5 Steps to a 5 AP Chemistry

Equilibrium Expressions

The reactant quotient can be written at any point during the reaction, but the most useful

point is when the reaction has reached equilibrium. At equilibrium, the reaction quotient

becomes the equilibrium constant, Kc(or Kpif gas pressures are being used). Usually this

equilibrium constant is expressed simply as a number without units, since it is a ratio of

concentrations or pressures. In addition, the concentrations of solids or pure liquids (not in

solution) that appear in the equilibrium expression are assumed to be 1, since their concen-

trations do not change.

Consider the Haber process for the production of ammonia:

The equilibrium constant expression would be written as:

If the partial pressures of the gases were used, then Kpwould be written in the follow-

ing form:

There is a relationship between Kcand Kp: Kp=Kc(RT)Δn, where Ris the ideal gas

constant (0.0821 L atm/mol K) and Δnis the change in the number of moles of gas in the

reaction.

Remember: Be sure that your value of Ris consistent with the units chosen for the

partial pressures of the gases.

For the following equilibrium Kp=1.90: C(s) =CO 2 (g) 2 CO(g) Calculate Kcfor

this equilibrium at 25°C.

The numerical value of the equilibrium constant can give an indication of the extent of

the reaction after equilibrium has been reached. If the value of Kcis large, that means the

numerator is much larger than the denominator and the reaction has produced a relatively

large amount of products (reaction lies far to the right). If Kcis small, then the numerator

is much smaller than the denominator and not much product has been formed (reaction

lies far to the left).

Kc=0 0777.

190

0 0826

.

[(.

[(

=

−

Kc

L atm)(298K)](2 1)

mol K)]

KKRTpc= ( )Δn

C( s ) + CO g2p() 219 CO( g) K =. 0



K

P

p PP

NH

NH

3

22

=

×

2

3

Kc^3

22

NH

NH

=

[]

[][]

2

3

Ng Hg) 2NHg) 22 ()+ 3 ( 3 (

Equilibrium  213

KEY IDEA