NAMIBIA UNIVERSITY OF SCIENCE AND TECHNOLOGY

FACULTY OF ENGINEERING

InSTEM

QUALIFICATION: INTRODUCTION TO SCIENCE, TECHNOLOGY, ENGINEERING AND MATHEMATICS

QUALIFICATION CODE: O4STEM LEVEL: 4

COURSE CODE: ICH402S COURSE NAME: INTRODUCTION TO CHEMISTRY B

SESSION: NOVEMBER 2019 PAPER: N/A

DURATION: 3 HOURS MARKS: 100

FIRST OPPORTUNITY EXAMINATION QUESTION PAPER

EXAMINER(S) Mr Victor Uzoma

MODERATOR: Prof Habauka M Kwaambwa

INSTRUCTIONS

DO AWN

PE 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 FINALANSWERS 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)

Question 1 [9]

dad

ddd

a lnk

1.13

1.1.4

LZ

1.2.1

1.2.2

1.2.3

Chlorate(l) ions undergo the following reaction under aqueous conditions:

2NH3 + C/O- — No2H4+ C/~ + H20

A series of experiments was carried out at different concentrations of C/ O- and N#3.

The table below shows the results obtained:

experiment a fmol [NH] initial rate /moldm? /moldm?s*

1 0.200 0.200 1.025

2 0.400 0.200 2.05

3 0.400 0.400 8.20 Use the data in the table to determine the order with respect to each reactant,

C/O- and NH3. Show your reasoning. (2)

Write the rate equation for this reaction. (1)

Use the results of experiment 1 to calculate the rate constant, k, for this reaction.

Include the units of k. (1)

Redraw the axes and sketch a graph to show how the value of k changes as

temperature is increased. (1)

In another experiment, the reaction between chlorate(I) ions and iodide ions in

aqueous alkali was investigated. A solution of iodide ions in aqueous alkali was added

to a large excess of chlorate(I) ions and [I"] was measured at regular intervals.

Describe how the results of this experiment can be used to confirm that the reaction

is first-order with respect to [I]. (2)

A three-step mechanism for this reaction is shown:.

Step 1 1C/O-+H20 — HC/O+OH-

Step 2 I-+HCIO > HIO+C/-

Step 3 HIO +OH~ — H20 +107

Use this mechanism to deduce the overall equation for this reaction. (1)

In the three step mechanism in 1.2.2, Identify a step that involves a redox reaction.

Explain your answer. (1)

Question 2 [12]

Entropy changes and free energy changes can be used to predict the feasibility of processes.

Dod.

2.2

2.3

2.4

2.4.1

2.4.2

Three processes are given below. For each process, predict and explain whether the

entropy change, AS, would be positive or negative. (3)

e The melting of iron.

e The reaction of magnesium with dilute sulfuric acid.

e The complete combustion of ethane: 2C2He(g) + 702(g) —- 4CO2(g) + 6H20(I)

Ammonia can be oxidised as shown in the equation below:

ANHa(g) + 502(g) — 4NO(g) + 6H20(g)

At 450°C, AH = -907 kJ mol and AG = —1041 kJ mol“. Calculate the standard

entropy change, AS, in J K+ mol, for this reaction. Show all your working. (2)

A reaction is not feasible at low temperatures but is feasible at high temperatures.

Deduce the signs of AH and AS for the reaction and explain why the feasibility

changes with temperature. (3)

The metal tungsten is obtained on a large scale from its main ore, wolframite.

Wolframite contains tungsten(VI) oxide, WO3. Tungsten is extracted from wolframite

by reduction with hydrogen:

WOs3(s) + 3H2(g) ~ W(s) + 3H20(g) AH = +115 kJmol

Standard entropies are given in the table below:

Substance | WOs(s) | H2(g) W(s) H20(g)

S°/JK?molt | 76 131 33 189

Calculate the free energy change, AG, in kJmol™, for this reation at 25°C. (2)

Calculate the minimum temperature, in K, at which this reaction becomes feasible.

(Show your working) (2)

Question 3 [16]

This question is about acids, bases and buffers solutions.

3.1

3.1.1

S12

8.18

wd

3.3

Barium hydroxide, Ba(OH)z, is a strong Brénsted—Lowry base. A student prepares

250.0 cm? of 0.1250 mol dm barium hydroxide.

Explain what is meant by the term Br@nsted-Lowry base. (1)

Calculate the mass of Ba(OH)2 that the student would need to weigh to two decimal

places to prepare 250.0 cm? of 0.1250 mol dm? Ba(OH)z. (3)

Calculate the pH of a 0.1250 mol dm? solution of Ba(OH)2. Give your answer to two

decimal places. (3)

The student attempts to prepare a buffer solution by mixing 200 cm? of

0.324 mol dm C2HsCOOH with 100 cm? of the 0.1250 mol dm-3 Ba(OH)2 prepared in (3.1). The equation for the reaction that takes place is shown below:

2C2HsCOOH(aq) + Ba(OH)2(aq) — (C2HsCOO)2Ba(aq) + 2H20(I)

Explain whether the student was successful in preparing a buffer solution. Include

all reasoning and any relevant calculations. (4)

Blood contains a mixture of carbonic acid, H2CO3, and hydrogen carbonate ions, HCO3-.

Explain how the carbonic acid—hydrogen carbonate mixture acts as a buffer. /n your

answer include the equation for the equilibrium in this buffer solution and explain how

this equilibrium system is able to contro! blood pH (5)

Question 4 [12]

4.1 20.0 cm? of 0.100 moldm-? NaOH were slowly added to a 10.0 cm? sample of 0.100 moldm? ethanoic acid, and the pH was measured throughout the addition.

4.1.1 Calculate the number of moles of NaOH remaining at the end of the addition. (2)

4.1.2 Calculate the [OH] at the end of the addition. (3)

4.1.3 Using the expression Kw = [H*][OH7] and your value in 4.1.2, calculate [H*] and the

pH of the solution at the end of the addition. (3)

4.2 Draw similar axes as below in your answer book, sketch how the pH will change

during the addition of a total of 20.0 cm? of 0.100 mol dm? NaOH. Mark clearly where the endpoint occurs. (3)

0 5 10 15 20

volume NaOH added/cm?®

4.3 From the following list of indicators, which one is most suitable for this titration? (1)

Indicator pH

Malachite green 0-1

Thymol blue 1-2

Bromophenol blue 3-4

Phenolphthalein 9-10

Question 5 [12]

Lead(Il) chloride, PbClz, can be used in the manufacture of some types of coloured glass.

PbCl2 is only sparingly soluble in water. The [Pb?*] in a saturated solution of PbCl2 can be

estimated by measuring the cell potential, Ece, of the following cell.

Table of cell potential values:

Half-reaction Cell potential/ E° (V)

Clo (g) + 2e Y 2 Cl 1.36

lo (s)+ 2e + Y¥ 20 0.53 2H* +2 & Y H2 (g) 0.00

Pb *? + 2e Y Pb(s) -13

Sn** + 2 & Y Sn(s) -0.14

A

H2(g)_ ——» latm, 298K

B salt bridge —|__ [ D co rr

“O. Ly a

OO se alt ae on a a ee me “ee = a saturated solution of PbCl>

C Solid PbCl,

5.1 State what the four letters A-D in the above diagram represent. (2)

5.2 In a saturated solution of PbCl2, The concentration [PbCl2(aq)] = 3.5 x 10°? moldm?.

5.2.1 The E° for the Pb* / Pb electrode is — 0.13V. Predict the potential of the right-hand

electrode in the diagram above, choosing from the box in the table below:

Electrode potential /

V

-0.1

-0.13

-0.09

-0.00

Explain your answer. (2)

5.2.2 Write an expression for the solubility product, Ksp, of PbClo. (1)

5.2.3 Calculate the value of Ksp, including units. (2)

5.5 The behaviours of Pblz2 and SnCl2 towards reducing agents are similar, but their

behaviours towards oxidising agents are very different.

Table of some electrode potentials:

Half-reaction Electrode potential (V)

Zn2* + 2 e& Y Zn(s) -0.76

Fe2* + 2e° Y Fe(s) -0.44

Pb2* + 2e YPb -0.13

Sn2* + 2e Y Sn -0.14

2H* + 2e -Y Ho 0.00

Sn +2e YSn 0.15

2h +2e Y kb 0.54

Agtte Y Ag 0.80

Br2(I) + 2e- Y 2Br 1.07

2cl + 2eY Ch 1.36

Pb** + 2e° Y Pb2* 1.69

5.3.1 Illustrate this comparison by quoting and comparing relevant E° values shown in the

table above for the two metals and their ions. Explain what the relative E° values

mean in terms of the ease of oxidation or reduction of these compounds.

5.3.2. Writing a balanced molecular or ionic equation in each case, suggest a reagent to

carry out each of the following reactions:

Pb2* > Pb (s) and Sn** > Sn?*

Question 6

Allyl bromide, CH2=CHCH2Br, is used in the production of polymers.

6.1 Part of the C=C double bond in allyl bromide is called a m-bond. Draw a labelled

diagram to show the formation of the m-bond.

6.2 Allyl bromide is a member of a homologous series. Compounds in this series have

the same general formula.

6.2.1 What is meant by the term homologous series?

6.2.2 What is the general formula of the homologous series that has allyl bromide as a

member?

6.2.3. Give the systematic name for allyl bromide.

[25]

(2)

(2)

(1) (1)

6.3 Reaction mechanisms use curly arrow and can involve electrophiles and nucleophiles.

6.3.1 What does a curly arrow represent in mechanisms?

6.3.2 What is meant by the term nucleophile?

6.4 Allyl bromide CH2=CHCH2Br, reacts with aqueous sodium hydroxide.

6.4.1 Outline the mechanism of this reaction. Include curly arrows, relevant dipoles and

final product(s).

6.4.2 Name the type of mechanism.

6.5 Allyl bromide, CH2=CHCH2Br, reacts with bromine, Brz.

(1)

(1)

(3)

(1)

6.5.1 Outline the mechanis of this reaction. Include curly arrows, relevant dipoles and the

intermediate and final product(s).

6.5.2 Name the type of mechanism. (4) (1)

6.6 Allyl bromine is reacted as shown below:

CH2=CHCH2Br—28 = CH3CH2CH2Br__§"*P2__ mixture of organic products 1-bromopropane = Cla

6.6.1 State the reagents and conditions for step 1. (1)

6.6.2 Instep 2, 1-brmopropane reacts with chlorine by readical substitution. Outline the

mechanism for the momochlorination of 1-bromopropane. In your mechanism, you

can show the formula of 1-bromopropane as C3H7Br. Include the names of the three

stages in this mechanism, state the essential conditions and all termination steps. (5)

6.6.3 Radical substitution produces a mixture of organic products. Suggest two reasons

why. (2)

Question 7 [14]

Ted Write the equations for the following decay processes:

7.1.1 The beta decay of uranium -237 (2)

7.1.2 Positron emission from silicon-26 (2)

7.2. The radioisotope fluorine-21 had an initial mass of 80 milligrams. 20 milligrams of

sample remained unchanged after 8.32 s. What is the half life of F-21? (2)

7.3 If 1.000 x 10°? mol of Cs-135 emits 1.390 x 10° beta-particles in 1.00 year, what is

the decay constant? (4)

7.4 Achemist determines that a sample of petrified wood has a carbon-14 decay rate of

6.00 counts per minute per gram. What is the age of the piece of wood in years?

The decay rate of carbon-14 in fresh wood today is 13.6 counts per minute per gram,

and the half life of carbon-14 is 5730 years. (4)

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