Additional Practice Problems
Set 1
Note: For some of these problems use the general expression for chemical equilibrium given
as Keq = conc. of products/conc. of reactants raised to their coefficients.
1. Hydrogen gas has ΔH°f = 0 kJ·mol-1 and ΔG°f = 0 kJ·mol-1, but its S° = 130.7 J·mol-1·K-1.
The enthalpy and free energy values are zero; why isn’t the entropy value also zero?
2. (a) Suppose that you have two opaque jars which contain a total of 8 identical marbles. What
is the entropy of this system? Express your answer as a multiple of kB.
(b) Suppose that you have two opaque jars which contain 4 red marbles and 4 blue marbles.
What is the entropy of this system?
(c) Does having two different kinds of marbles make a difference to the entropy of the
system?
3. There are two isomers of C3H6. Both are gases under standard conditions.
(a) Draw the Lewis structure of each isomer.
(b) Which isomer would you expect to have a larger entropy under standard conditions?
(assuming the same volume of sample) Why?
(c) What would be the sign of the entropy change for the condensation of either of these
gases?
4. The reaction 4HCl(g) + O2(g) → 2Cl2(g) + 2H2O(g) comes to equilibrium at 400oC in a 2L
flask. Analysis of the equilibrium mixture shows that it contains 60g of Cl2, 12g of H2O,
20g of HCl and 8g of O2. Calculate the equilibrium constant.
5. The equilibrium constant for the reaction H2(g) + I2(s) → 2HI(g) is 0.352 at 25oC. Suppose
than an excess of solid iodine is placed in a rigid flask with 0.400 bar of hydrogen gas
and 0.300 bar of hydrogen iodide. In what direction will the reaction proceed?
6. Calculate the equilibrium constant for the reaction CO(g) + Cl2(g) → COCl2(g) at 25oC.
7. Using the solubility product of barium sulfate and the standard free energies of formation
of solid barium sulfate and of the aqueous sulfate ion, calculate the standard free energy
of formation of the aqueous Ba2+ ion and compare the value obtained to that given in the
data tables in your textbook.
8. What is the vapour pressure of a solution made by dissolving 50g of ammonium sulphate
into 300g of water at 40oC? The vapour pressure of pure water at this temperature is
7373 Pascals.
9. In lab, we often rinse wet glassware with acetone to remove the water then use a stream
of air to evaporate off the acetone. The structure of acetone and vapour pressure curves
for acetone and water are shown below. (1 atm = 1.01325 bar = 760 mmHg)
(a) What type(s) of intermolecular forces are responsible for water’s solubility in acetone?
(b) What are the normal boiling points of acetone and water? (1 bar = 750 mmHg)
(c) What are the vapour pressures of acetone and water at room temperature (20 °C)?
(d) Briefly, justify the relative boiling points and vapour pressures of water and acetone.
(e) Why does blowing a stream of air over acetone-wet glassware accelerate evaporation?
(f) Why is blowing a stream of air over water-wet glassware much less effective?
(g) If you need dry glassware for the following week’s lab, is this method of drying
necessary? Why or why not?
10. Use Le Châtelier’s principle to describe the shift in equilibrium for the changes in the
systems below (questions 1 – 3):
):
1. H2(g) + I2(g)
2HI(g) ΔH
H = +52.7kJ/mol
a) increase [H2]
b) increase [HI]
c) increase the pressure by lowering the volume
d) add a catalyst
2. 2NO2(g)
N2O4(g)
a) increase [NO2]
b) decrease [N2O4]
c) decrease the pressure by raising the volume
d) add a catalyst
3. 2CO(g) + O2(g)
2CO2(g) + 566kJ
a) increase [CO2]
b) decrease [CO]
c) increase the pressure by adding He(g)
d) raise the temperature
11.
12.
13.
14.
Set 2
Set 3