III. Equilibrium (pg. 4-Review)
AP Chemistry
Equilibrium Constant
1) Determine the equilibrium constant for the reaction H2 + I2 ↔ 2 HI; if the equilibrium concentrations are: H2 0.9
moles/liters; I2 0.4 mole/liter; HI 0.6 mole/liter.
2) One of the two most important uses of ammonia is as a reagent in the first step of the Oswald process, a synthetic
route for the production of nitric acid. This first step proceeds according to the equation
4 HN3 (g) + 5 O2 (g) ↔ 4 NO (g) + 6 H2O (g).
What is the expression for the equilibrium constant of this reaction?
3) What is the Kc for the following reaction, 2 SO3 (g) ↔ 2 SO2 (g) + O2 (g), if Kp is 0.76 at 900 K?
4) Consider the following equilibrium, for which Kp = 0.0752 at 480 oC: 2 Cl2 (g) + 2 H2O (g)  4 HCl
(g) + O2 (g)
a) What is the value of Kp for the reaction 4 HCl (g) + O2 (g)  2 Cl2 (g) + 2 H2O (g) ?
b) What is the value of Kp for the reaction Cl2 (g) + H2O (g)  2 HCl (g) + ½ O2 (g)?
c) What is the value of Kc for the reaction in part b)?
ICE Tables
1) At 603 K, Kc for the reaction: NH4Cl (g) ↔ NH3 (g) + HCl (g) is 1.1 x 10-2. What is the equilibrium
concentration of NH3 (g) if the initial concentration of NH4Cl (g) is 0.500 M?
2) For the equilibrium: C (s) + CO2 (g) ↔ 2 CO (g) the value of Kp is 1.6 x10-5 at 1000 oC. If a 5.00 L
container initially contains 2.00 atm pressure of CO2 (g) and C (s) at 1000 oC, what are the partial pressures
of CO2 (g) and CO (g) at equilibrium?
3) A chemical system at 800 K initially contains 0.0729 M CO (g) and 0.1687 M H2 (g). The following
equilibrium is established: CO (g) + 3 H2 (g) ↔ CH4 (g) + H2O (g).
a) At equilibrium it is found that 72.6 % of the CO reacts. What is the value of the equilibrium
constant?
b) Another study at 1000 K finds Kc is 2.5 x 102. Is the reaction exothermic or endothermic?
Explain.
LeChatelier’s Principle
1) Hydrogen gas reacts with solid sulfur to produce hydrogen sulfide gas: H2 (g) + S (s) ↔ H2S (g)
∆H = -20.17 kJ/mol.
An amount of solid S and an amount of gaseous H2 are placed in an evacuated container at 25 oC, At
equilibrium, some solid S remains in the container. Predict and explain each of the following:
a) The effect on the equilibrium partial pressure of H2S gas when additional solid sulfur is
introduced into the container.
b) The effect on the equilibrium partial pressure of H2 gas when additional H2S gas is introduced
into the container.
c) The effect on the mass of solid sulfur present when the volume of the container is increased.
d) The effect on the mass of solid sulfur present when the temperature is decreased.
e) The effect of adding a catalyst to initial amounts of reactants.
2) According to LeChatelier’s principle, what changes in the composition of the equilibrium system
H2 (g) + Br2 (l) ↔ 2 HBr (g)
Occur when a) Br2 (l) is added; b) the partial pressure of H2 (g) is increased; c) the volume of the container that
contains the equilibrium system is increased at constant temperature?
3) Given the following information: 2 NOCl (g) ↔ 2 NO (g) + Cl2 (g) ∆Ho + 75 kJ
What change occurs in a system at equilibrium a) if the temperature is decreased; b) if the partial pressure of
chlorine gas is increased; and c) if the total volume is increased.
Equilibrium Constant and Reaction Quotient
1) At 100 oC the equilibrium constant for the reaction COCl2 (g)  CO (g) + Cl2 (g) has a value of Kc =
2.19 x 10-10. Are the following measurements of COCl2, CO, and Cl2 at 100 oC at equilibrium? If not,
indicate the direction that the reaction must proceed to achieve equilibrium.
a) [COCl2] = 2.00 x 10-3 M, [CO] = 3.3 x 10-6 M, [Cl2] = 6.62 x 10-6 M;
b) [COCl2] = 4.50 x 10-2 M, [CO] = 1.1 x 10-7 M, [Cl2] = 2.25 x 10-6 M;
c) [COCl2] = 0.0100 M, [CO] = [Cl2] = 1.48 x 10-6 M.
2) The Kp for the equilibrium N2 (g) + 3 H2 (g)  2 NH3 (g) is 4.51 x 10-5 at 450 oC. For each of the
mixtures listed here, indicate whether the mixture is at equilibrium at 450 oC. If it is not at equilibrium,
indicate the direction (toward product or toward reactants) in which the mixture must shift to achieve
equilibrium.
a) 105 atm NH3, 35 atm N2, 495 atm H2;
b) 35 atm NH3, 595 atm H2, no N2;
c) 26 atm NH3, 42 atm H2, and 202 atm N2