In the following reactions, which substances are acting as oxidising agents?
{C(s) + O_2(g) → CO_2(g)}
{2Fe^{3+}(aq) + 2I^{-}(aq) → Fe^{2+}(aq) + I_2(aq)}
{Mg(s) + 2H^+(aq) → Mg^{2+}(aq) + H_2(g)}
A. {C(s), Fe^{3+}(aq), H^{+}(aq)}
B. {C(s), Fe^{3+}(aq), Mg(s)}
C. {O_2(g), I(aq), H^+(aq)}
D.{ O_2(g), Fe^{3+}(aq), H^+(aq)}
E. {O_2(g), I^{-}(aq), Mg(s)}
To clarify, we can first say:
Oxidizing Agents reduce themselves, and so their oxidation number decreases.
In the first reaction-
Carbon is oxidized: C^0 -4e \rightarrow C^4
Therefore it is a reducing agent.
Oxygen is reduced: 2O + 4e \rightarrow 2O^{-2}
Therefore it is an oxidizing agent.
We can now narrow down are answer to C, D or E.
In the second reaction-
Iron is reduced: 2Fe^{3+} \rightarrow Fe^{2+}
Therefore it is an oxidizing agent.
Iodine is oxidized: 2I^- \rightarrow I_2^0
Therefore it is a reducing agent.
We can conclude our answer is D.
For the sake of chemistry, we can also revise the third reaction in which-
Hydrogen is reduced: 2H^1 + 2e \rightarrow 2H^0
Therefore it is an oxidizing agent.
Magnesium is oxidized: Mg^0 -2e \rightarrow Mg^2
Therefore it is a reducing agent.