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ICH402S - INTRODUCTION TO CHEMISTRY B - 2ND OPP - JAN 2020 |
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NAMIBIA UNIVERSITY
OF SCIENCE AND TECHNOLOGY
FACULTY OF ENGINEERING
InSTEM
QUALIFICATION: INTRODUCTION TO SCIENCE, TECHNOLOGY, ENGINEERING AND MATHEMATICS
QUALIFICATION CODE: O4STEM
COURSE CODE: ICH402S
LEVEL: 4
COURSE NAME: INTRODUCTION TO CHEMISTRY B
SESSION:
JANUARY 2020
PAPER:
N/A
DURATION:
3 HOURS
MARKS:
100
SECOND OPPORTUNITY EXAMINATION QUESTION PAPER
EXAMINER(S)
Mr Victor Uzoma
MODERATOR:
Prof Habauka M Kwaambwa
INSTRUCTIONS
Answer all questions.
Write all the answers in ink.
No books, notes, correction fluid (Tippex) or cell phones allowed.
Pocket calculators are allowed.
You are not allowed to borrow or lend any equipment or stationary.
All FINAL ANSWERS must be rounded off to TWO DECIMAL PLACES unless otherwise
stated.
7. Periodic table on page 9.
THIS QUESTION PAPER CONSISTS OF 9 PAGES (Excluding this front page)
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Question 1
[9]
i
Nitrogen(Il) oxide reacts with hydrogen according to the equation below:
2NO (g) + 2H2 (g) > No (g) + 2H20 (g)
A suggested mechanism for this reaction is:
Step 1: NO+ H2Y X
fast
Step 2:X +NO > Y+H20
slow
Step 3: Y + H2 > N2+ H20 fast
1.11 Identify the rate-determining step.
(1)
1.1.2 A student hypothesized that the order of reaction with respect to H2 is 2. Evaluate
this hypothesis by finding the orders with respect to hydrogen and nitrogen oxide
and write the rate expression for this reaction. (show all working)
(3)
1.2.1 Determine the orders of reaction with respect the reactants and the overall rate
expression for the reaction between 2-bromobutane and aqueous sodium hydroxide
using the data in the table below:
(2)
Experiment =| [NaOH]/moldm™® | [c,HsBr] / moldm™ | Rate / moldm™s*
1
1.00
1.00
2
0.50
1.00
3
0.25
0.25
4
1.00
0.50
1.66 x 10°
8.31 x 10%
41.02 x 10%
8.29 x 10%
1.2.2 Determine the rate constant, k, with its units, using the data from experiment 3.
(2)
1.2.3 Identify the molecularity of the rate-determining step in this reaction.
(1)
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Question 2
[3]
aed Iron (III) oxide is the main source of iron but the decomposition of Fe203(s) into its
elements is extremely difficult due to a large positive value of AG®. Consider the
following reactions:
Fe203 (s) > 2Fe(s) + “O2(g) AG® = +742 kJ mol
CO (g) +502 (g) > CO2(g) AG® = -257 kJ mot?
Suggest, with a reason, whether it is possible to produce iron by reacting Fe203
with CO.
(2)
Pod
The thermite reaction is one of the most exothermic reactions:
Fe203 (s) + 2Al (s) > 2Fe (I) + AlzO3 (s)
AH®= -825.2 kJ
Species
Al(s)
Al2zO3 (s)
Fe (I)
Fe203 (s)
S$°/ JK*mol*
#28.3
+50.9
+34.8
+87.5
AG;/°/ kiImol+
0
-1582
+10.0
-742
2.2.1 Calculate the standard free energy change, AG®, in kJ mol-1, by using values of
the standard free energy change of formation, AG;, from the table above.
(2)
2.2.2 Calculate the standard entropy change, AS, in J K? mol*, by using values of
standard entropy, S®, from the table.
(2)
2.2.3 Calculate the standard free energy change, AG®, for the reaction using AH® and AS®
values at 25°C.
(2)
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Question 3
[21]
3.1 20.0 cm? aqueous solutions of two bases, each with a concentration of
0.100 mol dm? were separately titrated with 0.100 mol dm? hydrochloric acid,
HCl (aq), and the following graph was obtained.
(eereerereere
eet
joe
eS
0
Ta
I—
0
2
4
6
8 10 12 14 16 18 20 22
Volume of HCL/ cm?
3.1.1 Deduce the pH at the equivalence points for base | and base Il.
(2)
3.1.2 Suggest why the titration curve for base | is different from base II.
(1)
3.1.3 State the formulas of two possible bases which could be used as base I.
(1)
3.1.4 Calculate, using data from the graph, the dissociation constant, Kp, of base Il,
showing your working.
(3)
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3.1.5 Table of Common Acid-Base Indicators:
Indicator
Thymol Blue
Pentamethoxy
red
Methyl red
Bromcresol
purple
pH Range
1.2-2.8
/[1.2-2.3
|Acid
red
red-violet
4.4-6.2
5.2-6.8
red
yellow
Base
yellow
{colorless
yellow
purple
Suggest an indicator from the table above, that can be used for both titrations.
(1)
Sub State what is meant by the term buffer solution.
(2)
B22 Calculate the pH of a solution prepared by mixing 40.0 cm? of 0.200 mol dm Nks (aq)
and 40.0 cm? of 0.100 mol dm- HCI (aq), showing your working.
(pKb NH3 = 4.75 at 298 K)
(3)
33 The equations of two acid-base reactions are given below:
ReactionA — H2COs(aq) + H2O(I) Y HCO3(aq) + H30 (aq)
ReactionB — HCOs"(aq) + H20(I) Y CO3*(aq) + H30*(aq)
3.3.1 Explain whether HCO3(aq) behaves as an acid or a base in each of the reactions
Aand B.
(1)
3.3.2 Deduce two conjugate acid-base pairs from reactions A and B.
(2)
3.4
Nitric acid, HNO3, and nitrous acid, HNOz, are described as strong and weak acids
respectively.
3.4.1 Distinguish between strong and weak acids.
(1)
3.4.2 A 1.00 g sample of solid magnesium carbonate, MgCO3, is added to separate
solutions of HNO3 and HNO2 of the same concentration and temperature. State one
similarity and one difference in the observations made in these reactions.
(2)
3.4.3 A solution of HNO3 has a pH of 1, while a solution of HNO2 has a pH of 5. Determine
the ratio of the hydrogen ion concentration in HNO3:HNOz.
(1)
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Question 4
[13]
lodine monobromide, IBr, has a permanent dipole. Alkenes react with IBr in a similar way
to the reactions of alkenes with HBr.
4.1 Propene reacts with IBr to make two possible organic products. One of these
products is 2-bromo-1-iodopropane.
4.1.1 Using the curly arrow model, complete the mechanism to make
2-bromo-1-iodopropane.
H
CH,
ene! ——_>
ee
H
H
(3)
HCH,|
Hoe
I Br
4.1.2 What is the name of this mechanism?
(1)
4.1.3 Draw the structure of the possible organic product of the reaction of propane with
IBr.
(1)
4.2
Methane reacts with IBr to form many products. Two of these products are
lodomethane and hydrogen bromide.
4.2.1 Suggest the essential condition needed for this reaction.
(1)
4.2.2 The mechanism of this reaction involves three steps, of which one is called
termination. Describe the mechanism of the reaction that forms iodomethane and
hydrogen bromide. Your answer must include:
The name of the mechanism
The names of the other two steps of the mechanism
Equations for these two steps of the mechanism
The type of bond fission
One equation for a termination step.
(7)
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Question 5
[9]
The electrons transferred in redox reactions can be used by electrochemical cells to
provide energy. Some electrode half-equations and their standard electrode potentials
are shown in the table below:
Half-equation
Cr2O7* (aq) + 14H (aq) + 6e > 2Cr (aq) + H20 (I)
Fr3* (aq) + e' > Fe?* (aq)
2H* (aq) + 2e° > Hp (g)
Fe** (aq) + 2e° > Fe (s)
Lit (aq+ &)-> Li(s)
E/V
1.33
+0.77
0.00
-0.44
-0.304
5.1 Describe a standard hydrogen electrode.
(4)
5.2. Aconventional representation of a lithium cell is given below. This cell has an e.m.f.
of +2.91 V.
Li(s) | Lit(aq) | | Li*(aq) | MnO2(s) , LiMnO2(s) | Pt(s)
Write a half-equation for the reaction that occurs at the positive electrode of this cell.
Calculate the standard electrode potential of this positive electrode, and the change
in gibbs free energy for the cell. Is the reaction feasible or not, explain why.
(5)
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Question 6
[25]
6.1 Electrolysis of an aqueous solution of copper(II) sulfate, CuSOa, can be carried out
using platinum electrodes.
6.1.1 State an equation for the half-reaction occurring at the positive electrode (anode)
and one observation that could be made as a result.
(2)
6.1.2 State an equation for the half-reaction occurring at the negative electrode
(cathode).
(2)
6.1.3 Describe two changes or observations in the electrolyte as result of these
half-reactions.
(2)
6.1.4 Determine the relative amount, in moles, of products formed at each electrode.
(1)
6.1.5 Identify another compound which will form the same products at the positive and
negative electrodes.
(1)
6.1.6 The same process is carried out using copper electrodes instead of the platinum
electrodes. Describe the changes or observations that take place at both the
electrodes and in the electrolyte.
(3)
6.2 Identify two factors that affect the quantity of copper produced during the
electrolysis of an aqueous copper(II) sulfate solution.
(1)
6.3 Consider the following half cell reactions and their standard electrode potentials:
Ni** (aq) + 2e- Y Ni(s)
£9 =-0.2V
MnOsg (aq) + 8H* (aq) +S5e Y Mn2* (aq) + 4H20 (I) £°=41.51v
6.3.1 State the conditions needed for the electrode potentials to be described as standard.(1)
6.3.2 Deduce a balanced equation for the overall reaction which will occur spontaneously
when the two half cells are connected.
(2)
6.3.3 Identify the reducing agent in the above reaction and determine the change in
oxidation number for the oxidizing agent.
(2)
6.3.4 Determine the cell potential when the two half cells are connected.
(1)
6.3.5 Draw and label a diagram of the voltaic cell from part 6.3. Indicate the anode,
cathode, the direction of the electron movement and ion flow.
(5)
6.3.6 Outline two differences between an electrolytic cell and a voltaic cell.
(2)
7
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Question 7
[15]
7.1 Crude oil is a source of many hydrocarbons. The skeletal formulae of some of
these hydrocarbons are shown below:
OO
A.
A
B
Cc
i i i
CO
D
E
AND
F
G
H
I
7.1.1 Explain why compound A is both saturated and a hydrocarbon.
(2)
7.1.2 What is the empirical formula of compound C?
(1)
7.1.3 Give the letters, of two hydrocarbons that are structural isomers of each other.
(2)
7.1.4 Explain why hydrocarbon D has a higher boiling point than hydrocarbon C.
(2)
7.1.5 Hydrocarbons G and H are stereoisomers of each other. What is meant by the term
stereoisomerism?
(2)
7.1.6 Construct the equation for the complete combustion of hydrocarbon C.
(2)
7.1.7. Compound | does not contain a functional group.
7.1.7.1 What is meant by the term functional group?
(1)
7.1.7.2 Give the IUPAC name for the structure.
(1)
7.1.7.3 Compound | reacts with chlorine in the presence of ultraviolet radiation to give
several structural isomers of CgHi7Cl. How many structural isomers could be formed
in this reaction?
(1)
7.1.7.4The mechanism of the reaction involves radicals. What is meant by the term radical? (1)
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