell-Boltzmann distribution, the curve itself doesn't change, since the temperature hasn't changed. Instead, the Eₐ line moves to the left, and a larger area of the curve lies beyond it.

Spotting a catalyst in a mechanism

A catalyst appears as a reactant in an early step and as a product in a later step, so it cancels from the overall equation. An intermediate is the opposite: product first, reactant later. Topic 5.7 has the comparison.

Worked examples

Try each one yourself first, then open the solution.

  1. Example 1

    A catalyst in the rate law

    Iodide ions catalyze the decomposition of hydrogen peroxide. Step 1 (slow): H₂O₂ + I⁻ → H₂O + IO⁻. Step 2 (fast): H₂O₂ + IO⁻ → H₂O + O₂ + I⁻. Find the overall reaction, the catalyst, the intermediate and the rate law.

    Show the solution
    1. Step 1: Add the steps: 2H₂O₂ + I⁻ + IO⁻ → 2H₂O + O₂ + IO⁻ + I⁻. Cancel I⁻ and IO⁻.
    2. Step 2: Overall: 2H₂O₂ → 2H₂O + O₂.
    3. Step 3: I⁻ is used in step 1 and regenerated in step 2: catalyst. IO⁻ is made in step 1 and used in step 2: intermediate.
    4. Step 4: The slow first step is elementary, so rate = k[H₂O₂][I⁻]. The catalyst appears in the rate law because it takes part in the slow step.

    Answer: 2H₂O₂ → 2H₂O + O₂; catalyst I⁻; intermediate IO⁻; rate = k[H₂O₂][I⁻]

  2. Example 2

    What a catalyst doesn't change (classic trap)

    A student claims that adding a catalyst to an exothermic reaction makes it release more heat, because the reaction becomes 'easier'. Evaluate the claim using an energy profile.

    Show the solution
    1. Step 1: A catalyst lowers the peak (the activation energy) by providing a different pathway.
    2. Step 2: The reactants and products are the same substances at the same energies, so the gap between them, ΔH, is unchanged.
    3. Step 3: The reaction releases the same amount of heat per mole; it just releases it faster.

    Answer: The claim is wrong. A catalyst lowers Eₐ but leaves the energies of reactants and products, and therefore ΔH, unchanged.

Common mistakes

  • Saying a catalyst isn't involved in the reaction. It takes part in the mechanism but is regenerated.
  • Saying a catalyst changes ΔH, or produces more product.
  • Drawing a catalyzed Maxwell-Boltzmann curve that's shifted. The curve stays the same; the Eₐ line moves left.
  • Mixing up catalysts and intermediates in a mechanism.

On the exam

  • Expect to identify a catalyst in a mechanism, explain how it increases the rate (lower activation energy pathway) and sketch the catalyzed path on an energy profile.
  • When explaining a catalyst's effect, a strong answer says the new pathway has a lower Eₐ, so a larger fraction of collisions have enough energy to react.

Connected topics

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Check yourself

4 questions on 5.11 Catalysis. Pick an answer to see if you got it, and why.

Question 1 of 4

In a car's catalytic converter, CO and NO molecules stick to the surface of a platinum-group metal, where they react to form CO₂ and N₂. Which of the following statements about this catalyst is correct?

Hydrogen peroxide decomposes slowly on its own: 2H₂O₂(aq) → 2H₂O(l) + O₂(g). Adding a little KI makes it decompose quickly. A proposed mechanism is shown below.

Step 1 (slow): H₂O₂(aq) + I⁻(aq) → H₂O(l) + IO⁻(aq)

Step 2 (fast): H₂O₂(aq) + IO⁻(aq) → H₂O(l) + O₂(g) + I⁻(aq)

Proposed reaction mechanism

Question 2 of 4

Which of the following correctly identifies the roles of I⁻ and IO⁻ in this mechanism?

Question 3 of 4

Which rate law is consistent with this mechanism?

Question 4 of 4

On a Maxwell-Boltzmann distribution of molecular energies, the molecules that can react are those with energies at or above the activation energy, Eₐ. Which of the following correctly describes how raising the temperature and adding a catalyst each increase the reaction rate?

0 of 4 answered