Disproportionation
A concise revision guide to disproportionation: one element in a single species oxidised and reduced at the same time, how to spot it with oxidation states, the chlorine and hydrogen peroxide examples, and which species can and cannot disproportionate.
- 3.1.7iv
- 3.2.3.2i-a
- 3.2.3.2i-b
What these spec points say
- 3.1.7iv work out the oxidation state of an element in a compound or ion from the formula
- 3.2.3.2i-a know the reaction of chlorine with water to form chloride ions
- 3.2.3.2i-b know the reaction of chlorine with water to form chlorate(I) ions
What Disproportionation Is
Disproportionation is a redox reaction in which the same element in a single species is both oxidised and reduced at the same time.
Some of the atoms of that element go up in oxidation state while others go down. The element must be in an intermediate oxidation state to begin with, so that it has room to move in both directions.
Chlorine gas reacting with water is the classic case. Chlorine starts at 0; in the products it is −1 in hydrochloric acid and +1 in chloric(I) acid:
Cl₂(g) + H₂O(l) ⇌ HCl(aq) + HClO(aq)
One chlorine atom has been reduced (0 to −1) and the other oxidised (0 to +1). Chlorine is both the oxidising agent and the reducing agent.
Definition: A disproportionation reaction is one in which an element in a single species is simultaneously oxidised and reduced.
Quick Check: Is the Element Really Doing Both Jobs?
Pick the one accurate statement in each set, using reactions that are new to this page.
Spotting It with Oxidation states
To decide whether a reaction is a disproportionation, assign oxidation states to every atom on both sides and look for one element that appears at one value on the left and at two different values on the right, one higher and one lower.
From the 0 rung, chlorine goes down to −1 and up to +1 at the same time, which is the pattern that defines disproportionation.
Worked example: copper(I) ions in solution. 2Cu⁺(aq) → Cu(s) + Cu²⁺(aq). Copper starts at +1 and finishes at 0 (reduced) and +2 (oxidised). This is disproportionation, and it is why copper(I) sulfate cannot exist in water.
Worked example: hydrogen peroxide decomposing. 2H₂O₂ → 2H₂O + O₂. Oxygen starts at −1 and finishes at −2 in water (reduced) and 0 in oxygen gas (oxidised). Disproportionation again.
Exam focus: Write the oxidation state above every atom of the element concerned on both sides, then state both changes with their numbers: “chlorine is reduced from 0 to −1 and oxidised from 0 to +1”.
Check: Is It Disproportionation?
Assign oxidation states in reactions not shown above and decide which are disproportionation.
Why Chlorine Disproportionates
Chlorine sits at 0, and it forms stable compounds both below (chloride, −1) and above (chlorate(I) +1, chlorate(V) +5). A reaction that sends some atoms each way can therefore be favourable.
Which higher state forms depends on the conditions. With water or cold dilute alkali chlorine reaches +1, but with hot concentrated alkali it goes to +5, forming chlorate(V) ions, ClO₃⁻, alongside chloride.
These reactions are so important in water treatment and bleach manufacture that they have a page of their own in the Group 7 (17) section: Disproportionation Reactions of Chlorine. Here the point is the pattern, not the industrial detail.
Key idea: An element can only disproportionate if it is in an intermediate oxidation state. Fluorine cannot, because it has no positive oxidation states; sodium cannot, because it has no negative ones.
The Reverse Process
The reverse of disproportionation is called comproportionation. Two species containing the same element at different oxidation states react to give one product at an intermediate value.
Iodate(V) ions and iodide ions in acid give iodine: IO₃⁻ + 5I⁻ + 6H⁺ → 3I₂ + 3H₂O. Iodine goes from +5 and −1 to 0.
The name is rarely examined, but recognising the pattern helps when an unfamiliar equation appears.
Remember: Disproportionation: one element, one starting value, two finishing values. Comproportionation: one element, two starting values, one finishing value.
In disproportionation one starting oxidation number splits into two; in comproportionation two starting values meet at one, as when iodate(V) and iodide ions give iodine.
Check: Which Elements Can Disproportionate?
Use the idea of intermediate oxidation numbers to decide which of a list of species could disproportionate.
Quick Check: Classify the Equation
Assign the numbers on paper and name the type of reaction, then flip the card.
Common Exam Points
Define disproportionation
A reaction in which an element in a single species is simultaneously oxidised and reduced.
Show that a reaction is a disproportionation
Give the oxidation state of the element in the reactant and in each product, and state which change is oxidation and which is reduction.
Do not say
“Two different elements are oxidised and reduced” (that is ordinary redox); “chlorine is neutralised”; “the oxidation state of chlorine in HClO is −1”.
FAQs
Use these quick answers to check the disproportionation idea.
What is disproportionation?
A redox reaction in which the same element in one species is both oxidised and reduced at the same time.
How do I recognise it in an equation?
Assign oxidation states. If one element starts at a single value and finishes at two different values, one higher and one lower, the reaction is a disproportionation.
Why can chlorine disproportionate but fluorine cannot?
Chlorine starts at 0 and has stable oxidation states both above and below it. Fluorine has no positive oxidation states, so it can only be reduced.
Is the reaction of chlorine with water reversible?
Yes. Chlorine water contains chlorine, hydrochloric acid and chloric(I) acid in equilibrium. With alkali the acids are neutralised and the reaction goes to completion.
Is a reaction where two different elements are oxidised and reduced a disproportionation?
No. That is an ordinary redox reaction. Disproportionation needs the same element going both ways.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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