How is the reaction rate related to equilibrium?

Dec 16, 2015

We need to distinguish between static and dynamic equilibrium.

The common equilibrium you see in chemistry is dynamic equilibrium:

The rate of the forward and reverse reactions are equal, thus resulting in no net change in the concentrations of products or reactants.

Example:

$\text{H"_2(g) + "I"_2(g) stackrel(k_1)(rightleftharpoons) 2"HI} \left(g\right)$
${\textcolor{w h i t e}{a a a a a a a a a a a a}}^{{k}_{- 1}}$
where ${k}_{1} = {k}_{- 1}$.

The less common "equilibrium" is static equilibrium:

A reaction has not occurred or has previously finished, yet no motion occurs at that time. The rate of a reaction in either direction at static equilibrium is $0$.

NOTE: this does NOT necessarily mean that the concentrations of products is equal to the concentrations of reactants.

Example:

"C" ("graphite") -> "C" ("diamond") rightleftharpoons "C" ("diamond")

Here, ["C" ("diamond")] > 0, but ["C" ("graphite")] = 0.