UNIVERSITY EXAMINATIONS
January/February 2023
CHE2612
Physical Chemistry II (Theory)
100 Marks
Duration 2.00 hours
First Examiner: Dr BS Dladla
Moderator:
Dr MG Smith
Instructions:
This is a closed book examination.
Use a ball point pen, preferably black.
This question paper consists of 9 pages.
A Resource section appears on page 7.
Name all symbols used.
Use ONLY SI units unless instructed otherwise.
Answer ALL questions. Each question counts 25 Marks.
The use of non-programmable calculator is permissible.
Guidelines for uploading the answer file on myExams can be found on page 9.
Additional Instructions:
1. Students must upload their answer scripts in a single PDF file (answer scripts must not be password
protected or uploaded as “read only” files)
2. Incorrect file format and uncollated answer scripts will not be considered.
3. NO emailed scripts will be accepted.
4. Students are advised to preview submissions (answer scripts) to ensure legibility and that the correct
answer script file has been uploaded.
5. Incorrect answer scripts and/or submissions made on unofficial examinations platforms (including the
invigilator cell phone application) will not be marked and no opportunity will be granted for resubmission.
6. Mark awarded for incomplete submission will be the student’s final mark. No opportunity for resubmission
will be granted.
1
7. Mark awarded for illegible scanned submission will be the student’s final mark. No opportunity for
resubmission will be granted.
8. Submissions will only be accepted from registered student accounts.
9. Students who have not utilized invigilation or proctoring tools will be subjected to disciplinary processes
(only include if applicable).
10. Students suspected of dishonest conduct during the examinations will be subjected to disciplinary
processes. UNISA has a zero tolerance for plagiarism and/or any other forms of academic dishonesty.
11. Students are provided one hour to submit their answer scripts after the official examination time.
Submissions made after the official examination time will be rejected by the examination regulations and will
not be marked.
12. Students experiencing network or load shedding challenges are advised to apply together with supporting
evidence for an Aegrotat within 3 days of the examination session.
13. Students experiencing technical challenges, contact the SCSC 080 000 1870 or email
Examenquiries@unisa.ac.za or refer to URL link for the list of additional contact numbers. ONLY
communication from your myLife account will be considered.
Connectivity to the internet and the IRIS invigilation system is compulsory.
Upon submission remember to tick the Honesty Declaration.
2
Question 1 [25]
1.1
Consider the following pressure-temperature phase diagram of a pure substance and identify
by name:
i.
1.2
The intersection at 1.
[1]
ii. The regions 2, 3, and 4.
[3]
iii. The endpoint at 5.
[1]
iv. The lines 6, 7, and 8.
[3]
Show by calculating Gibbs energy of reaction whether evaporation or condensation occurs
spontaneously at 298 K where standard molar Gibbs energy of formation of water vapour is 229 kJ/mol and that of liquid water is -237 kJ/mol.
1.3
1.4
[4]
Consider a gas with a molar volume 1.5 times that of a perfect gas.
a.
Calculate the compression factor.
[3]
b.
Interpret the value obtained in a.
[2]
Explain the following terms:
i.
Incongruent melting.
[2]
ii.
Enthalpy of formation
[2]
iii. Osmosis
[2]
iv. Partial molar volume
[2]
3
Question 2 [25]
2.1
Consider two initially separate metal blocks of aluminium (Mr = 26.98 g/mol) and lead
(Mr = 207.19 g/mol) in a chamber with adiabatic walls. The lead block of mass 3 g is initially
at 1000C and that of aluminium of mass 4 g is initially at 250C. The blocks are then placed in
contact and allowed to thermally equilibrate. The molar heat capacities of the metals are
considered constant over the temperature of experiment. At 1 bar, Cp,m(Al) = 24.35 JK-1mol-1
and Cp,m(Pb) = 26.44 JK-1mol-1
Calculate
2.2
a.
the final temperature.
[4]
b.
the change in the entropy of the blocks in contact.
[4]
c.
identify evidence that the process occurred spontaneously or not.
[2]
Consider the following reactions at 298 K.
2 O3(g) → 3 O2(g)
O2(g) → 2 O(g)
NO(g) + O3(g) → NO2(g) + O2(g)
∆𝑟𝑟 𝐻𝐻 ⦵ = −427.0 𝑘𝑘𝑘𝑘/𝑚𝑚𝑚𝑚𝑚𝑚
∆𝑎𝑎𝑎𝑎 𝐻𝐻 ⦵ = 495.0 𝑘𝑘𝑘𝑘/𝑚𝑚𝑚𝑚𝑚𝑚
∆𝑟𝑟 𝐻𝐻 ⦵ = −199.0 𝑘𝑘𝑘𝑘/𝑚𝑚𝑚𝑚𝑚𝑚
Apply Hess’s law to calculate the standard reaction enthalpy for the reaction
2.3
NO(g) + O(g) → NO2(g)
[14]
Choose one of the words in bold to give the sentence the correct scientific meaning.
At constant temperature and pressure, spontaneous change in a process tends to occur in the
direction of increasing/decreasing Gibbs energy.
[1]
4
Question 3 [25]
3.1
Consider a state for which the molar volume is 4.00 × 10-4 m3/mol at 288 K. If the van der
Waals parameters a and b are 0.76 Pa m6 mol-2 and 1.26 × 10-4 m3/mol respectively, calculate
the gas pressure in Pa, using i) the perfect gas law and ii) van der Waals equations of state.
3.2
Express your answers in kPa and in 5 significant figures
[4]
Explain the difference in the values obtained in 3.1.
[2]
3.3
The graph in the figure above depicts a temperature-composition solid-liquid phase diagram.
1.
Name the phases in the regions 1-8
[8]
2.
At what temperatures do the pure components solidify.
[2]
3.
Name the line curve marked 9.
[1]
4.
Write the probable formula for the incongruent-melting compound.
[2]
5
5.
Write the probable formula for the congruent-melting compound.
[2]
6.
Identify the lines marked 4, 5, and 10 as either peritectic or eutectic.
[3]
7.
Name the phase above line curve marked 9.
[1]
Question 4 [25]
4.1
Calculate total vapour pressure above an ideal liquid mixture for which the vapour
composition of one component is 5.32. The vapour pressures of the pure components at 298 K
are 75 Torr and 21 Torr.
4.2
[7]
Consider 1.5 mol of methane gas in a 32.414 dm3 vessel at 298 K. The van der Waals constants
for methane gas are a /atm L2 mol-2 = 2.273, and b /10-2 L mol-1 = 4.31. The universal gas
constant is R /J mol-1 K-1 = 8.3145.
i. Express each van der Waals constant in SI units.
[2]
ii. Calculate perfect gas pressure of sample in kPa. Express your result in SI units.
[2]
iii. Calculate as van der Waals gas the pressure. Express your result in SI units.
[4]
Express all your answers in 5 significant figures
4.3
A 125 mg sample of butane, (Mr/g mol-1 = 58.13) was burnt in a bomb calorimeter and an
increase in temperature of 3.25 K was observed. ∆cH⦵(C4H10, g) = –2878 kJ/mol,
∆cH⦵(C5H12, g) = –3537 kJ/mol at 298,15 K.
Calculate
i. The calorimeter constant.
[6]
ii. The change in temperature upon combustion in the same bomb calorimeter of 125 mg
pentane (Mr/g mol-1 = 72.15)
[4]
Express your answers in 5 significant figures.
END
6
RESOURCES SECTION
Van der Waals equation
𝑛𝑛𝑛𝑛𝑛𝑛
𝑉𝑉−𝑛𝑛𝑛𝑛
−
𝑛𝑛2 𝑎𝑎
𝑉𝑉 2
First law of Thermodynamics
𝑝𝑝 =
Internal energy change
ΔU = n CV,m ΔT
Standard reaction enthalpy
∆𝑟𝑟 𝐻𝐻 ⊖ = ∑ 𝜈𝜈∆𝑓𝑓 𝐻𝐻 ⊖ (𝑃𝑃𝑃𝑃) − ∑ 𝜈𝜈∆𝑓𝑓 𝐻𝐻 ⊖ (𝑅𝑅𝑅𝑅)
∆U = q + w
Dalton’s law
∆𝑟𝑟 𝐺𝐺 ⊖ = ∑ 𝜈𝜈∆𝑓𝑓 𝐺𝐺 ⊖ (𝑃𝑃𝑃𝑃) − ∑ 𝜈𝜈∆𝑓𝑓 𝐺𝐺 ⊖ (𝑅𝑅𝑅𝑅)
Partial pressure
pi = xi ptot
Standard reaction Gibbs energy of reaction
Compression factor
Reversible work
Irreversible work
Entropy of mixing
Partial molar volume definition
Ideal gas law
𝑝𝑝 = ∑ 𝑝𝑝𝑖𝑖
𝑍𝑍 =
𝑉𝑉𝑚𝑚
𝑜𝑜
𝑉𝑉𝑚𝑚
𝑤𝑤 = −𝑝𝑝𝑒𝑒𝑒𝑒𝑒𝑒 (𝑉𝑉2 − 𝑉𝑉1 )
∆𝑚𝑚𝑚𝑚𝑚𝑚 𝑆𝑆 = −
∆𝑚𝑚𝑚𝑚𝑚𝑚 𝐺𝐺
𝑇𝑇
𝜕𝜕𝜕𝜕
�
𝜕𝜕𝑛𝑛𝑋𝑋 𝑝𝑝,𝑇𝑇,𝑛𝑛
𝑉𝑉𝑋𝑋 = �
pV = nRT
Maximum work at constant temperature
∆A = ∆U - T∆S
Reversible expansion entropy
∆𝑆𝑆 = 𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛 𝑉𝑉𝑓𝑓
Entropy variation with temperature
at constant pressure
Temperature dependence of enthalpy
Universal gas constant, R
Critical compression factor
Reduced variables
Osmometry
Total volume
Composition of vapour in equilibrium
with an ideal liquid mixture
𝑉𝑉2
𝑉𝑉1
𝑤𝑤 = −𝑛𝑛𝑛𝑛𝑛𝑛 ln
𝑉𝑉
𝑌𝑌
𝑖𝑖
𝑇𝑇 𝐶𝐶𝑝𝑝
𝑆𝑆�𝑇𝑇𝑓𝑓 � = 𝑆𝑆(𝑇𝑇𝑖𝑖 ) + ∫𝑇𝑇 𝑓𝑓
𝑖𝑖
𝑇𝑇
𝑑𝑑𝑑𝑑
𝑇𝑇
∆𝐻𝐻 = ∫𝑇𝑇 𝑓𝑓 𝑛𝑛 𝐶𝐶𝑝𝑝,𝑚𝑚 𝑑𝑑𝑑𝑑
𝑖𝑖
8.3145 J/K mol
𝑍𝑍𝑐𝑐 =
𝑉𝑉𝑟𝑟 =
Π
𝑐𝑐𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚
𝑝𝑝𝑐𝑐 𝑉𝑉𝑐𝑐
𝑅𝑅𝑇𝑇𝑐𝑐
𝑉𝑉𝑚𝑚
𝑉𝑉𝑐𝑐
=
=
𝑅𝑅𝑅𝑅
𝑀𝑀
3
8
+
𝑝𝑝𝑟𝑟 =
𝑝𝑝
𝑝𝑝𝑐𝑐
𝐵𝐵𝐵𝐵𝐵𝐵
𝑐𝑐
𝑀𝑀 2 𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚
𝑇𝑇𝑟𝑟 =
𝑇𝑇
𝑇𝑇𝑐𝑐
𝑉𝑉 = 𝑛𝑛𝑀𝑀 𝑉𝑉�𝑀𝑀 + 𝑛𝑛𝑁𝑁 𝑉𝑉�𝑁𝑁
𝑥𝑥 𝑝𝑝∗
𝐴𝐴
𝑦𝑦𝐴𝐴 = 𝑝𝑝∗ +�𝑝𝑝𝐴𝐴∗ −𝑝𝑝
∗
𝐵𝐵
𝐴𝐴
𝐵𝐵 �𝑥𝑥𝐴𝐴
7
Conversion factors
1 atm
101.325 kPa
1 litre
1 dm3
1 bar
105 Pa
Thermodynamic data
Gravitational acceleration for free fall, g
9.80665 m/s2
Liquid water density at 293 K
0.997 g/cm3
At 298 K
H2(g)
∆𝑓𝑓 𝐻𝐻 ⊖ /kJ mol-1
0
∆𝑓𝑓 𝐺𝐺 ⊖ /kJ mol-1
O2(g)
0
0
H2O(l)
-285.83
-237.13
0
Temperature variation of molar heat capacities,
𝐶𝐶𝑝𝑝,𝑚𝑚
(𝐽𝐽𝐾𝐾−1 𝑚𝑚𝑚𝑚𝑚𝑚 −1 )
a
N2(g)
28.58
𝑐𝑐
= 𝑎𝑎 + 𝑏𝑏𝑏𝑏 + 𝑇𝑇 2
b / (10-3 K-1)
c / (105 K2)
3.77
– 0.50
8
GUIDELINES FOR UPLOADING THE ANSWER FILE ON myEXAMS
1. Access myExams at https://cset.myexams.unisa.ac.za/my/ and login using your
student number and myUnisa password.
2. Go to the "CHE2612 Exam January 2023" examination site through the site tabs
on the horizontal navigation bar.
3. Once the site has loaded, select the eAssessment tool from the left navigation
menu.
4. Select the ‘CHE2612 Examination’ file by clicking on the title of the assessment in
the list. A new page will open.
5. Scroll down to Submission
6. Under Attachments, click on the Choose File button to browse for a file on your
device.
Once you have attached your answer file, the name of the file, as well as the file size
and upload time stamp will be displayed under Attachments.
7. Click the honour pledge.
8. Submit your examination answer file.
© UNISA 2023
9