The equilibrium constant Kc for the reaction N2 (g) + 3H2 (g)⇋ 2NH3 (g) is:
[2NH3][N2][3H2]
[NH3][N2][3H2]3
[NH3]2[N2][3H2]
[NH3]2[N2][H2]3
N2 (g) + 3H2 (g)⇋ 2NH3 (g)
We can write Rate of forward reaction, rf = kf [N2]1 [H2]3
And the rate of backward reaction, rb = kb [NH3]2
3H2 (g) + N2 (g) ⇌ 2NH3 (g)