Questions 32-34 refer to the following. 5 H2O2(aq) + 2 MnO4 −(aq) + 6 H+(aq) → 2 Mn2+(aq) + 8 H2O(l) — Electrochemistry Chemistry Question

STEPS:

1. Understand oxidation in terms of oxidation numbers: Oxidation is defined as the loss of electrons, which mathematically corresponds to an increase in the oxidation number of an atom.
2. Determine the initial oxidation numbers in the reactants:
* In $\text{H}_2\text{O}_2(aq)$ (hydrogen peroxide): Hydrogen is assigned its standard oxidation state of $+1$. Since hydrogen peroxide is a neutral compound, the sum of all oxidation numbers must equal $0$. This forces the two oxygen atoms to have a combined state of $-2$, meaning each oxygen atom possesses an oxidation number of $-1$ (a classic exception to oxygen's typical $-2$ state).
* In $\text{MnO}_4^-(aq)$ (permanganate ion): Oxygen is assigned its standard oxidation state of $-2$. For the polyatomic ion to maintain its overall charge of $-1$, the manganese ($\text{Mn}$) atom must have an oxidation number of $+7$ ($\text{Mn} + 4(-2) = -1$).
* In $\text{H}^+(aq)$: The hydrogen ion has an oxidation number of $+1$.
3. Determine the final oxidation numbers in the products:
* In $\text{Mn}^{2+}(aq)$: For a monoatomic ion, the oxidation state is equal to its ionic charge, meaning manganese has an oxidation number of $+2$.
* In $\text{H}_2\text{O}(l)$: Hydrogen remains at $+1$ and oxygen is at its standard state of $-2$.
* In $\text{O}_2(g)$: For any element in its pure, neutral elemental state, the oxidation number is $0$.
4. Identify which element undergoes oxidation (an increase in oxidation number):
* Manganese changes from $+7$ to $+2$. This is a decrease in oxidation state (gain of electrons), which represents reduction.
* Hydrogen remains at $+1$ throughout.
* Oxygen changes from $-1$ in $\text{H}_2\text{O}_2$ to $0$ in $\text{O}_2$. This is an increase in oxidation number (loss of electrons), confirming that oxygen is the element being oxidized.
5. Select the matching option: This process proves that oxygen is oxidized and its oxidation number changes from $-1$ to $0$, verifying Option A as the correct choice.

*

WHY_OTHERS_WRONG:

• B is incorrect: While B correctly identifies oxygen as the element of interest, it claims the change in oxidation number is from $0$ to $-2$. A change from $0$ to $-2$ is a decrease in oxidation number, which represents reduction, not oxidation. Furthermore, the oxygen in the reactant $\text{H}_2\text{O}_2$ starts with an oxidation state of $-1$, not $0$.
• C is incorrect: This option incorrectly claims that manganese is the species being oxidized and that it begins with an oxidation state of $-1$. In reality, manganese is reduced, and its initial oxidation state in permanganate is $+7$.
• D is incorrect: Although this option correctly states that manganese changes from $+7$ to $+2$, this represents a reduction process (a decrease in oxidation number), not oxidation.