Catalysts

Learning Objectives (OCR Spec 3.2.2)

By the end of this lesson you should be able to:

• Define a catalyst and explain how it works by providing an alternative reaction pathway with lower activation energy
• Show the effect of a catalyst on a Boltzmann distribution and on an energy profile diagram
• Distinguish heterogeneous and homogeneous catalysts with named examples
• Describe industrial uses of catalysts and their economic and environmental importance
• Appreciate that catalysts are not consumed overall but may take part in the mechanism

Definition of a Catalyst

A catalyst is a substance that increases the rate of a chemical reaction without itself being used up in the process. A catalyst provides an alternative pathway with a lower activation energy than the uncatalysed route.

Key facts to remember:

• A catalyst does not change the position of equilibrium (see Lesson 10) - it speeds up the forward and reverse reactions equally.
• A catalyst is not consumed; if it appears in an elementary step of the mechanism it is regenerated in a later step.
• A catalyst does not change ΔH - only Ea.
• A catalyst is specific - different reactions require different catalysts.

Why a Lower Ea Gives a Faster Rate

When Ea is lowered, a greater proportion of molecules at any given temperature have kinetic energy ≥ Ea. The rate is therefore increased.

graph LR
  A[Boltzmann curve at fixed T] --> B[Ea uncatalysed: small shaded area]
  A --> C[Ea catalysed: much larger shaded area]
  B --> D[Few successful collisions]
  C --> E[Many successful collisions]

This is a crucial graph for OCR: draw the Boltzmann distribution once, and mark two vertical lines - one at the uncatalysed Ea and one at the lower catalysed Ea. Shade both areas to show that more molecules exceed the catalysed Ea. You do not redraw the curve - only the Ea line moves.

Energy Profile Diagram with a Catalyst

On an energy profile diagram, a catalysed reaction has a lower peak than the uncatalysed route. Both curves connect the same reactants and products:

graph LR
  R[Reactants] -->|Ea uncat| TS1[Transition state high]
  TS1 --> P[Products]
  R -->|Ea cat| TS2[Transition state low]
  TS2 --> P

Both paths start and end at the same levels - ΔH is unchanged - but the peak is lower for the catalysed route. Sometimes the catalysed route has more than one step (multiple small peaks), because the catalyst breaks the reaction into a series of easier elementary steps.

Heterogeneous Catalysts

A heterogeneous catalyst is in a different physical state from the reactants. Most commonly the catalyst is a solid and the reactants are gases or in solution.

How Heterogeneous Catalysts Work

1. Adsorption - reactant molecules stick to active sites on the catalyst surface, weakening their bonds.
2. Reaction - the weakened bonds make new bonds form more easily.
3. Desorption - product molecules leave the surface, freeing the active site for the next cycle.

The Ea is lowered because the adsorption step effectively pre-activates the reactant bonds.

Named Industrial Examples

Process	Reaction	Catalyst	Conditions
Haber	N2 + 3H2 ⇌ 2NH3	Iron (Fe, with K2O promoter)	450 °C, 200 atm
Contact	2SO2 + O2 ⇌ 2SO3	Vanadium(V) oxide V2O5	450 °C, 1-2 atm
Catalytic cracking	Long-chain alkanes → short alkanes + alkenes	Zeolite (alumino-silicate)	500 °C, slight pressure
Hydrogenation of alkenes	C2H4 + H2 → C2H6	Nickel (Ni)	150 °C, 5 atm
Catalytic converter (car)	2NO + 2CO → N2 + 2CO2	Platinum, palladium, rhodium	Exhaust temperatures
Manufacture of HNO3 (Ostwald)	4NH3 + 5O2 → 4NO + 6H2O	Platinum-rhodium gauze	900 °C, 4-10 atm

Catalyst Poisoning

Impurities can bind irreversibly to active sites, poisoning the catalyst. The Haber process uses very pure N2 and H2 because sulfur compounds poison iron catalysts. Lead in petrol poisoned early catalytic converters, which is why modern petrol is unleaded.

Homogeneous Catalysts

A homogeneous catalyst is in the same physical state as the reactants - typically both dissolved in solution.

How Homogeneous Catalysts Work

The catalyst reacts with one of the reactants to form an intermediate, which then reacts further to give the product and regenerate the catalyst. Each elementary step has a lower Ea than the uncatalysed route.

Example: acid catalysis of ester hydrolysis, CH3COOC2H5 + H2O → CH3COOH + C2H5OH, catalysed by H⁺. The mechanism proceeds through a protonated intermediate that is more susceptible to nucleophilic attack.

Named Examples