In the following reactions:
a. `3MnO_(2)+4Alrarr3Mn+2Al_(2)O_(3)`
b. `2MnO_(4)^(ө)+16H^(o+)+10Cl^(ө)rarr2Mn^(2+)+5Cl_(2)+8H_(2)O`
Which species is reduced and which is oxidised?

Explanation

To determine which species is reduced and which is oxidized in the given reactions, we need to analyze the changes in oxidation states of the elements involved. Let's break down the reactions step by step.

### Reaction a:
**Reaction:**
\[ 3 \text{MnO}_2 + 4 \text{Al} \rightarrow 3 \text{Mn} + 2 \text{Al}_2\text{O}_3 \]

1. **Assign Oxidation States:**
- In \(\text{MnO}_2\):
- Oxygen (O) has an oxidation state of -2.
- Let the oxidation state of Manganese (Mn) be \(x\).
- Therefore, \(x + 2(-2) = 0 \Rightarrow x - 4 = 0 \Rightarrow x = +4\).
- For Aluminum (Al):
- In elemental form, Al has an oxidation state of 0.
- In \(\text{Al}_2\text{O}_3\):
- Oxygen (O) is -2, and let the oxidation state of Aluminum be \(y\).
- Therefore, \(2y + 3(-2) = 0 \Rightarrow 2y - 6 = 0 \Rightarrow 2y = 6 \Rightarrow y = +3\).
- In elemental form, Mn has an oxidation state of 0.

2. **Identify Changes in Oxidation States:**
- Mn changes from +4 (in MnO2) to 0 (in elemental Mn) → **Reduction** (decrease in oxidation state).
- Al changes from 0 (elemental Al) to +3 (in Al2O3) → **Oxidation** (increase in oxidation state).

### Conclusion for Reaction a:
- **Oxidized Species:** Aluminum (Al)
- **Reduced Species:** Manganese (Mn)

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### Reaction b:
**Reaction:**
\[ 2 \text{MnO}_4^{-} + 16 \text{H}^{+} + 10 \text{Cl}^{-} \rightarrow 2 \text{Mn}^{2+} + 5 \text{Cl}_2 + 8 \text{H}_2\text{O} \]

1. **Assign Oxidation States:**
- In \(\text{MnO}_4^{-}\):
- Oxygen (O) is -2, let the oxidation state of Mn be \(x\).
- Therefore, \(x + 4(-2) = -1 \Rightarrow x - 8 = -1 \Rightarrow x = +7\).
- For Hydrogen (H):
- In \(\text{H}^{+}\), the oxidation state is +1.
- For Chlorine (Cl):
- In \(\text{Cl}^{-}\), the oxidation state is -1.
- In \(\text{Mn}^{2+}\):
- The oxidation state of Mn is +2.
- In \(\text{Cl}_2\):
- In elemental form, Cl has an oxidation state of 0.

2. **Identify Changes in Oxidation States:**
- Mn changes from +7 (in MnO4-) to +2 (in Mn2+) → **Reduction** (decrease in oxidation state).
- Cl changes from -1 (in Cl-) to 0 (in Cl2) → **Oxidation** (increase in oxidation state).

### Conclusion for Reaction b:
- **Oxidized Species:** Chlorine (Cl)
- **Reduced Species:** Manganese (Mn)

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### Final Summary:
- **Reaction a:**
- Oxidized: Aluminum (Al)
- Reduced: Manganese (Mn)

- **Reaction b:**
- Oxidized: Chlorine (Cl)
- Reduced: Manganese (Mn)