5 Steps to a 5 AP Chemistry 2019

214 ❯ STEP 4. Review the Knowledge You Need to Score High

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 Kp if 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 con-

centrations do not change.

Consider the Haber process for the production of ammonia:

N 22 (g)3+ H(g)2NH( 3 g)

The equilibrium constant expression would be written as:

K =

[NH]

c [N][H]

3

2

22

3

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

ing form:

=

×

K

P

p PP

NH

2

NH

3

3

22

There is a relationship between Kc and Kp: Kp = Kc(RT)Dn, where R is the ideal gas

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

the reaction.

Remember: Be sure that your value of R is 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 Kc for

this equilibrium at 25°C.

C(s)+=CO (g)22p CO(g) K 1.90

KKpc= ()RT ∆n

=

−

K

K

1.90

[(0.0826Latm)(298K)]

c [(mol)]

(2 1)

Kc=0.0777

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 Kc is 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 Kc is small, then the numerator

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

lies far to the left).

KEY IDEA