Identify the oxidant and the reductant in the following reactions:
a. `Zn(s)+(1)/(2)O_(2)(g)rarrZnO(s)`
b. `Zn(s)+2H^(o+)(aq)rarrZn^(2+)(aq)+H_(2)(g)`

Explanation

To identify the oxidant and reductant in the given reactions, we need to analyze the changes in oxidation states of the elements involved. Let's break down each reaction step by step.

### Reaction a:
**Zn(s) + (1/2)O2(g) → ZnO(s)**

1. **Identify the oxidation states:**
- In elemental zinc (Zn), the oxidation state is 0.
- In zinc oxide (ZnO), zinc has an oxidation state of +2 (since oxygen is -2).

2. **Determine the change in oxidation state:**
- Zinc goes from 0 to +2, indicating it is losing electrons (oxidation).
- Oxygen in O2 (oxidation state 0) is reduced to -2 in ZnO.

3. **Identify the oxidant and reductant:**
- Since zinc is losing electrons, it is the **reductant**.
- Oxygen, which is gaining electrons, is the **oxidant**.

### Reaction b:
**Zn(s) + 2H⁺(aq) → Zn²⁺(aq) + H2(g)**

1. **Identify the oxidation states:**
- In elemental zinc (Zn), the oxidation state is 0.
- In zinc ions (Zn²⁺), the oxidation state is +2.
- In hydrogen ions (H⁺), the oxidation state is +1.
- In molecular hydrogen (H2), the oxidation state is 0.

2. **Determine the change in oxidation state:**
- Zinc goes from 0 to +2, indicating it is losing electrons (oxidation).
- Hydrogen goes from +1 in H⁺ to 0 in H2, indicating it is gaining electrons (reduction).

3. **Identify the oxidant and reductant:**
- Zinc is the **reductant** as it is losing electrons.
- Hydrogen ions (H⁺) are the **oxidant** as they are gaining electrons.

### Final Answers:
- **Reaction a:**
- Oxidant: O2
- Reductant: Zn
- **Reaction b:**
- Oxidant: H⁺
- Reductant: Zn