Question

2. Is the following reaction a redox reaction? Justify your answer by showing the oxidation numbers for each element and show your work. (5 Marks)

S(s) + HNO3 (aq ) → SO2 (g) + NO (g) + H2O (l)

Answer 1

To determine if the reaction \( S(s) + HNO_3(aq) \rightarrow SO_2(g) + NO(g) + H_2O(l) \) is a redox reaction, we can analyze the oxidation states (oxidation numbers) of the elements involved before and after the reaction.

### Step 1: Assign oxidation states

1. **Reactants:**
- **Sulfur (\( S \))**: In its elemental form, sulfur has an oxidation state of 0.
- **Hydrogen (\( H \))** in \( HNO_3 \): The oxidation state of hydrogen is +1.
- **Nitrogen (\( N \))** in \( HNO_3 \): The oxidation state of nitrogen can be calculated as follows:
- The total charge of \( HNO_3 \) is 0.
- \( H: +1 \)
- \( O: -2 \times 3 = -6 \)
- Let the oxidation state of \( N \) be \( x \).
- Therefore:
\[
+1 + x - 6 = 0 \implies x = +5
\]
- **Oxygen (\( O \))** in \( HNO_3 \): The oxidation state of oxygen is -2.

2. **Products:**
- **Sulfur (\( S \))** in \( SO_2 \):
- The total charge of \( SO_2 \) is 0.
- Let the oxidation state of \( S \) be \( y \).
- So we have:
\[
y - 2 \times 2 = 0 \implies y = +4
\]
- **Nitrogen (\( N \))** in \( NO \):
- The total charge of \( NO \) is 0.
- Let the oxidation state of \( N \) be \( z \).
- Therefore:
\[
z - 2 = 0 \implies z = +2
\]
- **Hydrogen (\( H \))** in \( H_2O \): The oxidation state of hydrogen is +1.
- **Oxygen (\( O \))** in \( H_2O \): The oxidation state of oxygen is -2.

### Step 2: Summary of oxidation states

- **Reactants:**
- \( S: 0 \)
- \( H: +1 \) (from \( HNO_3 \))
- \( N: +5 \) (from \( HNO_3 \))
- \( O: -2 \) (from \( HNO_3 \))

- **Products:**
- \( S: +4 \) (from \( SO_2 \))
- \( N: +2 \) (from \( NO \))
- \( H: +1 \) (from \( H_2O \))
- \( O: -2 \) (from \( H_2O \))

### Step 3: Determine changes in oxidation states

- Sulfur (\( S \)):
- Changes from 0 (reactant) to +4 (product) = **oxidation** (loss of electrons).

- Nitrogen (\( N \)):
- Changes from +5 (reactant) to +2 (product) = **reduction** (gain of electrons).

### Conclusion:

Since there is a change in oxidation states for both sulfur and nitrogen, this indicates that oxidation and reduction processes are occurring. Therefore, we identify this reaction as a **redox reaction**.

**Final Verdict:** Yes, this reaction is a redox reaction due to the changes in oxidation states of sulfur (oxidation) and nitrogen (reduction).