Everything Science Grade 12

3.7 CHAPTER 3. REACTION RATES

3.7 The equilibrium constant

DEFINITION: Equilibrium constant

The equilibrium constant (Kc), relates to a chemicalreaction at equi-

librium. It can be calculated if the equilibriumconcentration of each

reactant and product ina reaction at equilibriumis known.

Calculating the equilibrium constant ESCBA

Consider the following generalised reaction which takes place in a closed container at

a constant temperature:

A + B� C + D

We know from section 3.2 that the rate of the forward reaction is directlyproportional

to the concentration ofthe reactants. In other words, as the concentration of the reac-

tants increases, so doesthe rate of the forward reaction. This can be shown using the

following equation:

Rate of forward reaction∝ [A][B]

or

Rate of forward reaction= k 1 [A][B]

Similarly, the rate of thereverse reaction is directly proportional to the concentration of

the products. This can be shown using the following equation:

Rate of reverse reaction∝ [C][D]

or

Rate of reverse reaction= k 2 [C][D]

At equilibrium, the rateof the forward reactionis equal to the rate of the reverse reac-

tion. This can be shownusing the following equation:

k 1 [A][B] = k 2 [C][D]

or