A catalyst does not push molecules harder; it finds them an easier path. With a lower energy barrier, far more collisions succeed, and the catalyst comes out unchanged at the end.
A catalyst increases the rate of a reaction by providing an alternative route with a lower activation energy; it is chemically unchanged at the end. On the Boltzmann distribution, Ea moves left, so a larger proportion of molecules can react. ΔH is unchanged, and because forward and reverse rates rise equally, a catalyst does not move the position of equilibrium. A homogeneous catalyst is in the same phase as the reactants and usually forms an intermediate; a heterogeneous catalyst is in a different phase and works by adsorbing reactants onto its surface.
Fig. 1: reaction pathway with and without a catalyst
Section 01
Lowering the barrier.
Add a catalyst in the simulator and watch the activation energy line move left on the Boltzmann curve while the curve itself stays put.
Same curve: the temperature is unchanged, so the distribution of energies is unchanged.
Lower Ea: more molecules lie to the right of it, so more collisions succeed.
Pathway diagram: a lower hump between the same reactant and product levels, so ΔH is the same.
Equilibrium: reached sooner, with the same composition.
Section 02
Same phase: homogeneous.
Homogeneous catalysts take part in the reaction, forming an intermediate that is then used up, regenerating the catalyst.
Fe2+ with peroxodisulfate and iodide: S2O82− + 2I− → 2SO42− + I2 is slow because two negative ions repel. Fe2+ reduces S2O82− and becomes Fe3+; Fe3+ then oxidises I− and is reduced back to Fe2+.
Atmospheric NO2: NO2 + SO2 → NO + SO3, then NO + ½O2 → NO2. NO2 speeds up the formation of acid rain.
H+ ions catalyse esterification in solution.
Stage 1 · Learn
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How does a catalyst increase the rate of a reaction?
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What effect does a catalyst have on ΔH for a reaction?
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Iron catalysing the reaction of nitrogen and hydrogen gases is an example of:
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The first step of heterogeneous catalysis is:
Section 03
Different phase: heterogeneous.
Heterogeneous catalysts are usually solids with gases or liquids passing over them. The reaction happens on the surface.
Step
What happens
adsorption
reactant molecules form weak bonds to the catalyst surface
reaction
bonds within the reactants are weakened, and they react on the surface
desorption
product molecules leave the surface, freeing the site
Iron in the Haber process; V2O5 in the Contact process.
Catalytic converters: Pt, Pd and Rh on a ceramic honeycomb convert CO, NOx and unburned hydrocarbons into CO2, N2 and H2O, e.g. 2CO + 2NO → 2CO2 + N2.
Poisoning: lead or sulfur compounds adsorb strongly and block the active sites.
Examiner trap
A catalyst lowers the activation energy of the forward and reverse reactions, so it does not change ΔH, the position of equilibrium or the yield. On a Boltzmann diagram for a catalysed reaction, do not redraw the curve; just move the Ea line to the left.
Stage 2 · Exam
Exam-style questions
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Which statement about a catalyst in a reversible reaction is correct?
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Fe2+ catalyses S2O82− + 2I− → 2SO42− + I2. Why is the uncatalysed reaction slow?
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In a catalytic converter, CO and NO react on a platinum surface. Which reaction occurs?
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On a Boltzmann distribution, the effect of adding a catalyst is shown by:
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