How may of following are redox reactions:
-decomposition of magnesite.
-reaction of `F_(2)` with HBr.

Answer

To determine how many of the given reactions are redox reactions, we will analyze each reaction step by step.

### Step 1: Analyze the decomposition of magnesite (MgCO3)

1. **Write the decomposition reaction**:
\[ \text{MgCO}_3 \rightarrow \text{MgO} + \text{CO}_2 \]

2. **Determine the oxidation states**:
- In MgCO3:
- Magnesium (Mg) = +2
- Carbon (C) = +4 (calculated as follows: \( +2 + x - 6 = 0 \) → \( x = +4 \))
- Oxygen (O) = -2
- In MgO:
- Magnesium (Mg) = +2
- Oxygen (O) = -2
- In CO2:
- Carbon (C) = +4
- Oxygen (O) = -2

3. **Check for changes in oxidation states**:
- The oxidation state of Mg remains +2, C remains +4, and O remains -2 in both reactants and products.
- **Conclusion**: There is no change in oxidation states, so this is **not a redox reaction**.

### Step 2: Analyze the reaction of F2 with HBr

1. **Write the reaction**:
\[ \text{F}_2 + 2 \text{HBr} \rightarrow 2 \text{HF} + \text{Br}_2 \]

2. **Determine the oxidation states**:
- In F2:
- Fluorine (F) = 0
- In HBr:
- Hydrogen (H) = +1
- Bromine (Br) = -1
- In HF:
- Hydrogen (H) = +1
- Fluorine (F) = -1
- In Br2:
- Bromine (Br) = 0

3. **Check for changes in oxidation states**:
- Fluorine changes from 0 in F2 to -1 in HF (reduction).
- Bromine changes from -1 in HBr to 0 in Br2 (oxidation).
- **Conclusion**: Since there is both oxidation and reduction occurring, this is a **redox reaction**.

### Final Conclusion

- **Decomposition of magnesite**: Not a redox reaction.
- **Reaction of F2 with HBr**: Is a redox reaction.

Thus, **only one of the reactions is a redox reaction**.

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