University of KwaZulu-Natal
School of Chemistry & Physics (Pietermaritzburg)
EXAMINATIONS: June 2012
SUBJECT, COURSE, AND CODE:
Physical Chemistry,
CHEM733 P1
DURATION: 2 HOURS
TOTAL MARKS: 60
External Examiner: Professor S Paul, University of South Africa
Internal Examiner:
Professor I Nikolayenko
Students are requested, in their own interests, to write legibly
NOTE:
This paper consists of 4 pages, a data sheet, and the periodic table of
elements.
Please see that you have them all.
The use of non-programmable electronic calculators is permitted.
Graph paper will be provided.
Answer ALL questions.
Page 1 of 4
School of Chemistry & Physics (Pietermaritzburg)
Subject, Course, and Code:
EXAMINATIONS: June 2012
Physical Chemistry, CHEM733 P1
Question One
(a)
Define the compression factor, Z, and sketch its variation with pressure for an ideal
and a real gas.
(2)
(b)
A certain mass of N2 gas occupies a volume of 1.00 L at −50 °C and 800 bar.
(i) Determine the volume occupied by the same mass of gas at 100 °C and 200 bar
using the following experimental compression factors for N2:
At −50 °C and 800 bar: Z = 1.95
At 100 °C and 200 bar: Z = 1.10.
(3)
(ii) Compare this value to the one obtained from the ideal gas law and comment on
the difference between them.
(3)
(c)
Define the relative and the excess molar thermodynamic quantity.
(2)
(d)
A solution of bromine in carbon tetrachloride behaves as an ideal-dilute solution. At
298 K the vapour pressure of pure CCl4 is 33.85 Torr. Henry’s law constant, when
the concentration of Br2 is expressed as a mole fraction, is 122.36 Torr. Calculate the
vapour pressure of each component, the total vapour pressure, and the composition
of the vapour phase, when the mole fraction of Br2 is 0.050. Assume that the
conditions of an ideal-dilute solution are satisfied at this concentration. Give brief
justification of the equations you might be using.
(5)
(e)
The excess molar Gibbs energy of methylcyclohexane (MCH) - tetrahydrofuran
(THF) solutions at 305.15 K was found to fit the expression
GmE  RTx(1 − x)[0.4857 − 0.1077(2x − 1) + 0.0191(2x − 1)2]
where x is the mole fraction of the MCH. Calculate the Gibbs energy of mixing when
1.00 mol of MCH is added to 3.00 mol of THF at this temperature and allowed to
equilibrate.
(5)
[20]
Page 2 of 4
School of Chemistry & Physics (Pietermaritzburg)
Subject, Course, and Code:
EXAMINATIONS: June 2012
Physical Chemistry, CHEM733 P1
Question Two
(a)
Write a brief (not more than a page, a page and a half long) on the topic "The kinetic
salt effect". Make sure to address the matter of thermodynamic stabilization/
destabilization of a reaction intermediate in aqueous solution.
(6)
(b)
When a rod of metallic lead was dipped into a 0.0100 M aqueous solution of
[Co(en)3 ]3+ , 68 percent of the cobalt complex was reduced by the lead to [Co(en)3 ]2+ .
Note that en is an abbreviation for 1,2-ethylenediamine, a common chelating ligand.
(i) Estimate the value of the thermodynamic equilibrium constant for the following
reaction
[Co(en)3 ]3+ + 21 Pb2+
[Co(en)3 ]2+ + 21 Pb
Account for the mean ionic activity coefficients using the Debye-Hückel limiting
law. For water at 25.0 C, A = 0.509 .
(10)
(ii) Given E  (Pb2+ Pb) = 0.126 V , what is the value of the standard electrode
potential for the [Co(en)3 ]3+ [Co(en)3 ]2+ redox couple?
(4)
Clearly indicate any assumptions made.
[20]
Page 3 of 4
School of Chemistry & Physics (Pietermaritzburg)
Subject, Course, and Code:
EXAMINATIONS: June 2012
Physical Chemistry, CHEM733 P1
Question Three
(a)
(i) List essential features of a galvanic cell.
(1)
(ii) Outline key ideas employed in the theoretical derivation of the Nernst equation
for the cell emf.
(3)
Note: You are not expected to carry out actual derivation for any particular cell in
this question!
(b)
Give a brief classification of the electrode (half-cell) types. Provide examples, where
relevant.
(6)
(c)
(i) Discuss the design principles and chemistry behind the potentiometric NH3(g)
pressure sensor.
(6)
(ii) Chance an estimate of the lowest concentration of aqueous ammonia your
sensor should be able to discriminate. State clearly, what in your opinion
constrains the detection limit for ammonia, and show any numerical work
explicitly.
At 25.0 C for NH+4 , pKa = 9.25 .
(4)
[20]
Page 4 of 4