Written Assignment 4: Chemical Equilibrium

Answer all assigned questions and problems, and show all work.

1.      Consider the following reaction:

N₂(g) + O₂ (g) ⇌ 2NO(g)

If the equilibrium partial pressures of N₂, O₂, and NO are 0.15 atm, 0.33 atm, and 0.050 atm, respectively, at 2200ºC, what is K_P? (5 points)

(Reference: Chang 14.19)

2.      The equilibrium constant K_P for the reaction

PCl₅(g) ⇌ PCl₃(g)  +  Cl₂(g)

is 1.05 atm at 250ºC. The reaction starts with a mixture of PCl₅, PCl₃, and Cl₂ at pressures 0.177 atm, 0.223 atm, and 0.111 atm, respectively, at 250ºC. When the mixture comes to equilibrium at that temperature, which pressures will have decreased and which will have increased? Explain why. (5 points)

(Reference: Chang 14.23)

3.      Consider the following reaction at 1600ºC:

Br₂(g)  ⇌  2Br(g)

When 1.05 moles of Br₂ are put in a 0.980-L flask, 1.20 percent of the Br₂ undergoes dissociation. Calculate the equilibrium constant K_c for the reaction. (8 points)

(Reference: Chang 14.25)

4.      The following equilibrium constants have been determined for hydrosulfuric acid at 25ºC:

H₂S(aq) ⇌ H⁺(aq)  +  HS^–(aq)            K^′_c = 9.5 × 10^–8

HS^–(aq) ⇌ H⁺(aq)  +  S^2–(aq)              K″_c = 1.0 × 10^–19

Calculate the equilibrium constant for the following reaction at the same temperature:
(5 points)

H₂S(aq) ⇌ 2H⁺(aq)  +  S^2–(aq)

                (Reference: Chang 14.29)

5.      For the reaction

            H₂(g)  +  CO₂(g)  ⇌  H₂O(g)  +  CO(g)

at 700ºC, K_c = 0.534. Calculate the number of moles of H₂ that are present at equilibrium if a mixture of 0.300 mole of CO and 0.300 mole of H₂O is heated to 700ºC in a 10.0-L container. (10 points)

(Reference: Chang 14.41)

6.      The equilibrium constant K_c for the reaction

            H₂(g)  +  Br₂(g)  ⇌  2HBr(g)

is 2.18 × 10⁶ at 730ºC. Starting with 3.20 moles of HBr in a 12.0-L reaction vessel, calculate the concentrations of H₂, Br₂, and HBr at equilibrium. (10 points)

(Reference: Chang 14.43)

7.      The dissociation of molecular iodine into iodine atoms is represented as

I₂(g) ⇌ 2I(g)

At 1000 K, the equilibrium constant K_c for the reaction is 3.80 × 10^–5. Suppose you start with 0.0456 mol of I₂ in a 2.30-L flask at 1000 K. What are the concentrations of the gases at equilibrium? (10 points)

(Reference: Chang 14.44)

8.      Consider the following equilibrium system involving SO₂, Cl₂, and SO₂Cl₂ (sulfuryl dichloride):

SO₂(g) + Cl₂(g)  ⇌ SO₂Cl₂(g)

Predict how the equilibrium position would change if the temperature remains constant:

a.       Cl₂ gas were added to the system. (2 points)

b.      SO₂Cl₂ were removed from the system. (2 points)

c.       SO₂ were removed from the system. (2 points)

(Reference: Chang 14.53)

9.      Consider the exothermic equilibrium

2I(g) ⇌ I₂(g)

What would be the effect on the position of equilibrium of

a.       increasing the total pressure on the system by decreasing its volume. (2 points)

b.      adding gaseous I₂ to the reaction mixture. (2 points)

c.       decreasing the temperature. (2 points)

(Reference: Chang 14.57)

10.  Consider the reaction

2SO₂(g)  + O₂(g) ⇌ 2SO₃(g)                ΔH⁰ = –198.2 kJ/mol

Comment on the changes in the concentrations of SO₂, O₂, and SO₃ at equilibrium if we were to

a.       increase the temperature. (2 points)

b.      increase the pressure. (2 points)

c.       increase SO₂. (2 points)

d.      add a catalyst. (2 points)

e.       add helium at constant volume. (2 points)

            (Reference: Chang 14.59)

11.  At 1130ºC the equilibrium constant (K_c) for the reaction

2H₂S(g)  ⇌  2H₂(g)  +  S₂(g)

is 2.25 × 10^–4. If [H₂S] = 4.84 × 10^–3 M and [H₂] = 1.50 × 10^–3 M, calculate [S₂].
(5 points)

            (Reference: Chang 14.79)

12.  A mixture containing 3.9 moles of NO and 0.88 moles of CO₂ was allowed to react in a flask at a certain temperature according to the equation

NO(g)  +  CO₂(g)  ⇌  NO₂(g)  +  CO(g)

At equilibrium, 0.11 mole of CO₂ was present. Calculate the equilibrium constant K_c of this reaction. (10 points)

(Reference: Chang 14.89)

13.  The equilibrium constant K_c is 54.3 at 430°C for the following reaction:

H₂(g)  +  I₂(g)  ⇌ 2HI(g)        

Initially, 0.90 M H₂, I₂, and HI are introduced into a 5.0-L flask and allowed to come to equilibrium. What are the equilibrium concentrations of H₂, I₂, and HI in the flask?
(10 points)

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