Thermodynamics Review

Equilibrium constants

All chemical reactions reach a point where they settle down and won’t go any further. At that point, the reaction is said to be at equilibrium. This point is not when all the reactants have been used up (this is actually rarely seen), nor when all the components of the reaction are present in equal quantities. How much product and how much reactant are present at equilibrium depends on the specific chemical properties of each of the compounds involved. The equilibrium constant, K_eq, is a measure of the concentrations of the reactants and products when the chemical reaction has reached equilibrium.

The equilibrium constant K_eq is defined by the ratio of products to reactants when the reaction is at equilibrium. That is, K_eq is calculated from the concentrations of the reactants and products when the reaction has finished (reached equilibrium). You can do the reaction many times, starting out with different concentrations of reactants and products each time, but at the end of the reaction, the ratio of products to reactants will always end up the same. Thus, the K_eq of a certain reaction is a constant value, that is, it never changes (as long as the temperature remains the same).

Example:

Consider the reaction:

Glucose + ATP Glucose-6-phosphate + ADP

K_eq = 2 x 10³

What is the K_eq of the reverse reaction?

Solution:

The reverse reaction can be written as:

Glucose-6-phosphate + ADP Glucose + ATP

First, let’s compare the K_eq equation for both reactions:

Keq(forward) =

[G-6-phosphate][ADP] [Glucose][ATP]

Keq(reverse) =

[Glucose][ATP] [G-6-phospate][ADP]

Notice how K_eq(forward) is the reciprocal of K_eq(reverse):

K_eq(reverse) = 1/K_eq (forward) = 1/(2 x 10³) = 5 x 10^-4