Part 6: Heterogeneous, homogeneous and autocatalysis

Explain surfaces, variable-oxidation-state cycles and the named catalytic examples required by AQA.

A catalyst is regenerated by its reaction pathway

A catalyst offers an alternative pathway with lower activation energy and is regenerated overall. It may be chemically changed in one step and restored in another. It increases the rates of forward and reverse reactions, so it reaches equilibrium faster without changing the equilibrium constant, equilibrium yield or overall ΔH.

A heterogeneous catalyst is in a different phase from the reactants; a homogeneous catalyst is in the same phase. “Solid” is not a definition of heterogeneous, and “aqueous” is not a definition of homogeneous. Compare phases of catalyst and reactants in the stated reaction.

Adsorb, react, desorb

For a typical solid-surface catalyst, reactants adsorb at active sites, forming interactions with surface atoms. This can weaken bonds and bring reactants into a favourable arrangement. Reaction then forms products, which desorb, releasing sites for further reaction. Adsorption is attachment at a surface; absorption into a bulk material is different.

Adsorption must be strong enough for reaction but not so strong that products cannot leave. A high surface area exposes more active sites; dispersing an expensive metal on a suitable support can reduce the metal required and cost. An impurity that binds strongly or chemically blocks active sites poisons the catalyst, reducing activity and potentially requiring regeneration or replacement.

Iron catalyses the Haber equilibrium between gaseous nitrogen and hydrogen. Sulfur-containing impurities can poison iron, so gas purification matters. In vehicle converters, lead contamination is a classic poisoning example; precious metals on a porous support maximise accessible sites.

Vanadium(V) oxide in the Contact process

In the required simplified catalytic cycle, SO₂ reduces V(V) in V₂O₅ to V(IV) in V₂O₄ while being oxidised to SO₃. Oxygen then reoxidises V₂O₄, regenerating V₂O₅. Double the first step before adding it to the second: the vanadium oxides cancel, giving the overall SO₂ oxidation.

The catalyst is treated as heterogeneous in this syllabus model. The equations express the redox cycle rather than a complete molecular account of an industrial supported catalyst.

Iron ions catalyse the iodide–peroxodisulfate reaction

The uncatalysed reaction brings two negatively charged ions together, so electrostatic repulsion contributes to a high activation barrier. A Fe²⁺/Fe³⁺ cycle replaces it with steps involving positively and negatively charged reacting ions. Fe²⁺ reduces peroxodisulfate and is oxidised to Fe³⁺; Fe³⁺ then oxidises iodide and regenerates Fe²⁺. Both steps occur in aqueous solution.

Add the two equations and cancel both iron species to obtain S₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂. Starting with Fe³⁺ can also initiate the cycle, with the order of steps reversed. “Fe²⁺ never reacts” would be wrong: it reacts and is regenerated.

Using illustrative standard potentials for the actual named couples, +2.01 V for S₂O₈²⁻/SO₄²⁻, +0.77 V for Fe³⁺/Fe²⁺ and +0.54 V for I₂/I⁻, the two step potentials are +1.24 V and +0.23 V. The iron couple lies between the other two. This supports both electron transfers thermodynamically; a useful catalytic rate still requires a suitable mechanism.

A product can catalyse its own formation

In acidified manganate(VII) reacting with oxalate, Mn²⁺ is a product and a catalyst: this is autocatalysis. The initially slow reaction produces some Mn²⁺; its catalytic cycle then speeds the reaction. Near completion the rate falls as reactants are depleted. Added Mn²⁺ at the start reduces the slow induction period.

The simplified cycle goes through Mn³⁺. Multiply the first step below by two and the second by five; after cancellation, the net products include two Mn²⁺. Thus the catalytic species is regenerated and also produced overall. The steps represent the required redox scheme, not proof that every balanced line is an elementary collision.

Read the curve in the right direction

Monitoring manganate(VII) absorbance at a suitable wavelength gives a non-destructive way to follow its falling concentration, provided the calibration and other species do not interfere. An absorbance–time trace for the autocatalytic reaction falls slowly at first, then more steeply, then levels off as manganate is exhausted. Its most negative gradient corresponds to the fastest disappearance, not the largest absorbance.

Control temperature, initial concentrations, acid concentration, total volume and mixing when comparing runs with and without added Mn²⁺. The initial Mn²⁺ addition should change catalyst amount without introducing an unaccounted dilution. Optical monitoring avoids repeatedly removing substantial aliquots from a small reacting mixture.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.

Sources