The reaction quotient Q for :
`N_2(g)+3H_2(g) hArr 2NH_3(g)` is given by `Q=([NH_3]^2)/([N_2][H_2]^3)` The reaction will proceed in backward direction, when :

Options

A. `QgtK_c`
B. `Q=0`
C. `Q=K_c`
D. `QltK_c`

Explanation

To determine when the reaction \( N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) \) will proceed in the backward direction, we need to analyze the reaction quotient \( Q \) in relation to the equilibrium constant \( K_c \).

### Step-by-Step Solution:

1. **Understand the Reaction Quotient \( Q \)**:
The reaction quotient \( Q \) is defined as:
\[ Q = \frac{[NH_3]^2}{[N_2][H_2]^3} \]
This expression represents the ratio of the concentrations of products to reactants at any point in the reaction.

2. **Understand the Equilibrium Constant \( K_c \)**:
The equilibrium constant \( K_c \) is also defined similarly but specifically at equilibrium conditions:
\[ K_c = \frac{[NH_3]^2}{[N_2][H_2]^3} \]
At equilibrium, the concentrations of the reactants and products remain constant.

3. **Compare \( Q \) and \( K_c \)**:
- If \( Q < K_c \): The reaction will proceed forward to produce more products (NH3).
- If \( Q = K_c \): The system is at equilibrium, and there is no net change in the concentrations of reactants and products.
- If \( Q > K_c \): The concentration of products (NH3) is higher than at equilibrium, which will drive the reaction in the backward direction to produce more reactants (N2 and H2).

4. **Determine the Condition for Backward Reaction**:
From the comparison:
- The reaction will proceed in the backward direction when \( Q > K_c \).

### Conclusion:
Thus, the reaction \( N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) \) will proceed in the backward direction when:
\[ Q > K_c \]