EXAMINATION Semester Test 1: PROGRAMME: BSc
/ TEST: Physical Chemistry
MODULE NCHM518/512 DURATION: 1.5 Hours
CODE:
MODULE Physical Chemistry 1 MARKS: 40 marks
DESCRIPTION:
EXAMINER: Prof. N. Chaukura DATE: 12 October, 2024
Internal Dr. S. Mthembu TIME: 08:30 HRS
moderator
IMPORTANT INSTRUCTIONS
1. Answer ALL questions.
2. Show all your calculations.
3. Use Answers sheet provided.
4. The use of non-programmable calculator is allowed.
5. The use of laptops is not allowed.
6. The use of cellphones is not allowed.
1. Chemical thermodynamics
The Haber process involves the reaction between nitrogen gas and hydrogen gas to produce
ammonia.
1.1.1 Provide a balanced chemical equation for the reaction. [2 mark]
1.1.2 Write down the expression for the equilibrium constant (K) for the reaction.
[2 marks]
1.1.3 -TΔSuniverse is defined as the Gibbs free energy, ΔG. The relation between ΔG and K is shown
in Figure 1.
Fig 1: The variation of Gibbs free energy with equilibrium constant
Discuss the significance of the 3 segments of the curve in the figure. [6 marks]
2. Chemical kinetics
In a chemical reaction involving reactant [A], the decrease in the concentration of A ([A]) over
time can be used to study the rate of the reaction (Fig 2).
[A]0 is the initial concentration of A
Fig 2: The variation of the concentration of A ([A]) over time
2.1.1 Define half-life for a first order reaction. [2 marks]
2.2.2 Use a graphical sketch to illustrate how [A] varies with t. [8 marks]
3. Electrochemistry
3.1 Consider the redox reaction (Equation 1) which occurs under acidic conditions:
2+ 2+ +
MnO -4(aq) + Fe (aq) ⎯acid
⎯⎯→ Mn (aq) + Fe 3(aq)
(Equation 1)
3.1.1 Deduce the reduction and oxidation half equations [4 marks]
3.1.2 Determine the potential for the cell constructed from MnO4- and Fe2+ . [6 marks]
3.2 Consider the dental voltaic cell in Fig 3.
Fig 3: A dental voltaic cell
Use the half equations and the data provided in the table to determine the cell potential.
[10 marks]
End of question paper
