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PCH602S-PHYSICAL CHEMISTRY-JAN 2020 |
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NAMIBIA UNIVERSITY
OF SCIENCE AND TECHNOLOGY
FACULTY OF HEALTH AND APPLIED SCIENCES
DEPARTMENT OF NATURAL AND APPLIED SCIENCES
QUALIFICATION: VARIOUS
QUALIFICATION CODE: VARIOUS
COURSE NAME: : PHYSICAL CHEMISTRY
SESSION: JANUARY 2020
DURATION: 3 HOURS
LEVEL: 6
COURSE CODE: PCH602S
PAPER: THEORY
MARKS: 100
SUPPLEMENTARY/SECOND OPPORTUNITY EXAMINATION QUESTION PAPER
EXAMINER(S) | Prof Habauka M Kwaambwa
MODERATOR: | Prof Rajaram Swaminathan
INSTRUCTIONS
1. Answer ALL the questions.
2. Write clearly and neatly.
3. Number the answers clearly.
PERMISSIBLE MATERIALS
Non-programmable Calculators
ATTACHMENT
List of Useful Constants
THIS QUESTION PAPER CONSISTS OF 7 PAGES (Including this front page and attachment)
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SECTION A: MULTIPLE CHOICE QUESTIONS
[20]
There are 10 questions in this section. Choose the correct answer. Each question carries 2
marks.
1. Which of the following statements is not true about the First Law of Thermodynamics?
A. Although energy may be converted from one form to another, it cannot be created
or destroyed.
When a chemical system changes from one state to another, the net transfer of
energy to its surroundings must be balanced by a corresponding change in the
internal energy of the system.
Energy U is a state function, meaning that its value is completely determined by
the thermodynamic state of the system.
Energy is not conserved in an isolated system, i.e. energy of an isolated system is
not constant.
There are 2 distinct ways to transport energy into or out of a closed system that is
not isolated, through heat, g, and work, w
2. According to the First Law of Thermodynamics, work done in an adiabatic process
between a given pair of end states depends on
The end states only, i.e. initial and final state
Particular adiabatic process
The value ofy = Cp/Cy
The value of the heat transferred
Mass of the system
3. Which of the following is a spontaneous process?
Freezing water at 40°C
Melting of ice at -273°C
Freezing of water at -24°C
Melting of ice at -24°C
Melting of ice at 100°C
4. Which of the following processes is most likely to lead to an increase in the entropy of
the system?
No2(g) + 3H2(g) = 2NH3(g)
H20() = H20(s)
SiH4(g) + 202(g) —> SiO2(s) + 2H20(g)
NH4NO3(s) + H20()) —» NHa* (aq) + NOs (aq)
NO(g) + O3(g) + NO2(g) + O2(g)
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. The equilibrium constant for a reaction is 0.48 at 25°C. What is the value of AG° at this
temperature?
A. 1.8kJ
B. -4.2 kJ
C. 1.5x 102 kJ
D. 4.2 kJ
E. More information is needed
Consider the reaction A(g) + B(g) — 2C(g). For this reaction, AH° = -116 kJ mol and
the equilibrium constant Kp is 140 at 600 K. Calculate for this reaction (i) AG° and (ii)
AS° at 600 K.
A. (i) 24.7 kImol*
and
(ii) -60.4 kJ K+mol?
B. (i) -60 kJmol
and
(ii) -24.7 kJ Ktmol
C. (i) -24.7 kImolt
and
(ii) -154J K*mol
D. (i) -24.7 kJImol?
and
(ii) -60.4 kJ K*mol*
E. None of the above
If A$ (K*) = 73.5 Scm’mol?, 23 (Al’*) = 189 Scm?molt and 23 (SO;") = 160
Scm2mol?, then
A. A, (KAI, (SO,),) = 165 Scm*molt
A, (K,SO,) = 307 Scm?molt
A, (K,SO,) = 393.5 Scm?mol
A, (Al, (SO,) ) = 887 Scm*molt
3
None of the above
. The conductivity of 0.10 M KCI solution at 20°C is 0.0212 Scm™ and the resistance of
the cell containing this solution at 20°C is 55 Q. The cell constant is:
A. 3.173 cm
1.166 cm?
4.616 cm
3.324 cm?
317.3 cm?
. When the concentration of the reactants are measured in moldm® and time in
seconds, what are the appropriate units for (i) rate and (ii) rate constant for a second
order reaction?
A. (i) moldm-3s*
(ii) dm3molts
B. (i) moldms+
(ii) dm2molts
C. (i) moldm?
(ii) dm?mol*s+
D. (i) moldm-3s*
(ii) dm®mol*s
E. (i) moldm?s+
(ii) dm?molts*
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10. When the concentration of A in a simple reaction A + B was changed from 0.51
moldm- to 1.03 moldm?, the half-life dropped from 150 seconds to 75 seconds at
25°C. What is the (i) order of the reaction and (ii) the value of the rate constant?
A. (i)/n=0 and
(ii) k = 1.70 x 103 moldm?s?
B. (i)/n=1 and
(ii) k = 1.31 x 107 s+
C. (i)/n=2 and
(ii) k = 1.31 x 107 dm?mol*s+
D. (i)n=1 and
(ii) k = 4.62 x 103 s+
E. None of the above
SECTION B
[80]
There are FOUR questions in this section. Answer all Questions.
QUESTION 1
[16]
(a) Briefly state each of the following laws:
(4)
(i) Boyle’s law
(ii) Hess’s law
(iii) Zeroth law of thermodynamics
(iv) Kirchhoff’s law or equation
(b) State whether g, w, AU and AS are negative, zero or positive for heating of an ideal
gas at constant volume.
(4)
(c) The statements below are all false. For each statement either correct it or state briefly
the reason for its being false.
(6)
(i) Charles’ law states that the volume of a given amount of gas is directly
proportional to the absolute temperature on the Kelvin scale under any
conditions.
(ii) The total heat absorbed, q, for a cyclic process is equal to zero.
(iii) The Second Law of Thermodynamics states that the entropy of the system.
always increases during a spontaneous process.
(iv) Every closed system is isolated and every isolated system is closed.
(v) The work done for reversible process is greater than the work done for an
yo, -(© irreversible process.
(vi) C, -(3 )
(d) For each of the following reactions, show or explain whether the heat evolved at
constant pressure (AH) is smaller, larger than or the same as the heat evolved at
constant volume (AU).
(2)
(i) C,H, (g)+3H,(g)—> 2CH,(g)
(ii)
BaCl,(s)+ F,(g)—> BaF,(s)+ Cl, (g)
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QUESTION 2
[24]
(a) If one mole of an ideal gas, Cy = SR , is expanded adiabatically until the temperature
drops from 20°C to 10°C, calculate gq, w, AU and AH.
(8)
(b) In the figure below, if 17 moles of an ideal gas (, Cv= SR ) go from point 1, T: = 290 K,
to point 2, T2 = 358 K, what is its change in internal energy (in kJ).
(3)
(c) If P1 = 179 kPa, Vi = 7.37 dm? and P2 = 200 kPa, V2 = 10 dm?, what is the heat absorbed
or liberated (in kJ) for the path taken as shown by the arrows? Assume that the change
in internal energy is the same as your answer in Problem (b).
(4)
Ps
P2 |-------
T2
Py |----__>
Ti
Vi
(d) The linear form of the Clausius—Clapeyron equation may be written as:
InP =- AH, i, GC
RT
(i)
State the main assumptions in its derivation.
(3)
(ii)
The vapour pressure of chloromethane is represented by the equation
In(P/mmHg) = = +17.23
where T is the temperature in kelvins. Calculate the molar enthalpy of
vaporization. Does the molar entropy of vaporization for chloromethane
conform to Trouton’s rule? Show clearly your answer. (Normal boiling point of
chloroomethane is —23 °C)
(6)
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QUESTION 3
[20]
(a) Give the equations and SI units for the terms conductivity, k, and molar conductivity,
A, as applied to a solution of an electrolyte.
(4)
(b) Show using a schematic diagram how the molar conductivity can used to distinguish
between HCI (strong electrolyte), acetic acid (weak electrolyte) and sodium dodecyl
sulphate (surfactant).
(4)
(c) The limiting molar conductivities (Ao) of Nal, CH3COONa and Mg(COOCHs)z, in
mSm?2mol?, are 12.69, 9.10 and 18.78, respectively (all at 25°C). What is the limiting
molar conductivity of Mgl2 at this temperature?
(3)
(d) The molar conductivity at infinite dilution, Ao, of acetic acid is 390.7 Scm?mol. What
is the acid dissociation constant, Ka, if the molar conductivity, A, of 0.0100 moldm?
acetic acid is 16.5 Scm*mol?
(3)
(e) Suppose that you construct a Galvanic cell which combines the Ce**(aq), Ce*(aq)/Pt
cell have a standard reduction potential (E°) of 1.61 V with a Cu2*(aq)/Cu(s) cell having
a standard reduction potential of 0.158 V at 298 K.
(i)
Write balanced chemical equations for the reactions at the anode and
cathode, indicating which reaction occurs at a particular electrode, and the
resulting spontaneous overall reaction.
(3)
(ii)
Devise the cell notation and calculate standard emf (E°) of the cell.
(3)
QUESTION 4
[20]
(a) The reaction H2 + lz — 2Hl is first order with respect to [Hz] and [lz]. When [Ha] = 1
molL* and [l2] = 2 molL*, the following kinetic are observed.
ad|teHI = 1.78 x 10% moll ts? at 556 K andaa d| HI = 0.2572 molL‘s? at 700 K.
Calculate the rate constant at each of the temperatures and evaluate the activation
energy and the Arrhenius pre-exponential factor. [Assume that the pre-exponential
factor is constant]
(8)
(b) The reverse reaction, i.e. 2H — H2 + lz, has an activation energy of 183 kJmol"?. Does
this make the reaction H2 + lz — 2HI exothermic or endothermic? Explain your answer
with a diagram of the energy profile of the reaction.
(6)
(c) Hydrogen peroxide, H202, decomposes in water by a first order kinetics process. A
0.156 moldm® solution of H2O2 in water has an initial rate of 1.14 x 10° moldm?s7.
Calculate the rate constant, k, for the decomposition reaction and the half-life of the
decomposition.
(4)
END OF EXAM QUESTIONS
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LIST OF USEFUL EQUATION AND CONSTANTS
na RT
a
Van der Waals eq®. P= ni
~ 72
=
_2
Universal Gas constant
R
=
Boltzmann’s constant,
k
=
Planck’s constant
h
=
Debye-Hiickel’s constant, A
=
Faraday’s constant
Mass of electron
F
=
Me =
Velocity of light
c
=
Avogadro’s constant
Na
=
1 electron volt (eV)
=
8.314 J K+ molt
1.381 x 103 J K?
6.626 x 1034J s
0.509 (mol dm)? or mol®5kg®5
96485 C mol?
9.109 x 10°31 kg
2.998 x 108 ms?
6.022 x 1073
1.602 x 10°79J