Reviewed CSCA Chemistry question · Standard

Which of the following equations represents a redox reaction?

AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
2KClO3(s) → 2KCl(s) + 3O2(g)
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq)

Show the answer and explanation

Correct answer

B. 2KClO3(s) → 2KCl(s) + 3O2(g)

Principle or equation

A redox reaction involves a change in oxidation states of elements; oxidation is loss of electrons, reduction is gain of electrons.

Why this answer is correct

In 2KClO3(s) → 2KCl(s) + 3O2(g), chlorine is reduced from +5 in ClO3- to -1 in Cl-, and oxygen is oxidized from -2 in ClO3- to 0 in O2. The other reactions are metathesis (double displacement) with no oxidation state changes.

Example

Check oxidation states: In KClO3, K=+1, O=-2, so Cl=+5. In KCl, Cl=-1. In O2, O=0. Changes occur, so redox.

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