constant expression for the reduction of NO with H2
with a second-power dependence on H2 concentration in the denominator.
The reason for the apparent contradiction between this equilibrium-constant expression and the rate law is that any complications of mechanism that affect the forward reaction and its rate law also affect the reverse reaction in the same way. They both cancel out in the overall equilibrium-constant expression.
Although the correct rate law for the forward or reverse reaction cannot be deduced from the balanced chemical reaction alone, the equilibrium-constant expression can be.A classical example of complications in a rate expression is the reaction analogous to HI production, but with bromine instead of iodine:
The equilibrium-constant expression is: