For the reversible reaction
`N_(2) (g) +3H_(2) (g) hArr 2NH_(3) (g) + " Heat"`
The equilibrium shifts in forward direction

Options

A. by decreasing the concentrations of `N_(2)(g)` and `H_(2)(g)`
B. by increasing pressure and decreasing temperature
C. by increasing the concentration of `NH_(3)(g)`
D. by decreasing the pressure

Answer

To determine the conditions under which the equilibrium of the reaction

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

shifts in the forward direction, we can analyze the effects of changes in concentration, pressure, and temperature based on Le Chatelier's principle.

### Step-by-Step Solution:

1. **Identify the Nature of the Reaction**:
The given reaction is exothermic since it releases heat. This means that the forward reaction (formation of ammonia) releases heat, while the reverse reaction (decomposition of ammonia) absorbs heat.

**Hint**: Remember that exothermic reactions release heat, while endothermic reactions absorb heat.

2. **Analyze the Effect of Concentration Changes**:
- **Decreasing the Concentration of Reactants**: If we decrease the concentrations of \(N_2\) and \(H_2\), the equilibrium will shift to the left (backward direction) to increase the concentration of the reactants.
- **Increasing the Concentration of Products**: If we increase the concentration of \(NH_3\), the equilibrium will also shift to the left to reduce the concentration of the product.

**Hint**: According to Le Chatelier's principle, reducing the concentration of reactants shifts the equilibrium to the left, while increasing the concentration of products also shifts it to the left.

3. **Analyze the Effect of Pressure Changes**:
- The total number of moles of gas on the reactant side is 4 (1 mole of \(N_2\) and 3 moles of \(H_2\)), while on the product side, it is 2 (2 moles of \(NH_3\)).
- If we increase the pressure, the equilibrium will shift towards the side with fewer moles of gas, which is the forward direction (towards \(NH_3\)).

**Hint**: Increasing pressure favors the side of the reaction with fewer moles of gas.

4. **Analyze the Effect of Temperature Changes**:
- Since the reaction is exothermic, decreasing the temperature will favor the forward reaction (formation of \(NH_3\)) because the system will try to produce more heat.
- Conversely, increasing the temperature would favor the reverse reaction, as the system would absorb heat.

**Hint**: For exothermic reactions, lowering the temperature shifts the equilibrium to the right (forward direction).

5. **Conclusion**:
The equilibrium will shift in the forward direction under the following conditions:
- By increasing the pressure (favoring fewer moles).
- By decreasing the temperature (favoring the exothermic forward reaction).

Thus, the correct condition that shifts the equilibrium in the forward direction is **increasing the pressure and decreasing the temperature**.

### Final Answer:
The equilibrium shifts in the forward direction by **increasing the pressure and decreasing the temperature**.