A student dissolved a 0.139g sample of oxalic acid, H2C2O4, in water in an Erlenmeyer flask. Then the student titrated the H2C2O4 solution in the flask with a solution of KMnO4, which has a dark purple color. The balanced chemical equation for the reaction that occurred during the titration is shown above. (a) Identify the species that was reduced in the titration reaction. Justify your answer in terms of oxidation numbers.

MnO₄⁻ was reduced and C₂O₄²⁻ was oxidized in the titration.

The balanced chemical equation for the reaction can be expressed as:

A redox reaction can be described by a variation in the oxidation number of reacting species in the chemical reaction equation from the reactant side to the product side

Let consider Mn in MnO₄⁻, its oxidation number was decreased from +7 on the reactant side to +2 on the product side. This shows a gain of additional five electrons.

Thus, MnO₄⁻ is reduced.

Also in C₂O₄²⁻, the oxidation number of carbon increases from +3 on the reactant side to +4 on the product side. Since an electron is lost, then oxidation  occurs at C₂O₄²⁻  

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MnO4 − was reduced

C2 O4^2- was oxidized

Explanation:

2MnO4 −(aq)  +5H +(aq)  +2 C2 O4^2-(aq)  +6H+(aq)  →2Mn2+(aq) + 10CO2(g)  +8H2 O(l)

A redox reaction is characterized by a change in the oxidation number of reacting species from left to right in the reaction equation.

If we look at Manganese, its oxidation number was decreased from +7 on the left hand side to +2 on the right hand side. This implies that it gained five electrons. recall that reduction refers to electron gain.

If we look at carbon, its oxidation number increased from +3 on the left hand side to +4 on the right hand side. This indicates loss of one electron and oxidation refers to electron loss.

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