Doubling the concentration of HI molecules doubles the number of
molecules available for collision, and independently doubles the
rate at which each one collides with a neighbor. The result is a
second-power dependence, and a second-order reaction.
At equilibrium, synthesis and breakdown of HI are exactly in balance.
The forward and reverse reaction rates are equal, and
This is the equilibrium-constant expression for the HI reaction.
It tells us that regardless of whether the absolute concentrations
are high or low, at equilibrium the ratio of products to reactants
will be the same.