CHM2041 Exam #3

Name ______________________________ SS# _________________________ Section _________

Instructions: Put your name, social security number, and section number on the top of this exam page. Answer the following three (3) parts in the notation and conventions of the lecture for a total of 100 points. Read the entire exam. Work alone and consult no one, living or dead, about this test (except Brucat) until after you leave the room. Notebooks, textbooks and any other study material is not allowed in the exam room. You may use only your writing and calculating instruments; sharing of calculators is NOT allowed. Show proper units to receive credit. Your name on this exam states the assertion:
I hereby pledge that I have neither given nor received any unauthorized aid on this exam.

A mixture of 0.71 mol of O₂ and 1.20 mol of SeO₂ is placed in a 2.35 liter vessel at 545K. When the reaction

SeO_{2 (g)} + 1/2O_{2 (g)} <--> SeO_{3 (g)}

reaches equilibrium, the partial pressure of oxygen gas is found to be p(O₂) = 8.25 atm.

a)    What is the total pressure in the reaction vessel at equilibrium?

p_total = 31.2 atm

b)    What is the value of the equilibrium constant, K_p, for the reaction above at 545K?

K_p = 0.298

c)    Qualitatively, what is the effect on the reaction equilibrium upon doubling reactor vessel pressure at constant temperature by the addition of Helium gas. (Hint: the vessel volume remains unchanged).

No Effect on Equilibrium

Part II (20 points each for a total of 40 points)

a)    Consider the reaction

Na_{2 (g)} <--> 2 Na _(g)          K_p = 2.53 x 10^-2 at 600 K.

1.00 atm of pure Na₂ gas is introduced and sealed in a constant volume sample tube at 100 K. The sample tube is then heated to 600K and allowed to reach chemical equilibrium. What is the percent of sodium dimer that is dissociated at 600K?

3.2 % Dissociated

b)     Consider the reaction

CO + 2 H₂ <--> CH₃OH:          K_C = 0.0545 at 212 ^oC

A mixture initially consisting of 1.02 moles of CO and 2.12 moles of H₂ in a 15.0 L vessel is allowed to come to equilibrium at 212 ^oC. What is the concentration of CH₃OH at equilibrium?

[CH₃OH] = 7.38 x 10^-5 M

Part III (15 point each for a total of 30 points)

Consider the reaction

ZrOH₂ <--> ZrOH + 1/2 H₂

The following thermodynamic data is known for this reaction at 300 K:

G_rxn⁰ = 4.32 kJ/mol ; S_rxn ⁰ = 55.4 J/Kmol

a)     What is the equilibrium constant for this reaction at 300 K?

K_eq = 0.177

b)     What is the equilibrium constant for this reaction at 400 K? State any approximations made in this calculation.

K_eq = 1.44