Question

QUESTION 9 [13 marks]
Question 9.1 [2 marks]
Define the term reduction by referring to oxidation numbers.
Question 9.2 [2 marks]
Calculate the oxidation number of phosphorus in HPO4
2−
.
QUESTION 9.3
The reaction between permanganate ions ( MnO4
-
) and hydrogen sulphide (H2S)
is given below.
MnO4
−
(aq) + H
+
(aq) + H2S(g) → Mn
2+
(aq) + S(s) + H2O(ℓ)
Question 9.3.1 [2 marks]
Write down the formula of the reducing agent in this reaction.
Question 9.3.2 [2 marks]
Write down the reduction half-reaction.
Question 9.3.3 [2 marks]
Write down the oxidation half-reaction.

Answer 1

### Question 9.1 [2 marks]
**Definition of reduction in terms of oxidation numbers:**
Reduction refers to the process in which the oxidation number of an element decreases, indicating that the element gains electrons. An increase in the electron density around an atom in a compound typically results in a more negative oxidation state.

### Question 9.2 [2 marks]
**Calculating the oxidation number of phosphorus in HPO₄²⁻:**
1. The oxidation state of hydrogen (H) is +1.
2. The oxidation state of oxygen (O) is -2.
3. Let the oxidation state of phosphorus (P) be \( x \).

The sum of the oxidation states in the ion must equal the charge of the ion:
\[
x + 1 + 4(-2) = -2
\]
This simplifies to:
\[
x + 1 - 8 = -2
\]
\[
x - 7 = -2
\]
\[
x = +5
\]
Therefore, the oxidation number of phosphorus in HPO₄²⁻ is **+5**.

### Question 9.3.1 [2 marks]
**Formula of the reducing agent in this reaction:**
The reducing agent is the species that is oxidized and donates electrons in the reaction. In this case, the reducing agent is **H₂S**.

### Question 9.3.2 [2 marks]
**Reduction half-reaction:**
In this reaction, the permanganate ion (MnO₄⁻) is reduced to Mn²⁺. The half-reaction is:
\[
\text{MnO}_4^- + 8 \text{H}^+ + 5 \text{e}^- \rightarrow \text{Mn}^{2+} + 4 \text{H}_2\text{O}
\]

### Question 9.3.3 [2 marks]
**Oxidation half-reaction:**
In this reaction, hydrogen sulfide (H₂S) is oxidized to sulfur (S). The half-reaction is:
\[
\text{H}_2\text{S} \rightarrow \text{S} + 2 \text{H}^+ + 2 \text{e}^-
\]

These reactions clearly separate the reduction and oxidation processes occurring in the overall redox reaction.