Equilibrium Calculations

Lesson 7

How can we describe an equilibrium system mathematically?

reactants products⇌

reactants

The Keq is the equilibrium constant- a number that does not change.

Providing the temperature is kept constant.

products

= 3.0Keq =

Equilibrium Calculations An equilibrium system, at any given temperature, can be described by an equilibrium expression and equilibrium constant.

aA + bB ⇌ cC + dD

(aq) and (g) are included! (l) and (s) are not-constant concentration!

[A]a[B]b

[C]c[D]d

Keq =

Equilibrium Constant- a number Expression- mathematical equation

Keq =Products

Reactants

SO3(g) + H2O(g) ⇌ H2SO4(l)

1. at 25oC, [SO3] = 0.200 M. [H2O] = 0.480 M, and [H2SO4] = 24 M.

Calculate the Keq.

The Keq has no units but concentration units that go in the expression must be M!

= 10.4

(0.200)(0.480)

1

[SO3] [H2O]

1don’t count (l)! Use 1Keq =

=

At equilibrium

No ICE

2. 0.500 mole PCl5, 0.40 mole H2O, 0.200 mole HCl, and 0.400 mole POCl3 are found in

a 2.0 L container at 125 oC. Calculate the Keq.

PCl5(s) + H2O(g) ⇌ 2HCl(g) + POCl3(g)

[HCl] = 0.200 moles = 0.10 M2.0 L

[POCl3] = 0.400 moles = 0.20 M2.0 L

= 0.20 M0.40 moles

2.0 L

[H2O] =

Keq = [HCl]2[POCl3]

[H2O]

Keq = [0.10]2[0.20]

[0.20]

Keq = 0.010

No ICE

at equilibrium

3. If 0.600 mole of SO3 and 0.0200 mole of SO2 are found in a 2.00 L container at

equilibrium at 25 oC. Calculate the [O2].

[SO3] = 0.600 mole/2.00 L = 0.300 M

[SO2] = 0.0200 mole/2.00 L = 0.0100 M

Keq = [SO 3]2

[SO2]2[O2]

798 = (0.300)2

(0.0100)2[O2]

(0.3)2 = 798(0.01)2[O2]

[O2] = (0.3)2

798(0.01)2

= 1.13 M

2SO2(g) + O2(g) ⇌ 2SO3(g)Keq = 798

1

4. When 0.800 moles of SO2 and 0.800 moles of O2 a 2.00 L container

and allowed to reach equilibrium, the equilibrium [SO3] is to be 0.300 M. Calculate the

Keq value.

2SO2 (g) + O2 (g) ⇋ 2SO3 (g)

Keq = [SO3]

2 (0.3)2

= 36.0

[SO2]2[O2]

(0.1)2(0.25)

Equilibrium concentrations go in the equilibrium equation!

Implies initial and not equilibrium concentrations

0.300 M0.250 M0.100 M

00.400M0.400 MI

+0.300 M-0.150 M-0.300 M C

E

- ICE

=

are placed into

x1/2x 2/2