For the reaction
2H₂(g) + 2NO(g) → N₂(g) + 2H₂O(g)
the observed rate expression is, rate = K f [NO]²[H₂]. The rate expression for the reverse reaction is :

Solution

Chemistry ma’am · 4 steps

At equilibrium the forward and backward rates are equal, so build the reverse rate law from K_c.

1. Equilibrium condition. At equilibrium, forward rate equals backward rate. Also, K_c = K_f/K_b, and K_c comes from the balanced equation.

   K_c = [N₂][H₂O]²/[H₂]²[NO]²

2. Set rates equal. Rate of the backward reaction = K_f[NO]²[H₂] = K_b × K_f/K_b[NO]²[H₂] = K_b K_c[NO]²[H₂].
3. Substitute Kc. Putting in K_c, the [NO]² cancels and one [H₂] cancels. That leaves K_b [N₂][H₂O]²/[H₂].
4. So. The reverse rate expression is K_b[N₂][H₂O]²/[H₂], which is option (b).

The idea to keep. When the forward rate law is not the stoichiometric one, get the reverse rate law from K_c = K_f/K_b. Divide the full K_c expression by what the forward rate already contains.

Answer from the NTA final answer key. Solution by PrepWiser.