For the reaction, `4NH_(3)(g) + 5O_(2)(g)hArr4NO(g) + 6`
` H_(2)O(l)`, `Delta H =` positive. At equilibrium the factor that will not affect the concentration of `NH_(3)` is:

Options

A. change in pressure
B. change in volume
C. catalyst
D. None of these

Explanation

To solve the problem, we need to analyze the factors that affect the equilibrium of the given reaction:

**Given Reaction:**
\[ 4NH_3(g) + 5O_2(g) \rightleftharpoons 4NO(g) + 6H_2O(l) \]
**ΔH = positive (endothermic reaction)**

**Step 1: Identify the factors that can affect equilibrium.**
The factors that can affect the equilibrium of a reaction include:
- Change in concentration of reactants or products
- Change in pressure
- Change in volume
- Change in temperature
- Presence of a catalyst

**Step 2: Analyze the effect of each factor on the equilibrium of the reaction.**

1. **Change in Pressure:**
- According to Le Chatelier's principle, if the pressure is increased, the equilibrium will shift towards the side with fewer moles of gas.
- In this reaction, we have 9 moles of gas on the reactants' side (4 moles of \(NH_3\) + 5 moles of \(O_2\)) and 4 moles of gas on the products' side (4 moles of \(NO\)).
- Therefore, an increase in pressure will shift the equilibrium to the left, affecting the concentration of \(NH_3\).

2. **Change in Volume:**
- Volume is inversely related to pressure. If the volume is increased, the pressure decreases.
- A decrease in pressure will shift the equilibrium towards the side with more moles of gas, which is the reactants' side in this case.
- Thus, changing the volume will also affect the concentration of \(NH_3\).

3. **Change in Temperature:**
- For an endothermic reaction (positive ΔH), increasing the temperature will shift the equilibrium to the right (towards products), while decreasing the temperature will shift it to the left (towards reactants).
- Therefore, temperature changes will affect the concentration of \(NH_3\).

4. **Presence of a Catalyst:**
- A catalyst speeds up the rate at which equilibrium is reached but does not affect the position of the equilibrium.
- It does not change the concentrations of reactants or products at equilibrium.
- Hence, the presence of a catalyst will not affect the concentration of \(NH_3\).

**Step 3: Conclusion**
The factor that will not affect the concentration of \(NH_3\) is the presence of a catalyst.

**Final Answer:**
The factor that will not affect the concentration of \(NH_3\) is the presence of a catalyst.

---