Multiple choiceQuestion 1 · Recall
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By the end you can define disproportionation, recognise it from oxidation numbers, and explain the chlorine/alkali and hydrogen-peroxide examples.
Usually one element is oxidised and a different element is reduced.
Occasionally the same element does both in the same reaction. One atom of it loses electrons while another atom of the same element gains them.
This is called disproportionation, and it is a favourite exam topic because it looks impossible at first glance.
Disproportionation
Disproportionation is a redox reaction in which the same element is simultaneously oxidised and reduced.
In oxidation-number terms, atoms of the element start at one value and end at two different values, one higher and one lower.
The starting oxidation number must therefore be intermediate between the two products: it can go up or down from there.
Worked example: Worked example: chlorine in cold dilute alkali
Chlorine reacts with cold, dilute sodium hydroxide:
Show that this is disproportionation.
Chlorine in sits at oxidation number , which is between (chloride) and (chlorate(I)).
Because it is in the middle, it can move either way depending on what it meets. Some chlorine atoms meet a species that takes electrons; others meet a species that gives them.
In cold dilute alkali, both possibilities happen in the same flask.
The same logic explains hydrogen peroxide: oxygen at sits between (water) and (oxygen gas), so it can disproportionate too.
Worked example: Worked example: hydrogen peroxide
Hydrogen peroxide decomposes:
Show that this is disproportionation.