Which of the following reactions is endothermic ?
`CaCO_3(s) to CaO(s) + CO_2(s)`
`Fe(s) + S(s) toFeS(s)`
`NaOH (aq) + HCl (aq) to NaCl (aq) + H_2O(l)`
`CH_4(g) + 2O_2(g) to CO_29g) + 2H_2O(l)`.

Options

A. `CaCO_3(s) to CaO(s) + CO_2(s)`
B. `Fe(s) + S(s) toFeS(s)`
C. `NaOH (aq) + HCl (aq) to NaCl (aq) + H_2O(l)`
D. `CH_4(g) + 2O_2(g) to CO_29g) + 2H_2O(l)`.

Explanation

To determine which of the given reactions is endothermic, we need to analyze each reaction and understand the energy changes involved. An endothermic reaction is one that absorbs heat from the surroundings, while an exothermic reaction releases heat.

### Step-by-Step Solution:

1. **Identify the Reactions**:
- Reaction 1: \( \text{CaCO}_3(s) \rightarrow \text{CaO}(s) + \text{CO}_2(g) \)
- Reaction 2: \( \text{Fe}(s) + \text{S}(s) \rightarrow \text{FeS}(s) \)
- Reaction 3: \( \text{NaOH}(aq) + \text{HCl}(aq) \rightarrow \text{NaCl}(aq) + \text{H}_2O(l) \)
- Reaction 4: \( \text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2O(l) \)

2. **Analyze Each Reaction**:
- **Reaction 1**: The decomposition of calcium carbonate into calcium oxide and carbon dioxide requires energy input to break the bonds in the calcium carbonate. This is a characteristic of an endothermic reaction.
- **Reaction 2**: The formation of iron sulfide from iron and sulfur is an exothermic reaction, as it releases energy when the bonds are formed.
- **Reaction 3**: The neutralization reaction between sodium hydroxide and hydrochloric acid is also exothermic, as it releases heat when forming sodium chloride and water.
- **Reaction 4**: The combustion of methane in oxygen is an exothermic reaction, as it releases heat and light energy.

3. **Conclusion**:
Based on the analysis, the only reaction that requires energy (is endothermic) is:
- \( \text{CaCO}_3(s) \rightarrow \text{CaO}(s) + \text{CO}_2(g) \)

Thus, the answer is **Reaction 1**.