Given the equilibrium system
`NH_(4)Cl(s)hArr NH_(4)^(+)(aq)+Cl^(-)(aq) (DeltaH=+"3.5 kcal/mol").`
What change will shift the equilibrium to the right?

Options

A. Increase in temperature
B. Decrease in temperature
C. Addition of `NaCl` crystals
D. Addition of `NH_4OH` solution

Explanation

To determine what change will shift the equilibrium of the reaction

\[ \text{NH}_4\text{Cl}(s) \rightleftharpoons \text{NH}_4^+(aq) + \text{Cl}^-(aq) \]

with a given enthalpy change (\(\Delta H = +3.5 \, \text{kcal/mol}\)), we can apply Le Chatelier's principle.

### Step-by-Step Solution:

1. **Identify the Nature of the Reaction**:
- The reaction is endothermic since \(\Delta H\) is positive. This means that heat is absorbed when the reaction proceeds to the right (products).

2. **Apply Le Chatelier's Principle**:
- Le Chatelier's principle states that if an external change is applied to a system at equilibrium, the system will adjust to counteract that change and restore a new equilibrium.

3. **Consider the Effect of Temperature**:
- For an endothermic reaction, increasing the temperature will shift the equilibrium to the right (towards the products) because the system will try to absorb the added heat.

4. **Evaluate Other Possible Changes**:
- Adding a solid (like NH4Cl) does not affect the equilibrium position because solids do not appear in the equilibrium expression.
- Adding an inert gas at constant volume does not change the partial pressures of the reactants or products, so it will not shift the equilibrium.
- Decreasing the temperature would shift the equilibrium to the left (towards the reactants) for an endothermic reaction.

5. **Conclusion**:
- The only change that will shift the equilibrium to the right is an increase in temperature.

### Final Answer:
**Increasing the temperature will shift the equilibrium to the right.**