dismutation
Summary
Dismutation (disproportionation) is a redox reaction in which a single species is both oxidized and reduced, so one element ends up in a higher and a lower oxidation state, as in 2H₂O₂ → 2H₂O + O₂. Under standard conditions it is spontaneous when E°cell > 0.
Detail
Meaning: A dismutation (disproportionation) is a redox reaction in which one species is both oxidized and reduced. Its element sits in an intermediate oxidation state, and the products contain that element in one higher and one lower state. Examples: 2H₂O₂ → 2H₂O + O₂ (oxygen: −1 to −2 and 0) and Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O (chlorine: 0 to −1 and +1). The reverse is comproportionation.
Equation: ΔG° = −nFE°cell, with E°cell = E°(reduction half) − E°(oxidation half). ΔG° is in J/mol, n is moles of electrons transferred (mol), F is the Faraday constant, 96,485 C/mol, and E° is in V. Dismutation is spontaneous under standard conditions when E°cell > 0.
On the MCAT: Assign oxidation numbers to spot a dismutation, or use two reduction potentials to decide whether an intermediate species is unstable toward it. H₂O₂ decomposition, halogens in base and superoxide dismutase (2O₂⁻ + 2H⁺ → H₂O₂ + O₂) are typical settings.
Common confusion: Not every redox decomposition is a dismutation. In 2H₂O → 2H₂ + O₂, two different elements change; in a dismutation, one element from one reactant species goes both up and down in oxidation number.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
- Chemistry: The Molecular Nature of Matter and Change
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.