UNIVERSITY EXAMINATIONS
JAN/FEB 2025
CHE3702
Physical Chemistry III
100 Marks
Duration: 3 Hours
This paper is a Closed Book Examination
Welcome to the CHE3702 exam.
The examination will be a CLOSED book examination.
Examiner name: Dr ME Aphane
Internal moderator name: Dr MG Smith
External moderator name: Prof EM van der Merwe
This paper consists of 12 pages.
Total marks: 100
Instructions:
Information on formulae and constants are attached on page 6 to 12.
Use of a non-programmable SCIENTIFIC calculator is permissible.
Show all calculations.
Show all units since marks will be deducted for wrong or missing units.
THE EXAM PAPER CONTAINS SIX (6) QUESTIONS
Attempt ALL questions.
Connectivity to the Internet and IRIS invigilation is compulsory.
Please note that by starting (and submitting) this exam you fully agree to, and will
adhere to, the Honesty Declaration of the University of South Africa.
Please tick the Honour Pledge box before submitting the examination paper.
Plagiarism is a serious offence and will not be tolerated in this exam session!
Information on how to upload my examination answer file and preparing your answer file is
attached after page 12 of the exam question paper.
, Page 2 of 23
CHE3702
JAN/FEB 2025
Question 1 [25]
(a) The position of chemical equilibrium may be affected by the addition or removal of
reactants or products in a chemical reaction. Describe briefly how will this affect the
position of chemical equilibrium reaction. (5)
(b) Calculate the equilibrium constant of the reaction:
-1
Given that, for the production of liquid formaldehyde, ∆Gθ = 28.95 kJ mol at 298 K
and 1.0 atm and that the vapour pressure of formaldehyde is 1500 Torr at that
temperature.
The production of liquid formaldehyde is given by:
CO (g) + H2 (g) → H2CO (ℓ) ......... ΔrGθ (ℓ)
and for the formation of gaseous formaldehyde:
H2CO(ℓ) → H2CO(g) ................. ΔrGθ (vap)
(6)
(c) Consider the cell: Pt |H2(g, pƟ) | HCl(aq) | AgCl(s) | Ag,
at 25°C and a molality of HCl of 0.010 mol kg−1, E = +0.4658 V.
HINT: the half reactions are:
AgCl (s) + e- → Ag (s) + Cl- (aq)
2H+ (aq) + 2e- → H2 (g)
And assume that hydrogen gas is at standard pressure of 1.0 bar.
(i) Write the overall redox reaction for the above cell reaction. (2)
(ii) Write the Nernst equation for the cell where the activities are expressed in terms of
the molality (b) and the mean activity coefficient (γ±) (5)
(iii) Calculate ΔrG for the above cell reaction. (2)
(iv) Assuming that the Debye–Hückel limiting law holds at this concentration, calculate
EƟ (AgCl, Ag). (5)
[TURN OVER]
, Page 3 of 23
CHE3702
JAN/FEB 2025
Question 2 [15]
(a) Discuss how strong and weak electrolytes can be distinguished with respect to the
molecular motion of ions in aqueous solutions. Include relevant theories and equations.
(8)
(b) The molar conductivity (Λm) of the acetic acid is given as 5.27 x 10-4 S m2 mol-1 at a
concentration of 100 mol m-3. If the cell has a resistance of 33.01 S-1 when filled with 100
mol m-3 KCl(aq) and 300.0 S-1 when filled with 100 mol m-3 CH3COOH, calculate the
measured conductivity (κ) of 100 mol m-3 KCl(aq). The conductivity (κ) of water is 0.076
S m-1 at 25 º C. (7)
Question 3 [20]
(a) Briefly describe how rate laws are determined based on the method of initial rates
in conjunction with the isolation method. (7)
(b) Describe what is meant by the term “integrated rate law”. Explain how it can be used to
obtain the order of the reaction. (4)
(c) The following data have been obtained for the hydrolysis of sucrose, C12H22O11, to
glucose, C6H12O6, and fructose C6H12O6, in acidic solution:
C12H22O11 + H2O → C6H12O6 + C6H12O6
t / min [sucrose] / mol dm-3
0 0.316
14 0.300
39 0.274
60 0.256
80 0.238
110 0.211
(i) Graphically prove the order of the reaction and determine the rate constant of the
reaction. (7)
(ii) Determine the half-life, t½ for the hydrolysis of sucrose. (2)
[TURN OVER]
, Page 4 of 23
CHE3702
JAN/FEB 2025
Question 4 [15]
(a) Distinguish between reaction order and molecularity of a reaction. (4)
(b) Define / describe the following, and provide relevant equations and examples were
necessary:
(i) The rate-determining step (2)
(ii) The steady-state approximation (2)
(c) Consider the following elementary bimolecular gas-phase reactions below at 850K:
H2 (g) + Br2 (g) → 2 HBr(g)
(i) Calculate the reduced mass, µ. HINT: take mH2 = 2.016mu and mBr2 = 159.8mu (2)
(ii) Calculate the value of the second order rate constant for the reaction. The collision
cross-section is 0.36 nm2, and the activation energy is 171 kJ mol-1. (5)
Question 5 [15]
(a) Explain the difference in the following terms below: (3)
(i) adsorption
(ii) adsorbate
(iii) adsorbent
(b) Adsorption of the hexacyanoferrate (III) ion, [Fe(CN)6]3-, on γ-Al2O3 from aqueous
solution was examined. The adsorption was modelled using a modified Langmuir
isotherm, yielding the following values of K at pH = 6.5:
T/K 10-10 K
280 2.505
295 1.819
310 1.364
325 1.050
(i) Determine the enthalpy of adsorption, ΔadsHθ. (10)
(ii) If the reported value of entropy of adsorption, ΔadsSθ = 146 J K-1 mol-1 under the above
conditions, determine ΔadsGθ. (2)
[TURN OVER]
JAN/FEB 2025
CHE3702
Physical Chemistry III
100 Marks
Duration: 3 Hours
This paper is a Closed Book Examination
Welcome to the CHE3702 exam.
The examination will be a CLOSED book examination.
Examiner name: Dr ME Aphane
Internal moderator name: Dr MG Smith
External moderator name: Prof EM van der Merwe
This paper consists of 12 pages.
Total marks: 100
Instructions:
Information on formulae and constants are attached on page 6 to 12.
Use of a non-programmable SCIENTIFIC calculator is permissible.
Show all calculations.
Show all units since marks will be deducted for wrong or missing units.
THE EXAM PAPER CONTAINS SIX (6) QUESTIONS
Attempt ALL questions.
Connectivity to the Internet and IRIS invigilation is compulsory.
Please note that by starting (and submitting) this exam you fully agree to, and will
adhere to, the Honesty Declaration of the University of South Africa.
Please tick the Honour Pledge box before submitting the examination paper.
Plagiarism is a serious offence and will not be tolerated in this exam session!
Information on how to upload my examination answer file and preparing your answer file is
attached after page 12 of the exam question paper.
, Page 2 of 23
CHE3702
JAN/FEB 2025
Question 1 [25]
(a) The position of chemical equilibrium may be affected by the addition or removal of
reactants or products in a chemical reaction. Describe briefly how will this affect the
position of chemical equilibrium reaction. (5)
(b) Calculate the equilibrium constant of the reaction:
-1
Given that, for the production of liquid formaldehyde, ∆Gθ = 28.95 kJ mol at 298 K
and 1.0 atm and that the vapour pressure of formaldehyde is 1500 Torr at that
temperature.
The production of liquid formaldehyde is given by:
CO (g) + H2 (g) → H2CO (ℓ) ......... ΔrGθ (ℓ)
and for the formation of gaseous formaldehyde:
H2CO(ℓ) → H2CO(g) ................. ΔrGθ (vap)
(6)
(c) Consider the cell: Pt |H2(g, pƟ) | HCl(aq) | AgCl(s) | Ag,
at 25°C and a molality of HCl of 0.010 mol kg−1, E = +0.4658 V.
HINT: the half reactions are:
AgCl (s) + e- → Ag (s) + Cl- (aq)
2H+ (aq) + 2e- → H2 (g)
And assume that hydrogen gas is at standard pressure of 1.0 bar.
(i) Write the overall redox reaction for the above cell reaction. (2)
(ii) Write the Nernst equation for the cell where the activities are expressed in terms of
the molality (b) and the mean activity coefficient (γ±) (5)
(iii) Calculate ΔrG for the above cell reaction. (2)
(iv) Assuming that the Debye–Hückel limiting law holds at this concentration, calculate
EƟ (AgCl, Ag). (5)
[TURN OVER]
, Page 3 of 23
CHE3702
JAN/FEB 2025
Question 2 [15]
(a) Discuss how strong and weak electrolytes can be distinguished with respect to the
molecular motion of ions in aqueous solutions. Include relevant theories and equations.
(8)
(b) The molar conductivity (Λm) of the acetic acid is given as 5.27 x 10-4 S m2 mol-1 at a
concentration of 100 mol m-3. If the cell has a resistance of 33.01 S-1 when filled with 100
mol m-3 KCl(aq) and 300.0 S-1 when filled with 100 mol m-3 CH3COOH, calculate the
measured conductivity (κ) of 100 mol m-3 KCl(aq). The conductivity (κ) of water is 0.076
S m-1 at 25 º C. (7)
Question 3 [20]
(a) Briefly describe how rate laws are determined based on the method of initial rates
in conjunction with the isolation method. (7)
(b) Describe what is meant by the term “integrated rate law”. Explain how it can be used to
obtain the order of the reaction. (4)
(c) The following data have been obtained for the hydrolysis of sucrose, C12H22O11, to
glucose, C6H12O6, and fructose C6H12O6, in acidic solution:
C12H22O11 + H2O → C6H12O6 + C6H12O6
t / min [sucrose] / mol dm-3
0 0.316
14 0.300
39 0.274
60 0.256
80 0.238
110 0.211
(i) Graphically prove the order of the reaction and determine the rate constant of the
reaction. (7)
(ii) Determine the half-life, t½ for the hydrolysis of sucrose. (2)
[TURN OVER]
, Page 4 of 23
CHE3702
JAN/FEB 2025
Question 4 [15]
(a) Distinguish between reaction order and molecularity of a reaction. (4)
(b) Define / describe the following, and provide relevant equations and examples were
necessary:
(i) The rate-determining step (2)
(ii) The steady-state approximation (2)
(c) Consider the following elementary bimolecular gas-phase reactions below at 850K:
H2 (g) + Br2 (g) → 2 HBr(g)
(i) Calculate the reduced mass, µ. HINT: take mH2 = 2.016mu and mBr2 = 159.8mu (2)
(ii) Calculate the value of the second order rate constant for the reaction. The collision
cross-section is 0.36 nm2, and the activation energy is 171 kJ mol-1. (5)
Question 5 [15]
(a) Explain the difference in the following terms below: (3)
(i) adsorption
(ii) adsorbate
(iii) adsorbent
(b) Adsorption of the hexacyanoferrate (III) ion, [Fe(CN)6]3-, on γ-Al2O3 from aqueous
solution was examined. The adsorption was modelled using a modified Langmuir
isotherm, yielding the following values of K at pH = 6.5:
T/K 10-10 K
280 2.505
295 1.819
310 1.364
325 1.050
(i) Determine the enthalpy of adsorption, ΔadsHθ. (10)
(ii) If the reported value of entropy of adsorption, ΔadsSθ = 146 J K-1 mol-1 under the above
conditions, determine ΔadsGθ. (2)
[TURN OVER]
