The reaction
`3Fe(s)+4H_(2)O hArr Fe_(3)O_(4)(s) +4H_(2)(g)` is reversible if it is carried out

Options

A. at constant pressure
B. at constant temperature
C. in an open vessel
D. in a closed vessel.

Explanation

To determine under what conditions the reaction

\[ 3\text{Fe}(s) + 4\text{H}_2\text{O}(l) \rightleftharpoons \text{Fe}_3\text{O}_4(s) + 4\text{H}_2(g) \]

is reversible, we can analyze the different scenarios presented in the question.

### Step 1: Analyze the Reaction Conditions
- The reaction involves solid iron (Fe), liquid water (H2O), solid magnetite (Fe3O4), and gaseous hydrogen (H2).
- The key to determining reversibility lies in the system's ability to reach equilibrium.

### Step 2: Consider Constant Pressure
- If the reaction is carried out at constant pressure, the equilibrium can be affected by the change in the number of moles of gas.
- In this reaction, there are 4 moles of gaseous products (H2) produced from the reactants.
- Therefore, changing the pressure could shift the equilibrium position according to Le Chatelier's principle.
- Thus, constant pressure alone does not guarantee reversibility.

### Step 3: Consider Constant Temperature
- Conducting the reaction at constant temperature does not ensure reversibility either.
- While temperature is a crucial factor in determining the position of equilibrium, it does not alone establish the conditions for reversibility.

### Step 4: Consider Open Vessel
- Performing the reaction in an open vessel allows for the escape of gaseous products (H2).
- This loss of mass means that the system cannot reach a stable equilibrium, making the reaction irreversible in this scenario.

### Step 5: Consider Closed Vessel
- In a closed vessel, the system is isolated from the environment, meaning that no mass can escape.
- The reaction can reach equilibrium as the concentrations of reactants and products stabilize.
- Since mass remains constant and energy can still change, the reaction can achieve a state of dynamic equilibrium, making it reversible.

### Conclusion
The reaction is reversible when carried out in a **closed vessel**.

### Final Answer
**The reaction is reversible when carried out in a closed vessel.**

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