Chemistry:Comproportionation

Short description: Reaction between two compounds of an element

Comproportionation or synproportionation is a chemical reaction where two reactants containing the same element but with different oxidation numbers, form a compound having an intermediate oxidation number. It is the opposite of disproportionation.

Frost diagrams

The tendency of two species to disproportionate or comproportionate can be determined by examining the Frost diagram of the oxidation states; if a species' value of ΔG/F is lower than the line joining the two oxidation numbers on either side of it, then it is more stable and if in a solution, these two species will undergo comproportionation.

A Frost diagram is another way of displaying the reduction potentials for the various oxidation states of a given element, X. It shows nE against the oxidation number N: here, E is the reduction potential for the X(N)/X(0) couple, and n is the number of electrons transferred in the conversion of X(N) to X(0)

Examples

• In lead batteries, the spontaneous reaction is:

Pb + PbO₂ + 2 H₂SO₄ → 2 PbSO₄ + 2 H₂O

• The laboratory preparation of manganese dioxide involves comproportionation of Mn(II) and Mn(VII) reagents:

2 KMnO₄ + 3 MnSO₄ + 2 H₂O→ 5 MnO₂ + K₂SO₄ + 2 H₂SO₄

• 15 Se + SeCl₄ + 4 AlCl₃ → 2 Se₈[AlCl₄]₂
• In the Claus process, two gaseous compounds of sulfur comproportionate in the presence of a catalyst to give elemental sulfur:

2 H₂S + SO₂ → 3 S + 2 H₂O

• In halogen chemistry:

IO₃⁻ + 5 I⁻ + 6 H ⁺ → 3 I₂ + 3 H₂O

• In anammox chemistry:

NH₄⁺ + NO₂⁻ → N₂ + 2H₂O

• Iron(III) chloride reacts with iron powder to form iron(II) chloride:

2 FeCl
3 + Fe → 3 FeCl
2