Download MAIN EXAM 2012 - University of KwaZulu

School of Chemistry and Physics
UNIVERSITY OF KWAZULU-NATAL, WESTVILLE CAMPUS
MAIN EXAMINATION
CHEM 161: CHEMICAL ENGINEERING CHEMISTRY 1
DURATION: 3 Hours
TOTAL MARKS: 100
Internal Examiner: S. Singh
Internal Moderator: H. B. Friedrich
IMPORTANT: COMPLETE THIS SECTION IMMEDIATELY
SURNAME AND INITIALS (Optional): ________________________
SIGNATURE: _____________________
Student
Number
Seat
Number
INSTRUCTIONS
1.
Non programmable calculators may be used but all working must be shown.
2.
You must write legibly in black or blue ink. Pencils and Tipp-Ex are not allowed.
3.
Your answers must be written on the question paper in the spaces provided. The left-hand
pages may be used for extra space or for rough work.
4.
This question paper consists of 15 pages (including the cover page) and 6 questions. Please
check that you have them all. A periodic table and a data sheet are provided separately.
5.
You will also require a copy of Section A - Multiple Choice Questions.
Question No.
Internal Mark
External Mark
1
2
3
4
5
6
Total
School of Chemistry and Physics
University of KwaZulu-Natal, Westville Campus
CHEM161: Chemical Engineering Chemistry
May 2012 Main Examination
SECTION B
TOTAL FOR SECTION B: 70 MARKS
Answer All Questions
Question 1
(a)
The atomic masses of the two stable isotopes of bromine,
79Br
(50.69%) and
81Br
(40.31%),
are 78.9183 u and 80.9163 u respectively.
(i)
State the number of neutrons in the heavier isotope of bromine.
(1)
(ii)
Calculate the average relative atomic mass of bromine.
(2)
2
School of Chemistry and Physics
University of KwaZulu-Natal, Westville Campus
CHEM161: Chemical Engineering Chemistry
May 2012 Main Examination
(b)
An
organic solvent containing carbon, hydrogen and sulfur yields CO2, H2O and SO2 on
combustion. When subjected to combustion analysis, a 1.086 g sample produces 2.272 g of
CO2, 0.4650 g of H2O and SO2. Determine the empirical formula of the solvent.
3
(4)
School of Chemistry and Physics
University of KwaZulu-Natal, Westville Campus
CHEM161: Chemical Engineering Chemistry
May 2012 Main Examination
(c)
A 50.00 g sample of Mg(OH)2 solution is mixed with 70.00 g of H3PO4 solution. A neutralization
reaction takes place which is represented by the equation
3Mg(OH)2 + 2H3PO4 
Mg3(PO4)2
+
6H2O
(i)
Determine the limiting reagent. (Show all calculations).
(3½)
(ii)
Calculate the mass of excess reagent that remains after completion of the reaction.
(2½)
4
School of Chemistry and Physics
University of KwaZulu-Natal, Westville Campus
CHEM161: Chemical Engineering Chemistry
May 2012 Main Examination
(d)
A 0.4308 g sample of iron ore is dissolved in acid and the iron converted into Fe2+. This
solution is reacted with a solution of KMnO4. The reaction requires 27.35 mL of a 0.02469
mol dm-3 solution of KMnO4. Calculate the percentage of Fe in the ore. The equation for the
reaction is:
5Fe2+ + MnO4- + 8H+  5 Fe3+ + Mn2+ + 4H2O
(3)
[16]
5
School of Chemistry and Physics
University of KwaZulu-Natal, Westville Campus
CHEM161: Chemical Engineering Chemistry
May 2012 Main Examination
Question 2
(a)
Calculate the frequency (in s-1) of the electromagnetic radiation emitted by a hydrogen atom
when its electron undergoes a transition from the n = 4 level to the n = 2 level.
(b)
(3)
In the following assignments of the four quantum numbers, write acceptable values for the
missing ones. What are the orbitals corresponding to these quantum numbers?
(i)
n = ________
l=1
ml = 0
ms = ________
l=0
ml = ________
ms = +½
orbital = ________
(ii)
n = ________
orbital = ________
6
(3)
School of Chemistry and Physics
University of KwaZulu-Natal, Westville Campus
CHEM161: Chemical Engineering Chemistry
May 2012 Main Examination
(c)
Draw the Lewis structure and assign formal charges for nitric acid (HNO 3) in which the three O
atoms are bonded to the central N atom and the ionizable H atom is bonded to one of the O
atoms.
(d)
(4)
Showing all the relevant steps, use VSEPR theory to predict the molecular geometry of:
(i)
CH4
(2)
(ii)
NH3
(2)
[14]
7
School of Chemistry and Physics
University of KwaZulu-Natal, Westville Campus
CHEM161: Chemical Engineering Chemistry
May 2012 Main Examination
Question 3
(a)
A chemist has synthesized a greenish-yellow gaseous compound of chlorine and oxygen and
finds that its density is 7.71 g L-1 at 36 °C and 2.88 atm. Calculate the molar mass of the
compound and determine its molecular formula.
(b)
(4)
A mixture of 15 g methane (CH4) and 22 g ethane (C2H6) are forced into a container until the
total pressure is 2.2 atm. What are the partial pressures of methane and ethane?
8
(3)
School of Chemistry and Physics
University of KwaZulu-Natal, Westville Campus
CHEM161: Chemical Engineering Chemistry
May 2012 Main Examination
(c)
Calculate the standard enthalpy of formation of acetylene (C2H2) from its elements. The
equations for each step and the corresponding enthalpy changes are
(a)
C(graphite) + O2(g)  CO2(g)
Horxn = -393.5 kJ mol-1
(b)
H2(g) + ½O2(g)  H2O(l)
Horxn = -285.8 kJ mol-1
(c)
2C2H2(g) + 5O2(g)  4CO2(g) + 2H2O(l)
Horxn = -2598.8 kJ mol-1
(5)
[12]
9
School of Chemistry and Physics
University of KwaZulu-Natal, Westville Campus
CHEM161: Chemical Engineering Chemistry
May 2012 Main Examination
Question 4
(a)
The following data were determined for the reaction:
A(g) + B(g)  AB(g)
Experiment
[A] (M)
[B] (M)
Initial Rate (M/s)
1
5.0 × 10-3
2.0 × 10-3
1.3 × 10-5
2
10.0 × 10-3
2.0 × 10-3
5.0 × 10-5
3
10.0 × 10-3
4.0 × 10-3
10.0 × 10-5
Determine the rate expression and the value of the rate constant k.
10
(5)
School of Chemistry and Physics
University of KwaZulu-Natal, Westville Campus
CHEM161: Chemical Engineering Chemistry
May 2012 Main Examination
(b)
Iodine atoms combine to form molecular iodine in the gas phase
I(g) + I(g)  I2(g)
This reaction follows second-order kinetics and has a high rate constant of 7.0 × 109 M-1 s-1 at
23°C. The initial concentration of I was 0.086 M.
(i)
Calculate the concentration after 2.0 min.
(3)
(ii)
Calculate the half-life of the reaction.
(2)
[10]
11
School of Chemistry and Physics
University of KwaZulu-Natal, Westville Campus
CHEM161: Chemical Engineering Chemistry
May 2012 Main Examination
Question 5
(a)
The following equilibrium process has been studied at 230 oC:
2NO(g) + O2(g) ⇌ 2NO2(g)
The concentration of the reacting species at equilibrium was found to be [NO] = 0.0542 M,
[O2] = 0.127 M and [NO2] = 15.5 M. Calculate the equilibrium constant, Kc.
(b)
(2)
The equilibrium constant (Kc) for the reaction
N2O4(g) ⇌ 2NO2(g)
is 4.63 × 10-3 at 25 °C. What is the value of Kp at this temperature?
12
(2)
School of Chemistry and Physics
University of KwaZulu-Natal, Westville Campus
CHEM161: Chemical Engineering Chemistry
May 2012 Main Examination
(c)
In an experiment 1.50 mol of H2(g) and 1.50 mol of Br2(g) were initially placed in a 1.0 L flask
at 50
oC.
Calculate the equilibrium concentration of the reactants and products if the
equilibrium constant is 54.3.
(5)
[9]
13
School of Chemistry and Physics
University of KwaZulu-Natal, Westville Campus
CHEM161: Chemical Engineering Chemistry
May 2012 Main Examination
Question 6
(a)
Identify the acid-base conjugate pairs in the following reaction:
HClO + CH3NH2 ⇌ CH3NH3+ + ClO-
(b)
Determine the [OH-] in aqueous ammonia at a pH of 11.28.
14
(2)
(2)
School of Chemistry and Physics
University of KwaZulu-Natal, Westville Campus
CHEM161: Chemical Engineering Chemistry
May 2012 Main Examination
(c)
What is the pH of a 0.0250 M methylamine (CH3NH2) solution in water Kb = 4.2 × 10-4.
(5)
[9]
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