Transition-metal redox and catalysis

What you’ll learn

• Recognise selected oxidation-state and equilibrium colour changes.
• Explain heterogeneous, homogeneous and autocatalytic action.
• Track catalyst regeneration through redox cycles.

Understand the idea

Transition-metal oxidation states can be interconverted by redox reactions predicted using suitable electrode potentials. In acidic vanadium systems, the common sequence is yellow vanadium(V), blue vanadium(IV), green vanadium(III) and violet vanadium(II). Chromium also illustrates the need to distinguish redox from acid–base equilibrium: orange dichromate and yellow chromate both contain chromium(VI), and their balance shifts with acidity.

A heterogeneous catalyst acts at a surface: adsorption can weaken reactant bonds, then products desorb and free the sites. Catalytic converters promote reactions that remove carbon monoxide and nitrogen oxides. In a homogeneous cycle, a transition ion changes oxidation state in one step and is restored in another; adding the steps eliminates the catalyst and intermediate species.

Vanadium(V) oxide catalyses the contact-process oxidation by a V(V)/V(IV) cycle. Iron(II)/iron(III) can shuttle electrons between iodide and peroxodisulfate. In an autocatalytic reaction a product is itself a catalyst; manganese(II) accelerates the acidic manganate(VII)–ethanedioate reaction, so the rate can initially rise as the catalyst accumulates before reactant depletion dominates.

Key formulae

$$\mathrm{Cr_2O_7^{2-}+H_2O\rightleftharpoons2CrO_4^{2-}+2H^+}$$

$$\mathrm{V_2O_5+SO_2\longrightarrow V_2O_4+SO_3}$$

$$\mathrm{V_2O_4+\tfrac12O_2\longrightarrow V_2O_5}$$

$$\mathrm{2CO+2NO\longrightarrow2CO_2+N_2}$$

Worked example

Explain why adding acid to a chromate–dichromate equilibrium changes its colour without changing the oxidation number of chromium.

1. Use $\mathrm{Cr_2O_7^{2-}+H_2O\rightleftharpoons2CrO_4^{2-}+2H^+}$.
2. Increasing hydrogen-ion concentration shifts this equilibrium left, towards orange dichromate.
3. Chromium is +6 in both ions, so no electron transfer or oxidation-state change is required.

Answer: The colour shifts towards orange because the equilibrium composition changes; chromium remains in oxidation state +6.