8. Equilibrium Calculations

Type 1 (system at equilibrium)

What is the Keq for the reaction 2HI (g) H2 (g) + I2 (g) if there is

2.0 mol of HI, 3.0 mol of I2, and 3.5 mol of H2 in a 5.0L closed flask?

[H2] = 3.5/5 = 0.7 MKeq = [H2][I2]

[HI]2

[I2] = 3/5 = 0.6 M

Keq = [0.7][0.6] = 2.6

[HI] = 2/5 = 0.4 M [0.4]2

The Keq for the reaction 2AB (g) A2 (g) + B2 (g) is 3.0. The

equilibrium concentration of A2 is 0.50M and B2 is 4.5M. What is the equilibrium

concentration of AB?

Keq = [A2][B2]

[AB]2

3.0 = [0.5][4.5] [AB]2 = [0.5][4.5] = 0.75 [AB] = = 0.87 M

[AB]2 3.0

Type 2 (system changing to equilibrium --- Calculating the Keq)

Example 1: A(g) + 3B(g) 2C(g)

4 moles of A and 8 moles of B are placed in a 1.0L flask. When equilibrium is

reached, the flask contains 4 moles of C. What is Keq?

A / + 3B / / 2C
I 4 / 8 / 0 (assume!)
C -2 / -6 / +4
E 2
/ 2 / 4
Step 1 make ICE table Step 6

Step 2fill in info given Keq = [C]2

Step 3 find change [A][B]3

Step 4calculate other changes from stoichiometry

Step 5calculate final equilibrium values Keq = [4]2 = 1

[2][2]3

Example 2: 2NO (g) + O2(g) 2NO2(g)

4.0 mol of NO2 is added to a 2.0 L flask. After a while, equilibrium is

attained. At equilibrium, 0.50 mol of NO is found. What is Keq?

[NO] = 0.5 mol / 2.0L = 0.25 M[NO2] = 4.0 mol / 2.0L = 2.0 M

2NO / + 02 / / 2NO2
I 0 / 0 / 2.0 M
C + 0.25 / +0.125 / - 0.25
E 0.25 M
/ 0.125 M / 1.75 M

Keq = [1.75]2 = 392 = 390

[0.25]2[0.125]

Example 3: N2(g) + 3H2(g) 2NH3(g)

6.0 moles of NH3 are added to a 1.0 L flask. A few hours later equilibrium is obtained and 0.50 mol of N2 are found. What is Keq?

N2 / + 3H2 / / 2NH3
I 0 / 0 / 6.0 M
C + 0.5 / +1.5 / - 1.0
E 0.50 M
/ 1.5 M / 5.0 M

Keq = [5.0]2 = 14.8 = 15

[0.5][1.5]3

Example 4: PCl5(g) PCl3(g) + Cl2(g)

8.0 moles of PCl3 and 6.0 moles of Cl2 are placed in a 2.0 L flask. After

equilibrium, 0.50 moles of Cl2 are found. What is Keq?

[PCl3] = 8.0 mol / 2.0L = 4.0M ; [Cl2]eq = 0.50 mol / 2.0L = 0.25M ; [Cl2]init. = 6.0 / 2.0 = 3.0M

PCl5 / PCl3 + / Cl2
I 0 M / 4.0 M / 3.0M
C +2.75 / - 2.75 / - 2.75
E 2.75 M
/ 1.25 M / 0.25 M

Keq = [0.25][1.25] = 0.113 = 0.11

[2.75]

Do Questions # 47 – 49 page 70