For the reversible reaction
` N_2(g) +3H_2(g) hArr 2NH_3(g) +"Heat" `
The equilibrium shifts in forward directions.

Options

A. by increasing the concentration of `NH_3 (g)`
B. by decreasing the pressure
C. by decreasing the concentration of `N_2(g) and H_2(g)`
D. by increasing pressure and decreasing temperature

Answer

To solve the problem regarding the equilibrium shift of the reaction:

\[ N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) + \text{Heat} \]

we will analyze how changes in concentration, pressure, and temperature affect the equilibrium position according to Le Chatelier's principle.

### Step-by-Step Solution:

1. **Understanding the Reaction:**
- The reaction is a reversible process where nitrogen and hydrogen gases react to form ammonia and release heat.
- The balanced equation shows that 1 mole of nitrogen reacts with 3 moles of hydrogen to produce 2 moles of ammonia.

2. **Writing the Expression for Q:**
- The reaction quotient \( Q \) is given by:
\[ Q = \frac{[NH_3]^2}{[N_2][H_2]^3} \]
- Here, \( [NH_3] \), \( [N_2] \), and \( [H_2] \) are the molar concentrations of ammonia, nitrogen, and hydrogen, respectively.

3. **Analyzing the Options:**
- **Option A:** Increasing the concentration of \( NH_3 \).
- If we increase \( [NH_3] \), \( Q \) will increase. If \( Q > K \), the reaction will shift to the left (backward direction). **Incorrect.**

- **Option B:** Decreasing the pressure.
- Decreasing pressure shifts the equilibrium towards the side with more moles of gas. The reactants have 4 moles (1 N2 + 3 H2) and products have 2 moles (2 NH3). Thus, the reaction will shift to the left. **Incorrect.**

- **Option C:** Decreasing the concentration of \( N_2 \) and \( H_2 \).
- Decreasing the concentration of reactants will increase \( Q \). If \( Q > K \), the reaction will shift to the left. **Incorrect.**

- **Option D:** Increasing pressure and decreasing temperature.
- Increasing pressure shifts the equilibrium towards the side with fewer moles of gas (to the right, producing more NH3).
- Decreasing temperature in an exothermic reaction (which this is) shifts the equilibrium to the right to produce more heat. **Correct.**

4. **Conclusion:**
- The correct option that causes the equilibrium to shift in the forward direction is **Option D: Increasing pressure and decreasing temperature**.