A catalyst does not just lower a hump on a diagram: it opens up a new mechanism. Homogeneous catalysts work through intermediates in the same phase as the reactants; heterogeneous catalysts grab reactants on a surface, let them react, then let go.
A catalyst increases the rate by providing an alternative route with a lower activation energy, and is regenerated at the end. A homogeneous catalyst is in the same phase as the reactants and works by forming an intermediate, so its energy profile has two humps with a dip between them. Transition-metal ions are good homogeneous catalysts because they change oxidation state easily: Fe2+ or Fe3+ catalyses S2O82− + 2I−, and NO2 catalyses the oxidation of SO2 in the air. A heterogeneous catalyst is in a different phase, usually a solid with gaseous reactants, and works by adsorption, reaction on the surface and desorption. Adsorption must be neither too strong nor too weak. Enzymes are protein catalysts with highly specific active sites.
Fig. 1: energy profiles for the reaction of peroxodisulfate with iodide, uncatalysed and catalysed by Fe2+ ions
Section 01
Homogeneous catalysts change oxidation state.
Compare uncatalysed and catalysed routes in the simulator and look for the extra dip that marks an intermediate.
Reaction
Catalyst
Step 1
Step 2
S2O82− + 2I− → 2SO42− + I2
Fe2+(aq) (or Fe3+)
S2O82− + 2Fe2+ → 2SO42− + 2Fe3+
2Fe3+ + 2I− → 2Fe2+ + I2
2SO2 + O2 → 2SO3 (atmosphere)
NO2(g)
SO2 + NO2 → SO3 + NO
NO + ½O2 → NO2
Why the uncatalysed reaction is slow: S2O82− and I− are both negative, so they repel and collisions with enough energy are rare.
Why Fe works: each step is between oppositely charged ions. Using E° values, S2O82−/SO42− +2.01 V, Fe3+/Fe2+ +0.77 V, I2/I− +0.54 V: the Fe3+/Fe2+ value lies between the other two, so both steps are feasible and it does not matter whether Fe2+ or Fe3+ is added.
Acid rain link: NO2 is regenerated, so a small amount speeds up the conversion of SO2 to SO3, which forms H2SO4 in rain.
Section 02
Heterogeneous catalysts work on a surface.
Follow the three stages for the iron catalyst in the Haber process and for a catalytic converter.
Stage
What happens (catalytic converter: 2CO + 2NO → 2CO2 + N2 on Pt/Rh)
adsorption
CO and NO form weak bonds to metal atoms at active sites; bonds inside the molecules are weakened and the molecules are held close together in a good orientation
reaction
bonds break and new bonds form on the surface, by a route with a lower activation energy
desorption
the bonds to the surface break and CO2 and N2 leave, freeing the active sites
Haber process: N2 + 3H2 ⇌ 2NH3 on solid iron; adsorption weakens the very strong N≡N triple bond.
Strength of adsorption: too weak (e.g. silver) and few molecules are held; too strong (e.g. tungsten) and products cannot desorb, blocking the sites. Nickel, platinum and iron are in between.
Surface area: converters spread a thin layer of Pt/Pd/Rh on a ceramic honeycomb to maximise active sites; lead compounds poison the catalyst by adsorbing permanently.
Enzymes are protein catalysts. Substrates bind to a specific active site; high temperatures or pH changes denature the enzyme and the rate falls.
Examiner trap
A catalyst lowers Ea by providing a different mechanism; it does not give molecules more energy and does not change ΔH or the position of equilibrium. Homogeneous means the same phase, not the same substance. In heterogeneous catalysis, stronger adsorption is not always better: products must be able to desorb. The catalysed profile has two humps with an intermediate in the dip.
Stage 1 · Learn
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What is a homogeneous catalyst?
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Which ion catalyses the reaction between S2O82− and I−?
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What is the correct order of the stages in heterogeneous catalysis?
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What catalyst is used in the Haber process?
Stage 2 · Exam
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Why is the reaction between S2O82− and I− slow without a catalyst?
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Which pair of equations shows NO2 acting as a catalyst in the oxidation of SO2?
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Why is tungsten a poor catalyst for most gas reactions, even though it adsorbs gases well?
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Using E° values (S2O82−/SO42− +2.01 V, Fe3+/Fe2+ +0.77 V, I2/I− +0.54 V), why can Fe3+ as well as Fe2+ be added as the catalyst?
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