Chem 201B
Dr. L. Baxley
Equilibrium Worksheet
Answers are at the back. Do these on your own and correct yourself.
1. Write equilibrium constant expressions, Kc, for the following reactions:
a. 2 NO (g) + O2 (g) ⇌ 2 NO2 (g)
b. Zn (s) + 2 Ag+ (aq) ⇌ Zn2+ (aq) + 2 Ag (s)
c. Mg(OH)2 (s) + CO32− (aq) ⇌ MgCO3 (s) + 2 OH− (aq)
2. Equilibrium is established in the reversible reaction 2 A(aq) + B(aq) ⇌ 2 C(aq). The
equilibrium concentrations are [A] = 0.55 M, [B] = 0.33 M, [C] = 0.43 M. What is the value
of Kc for this reaction?
3. Calculate Kc for the reaction CH4(g) + H2O(g) ⇌ CO(g) + 3 H2(g), KP = 7.7 x 1024 at 298 K.
4. a. Calculate KP for the reaction NH4HS(s) ⇌ NH3(g) + H2S(g), Kc = 8.5 x 10–3 at 25 ºC.
b. If a closed container at 25ºC contained 12.7 g NH4HS and some NH3 gas with a partial
pressure of 0.510 atm and H2S gas with a partial pressure of 1.14 atm, will the reaction
produce more solid, or will it produce more gases, in order to reach equilibrium?
5. For the reaction: H2(g) + I2(g) → 2HI(g), Kc = 12.3 at a certain temperature. If at a given
moment in the reaction at that temperature, [H2] = [I2] = [HI] = 3.21 x 10–3 M, which
direction will the reaction proceed to reach equilibrium? Explain your reasoning.
Chem 201B
Dr. L. Baxley
6. A 0.0040-mole sample of S2 (g) is allowed to dissociate in a 0.500-L flask at 1000 K. When
equilibrium is established, 2.0 x 10−11 moles of S (g) is present. What is the value of Kc for
the reaction: S2 (g) ⇌ 2 S (g)
7. A 0.50 mole sample of N2O4 (g) is allowed to come to equilibrium with NO2 (g) in a 0.750 L
flask. At equilibrium, 0.42 mol N2O4 remains. Calculate the equilibrium concentrations of
N2O4 and NO2 and calculate the equilibrium constant, Kc, for the reaction.
N2O4 (g) ⇌ 2 NO2 (g)
8. For the reaction below, if the initial reaction mixture contains a partial pressure of 0.100 atm
for each of the gases, I2, Cl2 and ICl, at 25 ºC, determine the equilibrium partial pressure of
each gas.
I2(g) + Cl2(g) ⇌ 2 ICl(g)
Kp = 81.9 at 25 ºC
Chem 201B
Dr. L. Baxley
9. Solid ammonium carbamate decomposes into ammonia and carbon dioxide according to the
equation shown below. At 25 ºC, experiments show that the total pressure of gases in
equilibrium with the solid is 0.116 atm. What is the equilibrium constant KP?
NH4H2NCO2(s) ⇌ 2 NH3(g) + CO2(g)
10. For the reaction below: Kc = 85.0 at 460 °C. The following amounts of substances are
brought together and allowed to react at this temperature: 0.0025 M SO2, 0.0025 M NO2,
0.025 M NO, and 0.025 M SO3. In which direction will a net change occur? What will the
equilibrium concentrations be?
SO2(g) +
NO2(g) ⇌ NO(g) + SO3(g)
Chem 201B
Dr. L. Baxley
11. For the reaction: PCl5 (g) ⇌ PCl3 (g) + Cl2 (g), Kc = 0.0454 at 261°C. If a vessel is filled
with these gases such that the initial concentrations are [PCl3] = 0.50 M, [Cl2] = 0.50 M and
[PCl5] = 3.50 M, in which direction will a net change occur? What will the equilibrium
concentrations be?
12. At 450°C, ammonia gas will decompose according to the following equation:
2 NH3 (g) ⇌ N2 (g) + 3 H2 (g)
At 450.˚C, Kc = 6.30. An unknown quantity of NH3 is placed in a reaction flask (with no N2
or H2) and is allowed to come to equilibrium at 450. °C. The equilibrium concentration of H2
is then determined to be 0.342 M. Determine the initial concentration of NH3 placed in the
flask. (NOTE that you do not need to use the quadratic equation for any reason in this
calculation, but you can’t make any simplifying assumptions either.)
Chem 201B
Dr. L. Baxley
13. If the system given below is at equilibrium, predict each of the following (answers are
increase, decrease, or no change):
H2(g) + 1/8 S8(s) ⇌ H2S(g)
∆H = −21 kJ/mol
a. If more H2 is added, how will [H2S] change? How will the value of Kc change?
b. If a substance is added that reacts with H2, how will [H2S] change? How will the value
of Kc change?
c. If a substance is added that reacts with S8, how will [H2S] change? How will the value of
Kc change?
d. If the temperature is increased, how will [H2S] change? How will the value of Kc
change?
e. If the volume of the container is increased, how will moles of H2S change? How will the
value of Kc change?
14. If reaction 2 SO2(g) + O2(g) ⇌ 2 SO3(g) is at equilibrium and the volume of the container is
increased, which direction will the reaction shift? How will the value of Kp change?
Chem 201B
Dr. L. Baxley
15. One of the most important industrial sources of ethanol is the reaction of steam with ethane
derived from crude oil:
C2H4(g) + H2O(g) ⇌ C2H5OH(g)
∆Hrxn = –47.8 kJ
For this reaction, Kc = 9 x 103 at 330 °C. Calculate Kc at 180 °C.
16. From these values of Kc,
a. CO (g) + H2O (g) ⇌ CO2 (g) + H2 (g)
b. SO2 (g) + ½ O2 (g) ⇌ SO3 (g)
Kc = 23.2 at 600 K
Kc = 56 at 900 K
c. 2 H2S (g) ⇌ 2 H2 (g) + S2 (g)
Kc = 2.3 x 10-4 at 1405 K
determine values of Kc and Kp for the following reactions at the temperatures given:
a. CO2 (g) + H2 (g) ⇌ CO (g) + H2O (g)
b. 2 SO2 (g) + O2 (g) ⇌ 2 SO3 (g)
c. H2 (g) + ½ S2 (g) ⇌ H2S (g)
17. Given the equilibrium constants for the following reactions,
2 CH4(g) ⇌ C2H6(g) + H2(g)
Kc = 9.5 x 10−13
CH4(g) + H2O(g) ⇌ CH3OH(g) + H2(g) Kc = 2.8 x 10−21
Calculate the equilibrium constant, Kc, for the reaction,
2 CH3OH(g) + H2(g) ⇌ C2H6(g) + 2 H2O(g) Kc = ?
Chem 201B
Dr. L. Baxley
Answer Key
1. a. Kc =
[NO2]2
[NO]2[O2]
2+
b. Kc = [Zn + ]2
[Ag ]
c. Kc =
[OH−]2
[CO32−]
2. Kc = 1.9
3. 1.3 x 1022
4. a. 5.1 (remember that Δng is the change in gases, so solids are not included)
b. It will produce more gases (because QP = 0.58, which is less than KP, so the reaction will
proceed forward)
5. The reaction will proceed to the right because Qc = 1, which is less than Kc. Since Qc is less
than Kc, more products must be made so that Qc will increase and become equal to Kc.
6. Kc = 2.0 x 10−19
7. [N2O4]eq = 0.56 M, [NO2]eq = 0.21 M, Kc = 0.081
8. PI2 = PCl2 = 0.027 atm; PICl = 0.246 atm
9. 2.31 x 10–4
10. [SO2] = [NO2] = 0.0027 M
[SO3] = [NO] = 0.025 M
11. [PCl3] = [Cl2] = 0.40 M
[PCl5] = 3.60 M
12. 0.255 M
13. a. [H2S] will increase, Kc will not change (it’s a constant!)
a. [H2S] will decrease, Kc will not change (it’s a constant!)
b. [H2S] will not change (S8 is a solid), Kc will not change (it’s a constant!)
c. [H2S] will decrease, Kc will decrease (it’s only a constant at constant temperature)
d. [H2S] will not change (same moles of gas on both sides), Kc will not change (it’s a
constant!)
14. Shift left (towards more moles of gas at lower pressure), Kp will not change (it’s a constant!)
15. Kc = 2 x 105
16. a. Kc = 0.0431
Kp = 0.0431
3
b. Kc = 3.1 x 10
Kp = 42
Kp = 6.1
c. Kc = 66
29
17. Kc = 1.2 x 10
18.