st stithians boys’ college...page 5 of 15 d cl 2 is the ox idising agent. 1.10 a piece of tin...

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ST STITHIANS BOYS’ COLLEGE

DEPARTMENT OF PHYSICAL SCIENCE

PRELIMINARY EXAMINATIONS 2011

GRADE 12 PHYSICAL SCIENCE : PAPER II

Examiners : Mr B Nozaic and Mr P Mphaphuli (Boys‟ College)

Moderator : Mr B Nozaic (Boys‟ College)

Maximum time : 3 hours 200 marks

INSTRUCTIONS

1. Read the questions carefully.

2. Answer the questions in the answer booklet provided

3. It is in your own interest to write legibly and to present your work neatly.

4. Ensure that the numbering of the questions is correct.

5. Show all your working, where necessary.

6. A data sheet is provided which may be used to assist you in answering the questions

Learning Outcome 1 Scientific Inquiry and Problem solving Skills

Interpret data and Solve Problems.

Learning Outcome 2 Constructing and Applying Scientific Knowledge

Explain relationships.

Applying Scientific Knowledge.

Learning Outcome 3 The Nature of Science and its relationship to Technology and

Society.

Evaluate Knowledge claims.

Impact of science on society

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QUESTION 1 - Multiple Choice.

Write down only the letter corresponding to the correct answer in your answer booklet

1.1 Consider the following chemical reaction written below : 2SO2 (g) + O2 (g) 2SO3 (g)

The reaction is said to be in dynamic equilibrium. Which ONE of the following statements regarding this equilibrium is TRUE?

A The concentrations of the individual reactants and products are constant

B The concentrations of the individual reactants and products are equal.

C The concentrations of the individual reactants are zero.

D The concentrations of the individual products increase until the reaction stops.

1.2 In which of the following reactions would an increase in pressure have no effect on the amount of product at equilibrium?

A C2H4(g) + H2O(g) C2H5OH(g)

B CH3CHO(g) CH4(g) + CO(g)

C 4HCl(g) + O2(g) 2H2O(g) + 2Cl2(g)

D CO(g) + H2O(g) H2(g) + CO2(g)

1.3 A solution of sodium hydroxide, made in a 500cm3 container, has a concentration of 0,25 mol.dm-3. Which of the following substances WILL NOT have exactly the same concentration as the sodium hydroxide solution? A 5,5g of sodium chloride dissolved in 400cm3 of water

B 4,2dm3 of oxygen gas at STP dissolved 750cm3 of water

C 5,3g of chlorine gas dissolved in 600cm3 of water

D 2,8dm3 of carbon dioxide gas dissolved in 500cm3 of water

1.4 Which of the following conditions would produce the greatest reaction rate between two

gases?

A High pressure and high temperature

B High pressure and low temperature

C Low pressure and high temperature

D Low pressure and low temperature

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1.5 Consider the following energy profile

According to this profile, what would be the Activation Energy (EA) and Heat of

Reaction (ΔH) for the reverse reaction ?

„

Energy of Activation (kJ)

Heat of Reaction (kJ)

A

-20

+96

B

+40

+96

C

-136

-96

D

+136

+96

1.6 The correct name for the following compound is

A. methyl ethane

B. 1- methyl ethane

C. propane

D. ethyl methane

+60

+20

-76

0

Reaction co-ordinate

Potential

energy (Ep)

(kJ)

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1.7 Consider the Homologous Series : Alcohols. Which of the following information combinations will be true for this Homologous Series ?

Functional

Group

General

formula

Type of

chemical reactions

A

C C OH

CnH2n+2O

will undergo

substitution reactions

B

C C OH

CnH2n-1O

will undergo

substitution reactions

C

C O

CnH2n-1O

will undergo

elimination reactions

D

C C O

CnH2nO

will undergo

addition reactions

1.8 Which of the following organic molecules do not belong to the Alkyl halides?

A trifluoromethane

B tetrachloromethane

C tetrachloroethane

D trifluoroethene

1.9 In the reaction between chlorine and an aqueous solution of sodium bromide, the following electrochemical reaction takes place. Which of the statement listed below is true ? Cl2 (g) + 2Br – (aq) Br2 (l) + 2Cl – (aq) A Br - ions have been reduced to Br2.

B Cl2 has been oxidised to Cl-.

C Br – ions are the oxidising agent.

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D Cl2 is the ox idising agent.

1.10 A piece of tin metal is placed in each of the following four test tubes containing an

aqueous solution is indicated below. In which test-tube will a metal deposit be seen on

the surface of the tin metal ?

A B C D

QUESTION 2

2.1 Define the following terms:

2.1.1 Alkyl substituent (2)

2.1.2 Functional group (2)

2.2 Consider the following representations of organic molecules A, B, C and D

listed in the table below:

A CH2 ―C―CH2 ―CH2 ―CH3

O

B

C3H7OH

PbCl2 NiSO4 ZnSO4 MgCl2

10 x 2 = 20 marks

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C

CnH2n-2

D chloroethane

Identify to which Homologous Series the molecules A, B, C and D belong (4)

2.3 Draw structural formulae for each of the following organic compounds:

2.3.1 2,2 dimethyl pentane (2)

2.3.2 3 flouro hex-2,4-diene (2)

2.4 Name the following compounds.

2.4.1 OH H H H H CH3 H

H C C C C C C C H

H H H H OH H H (3)

2.4.2 CH3 CH2 CH2 CH2 CH2 COOH (2)

QUESTION 3

Consider the following organic chemical reactions as shown below:

A n CH2 ═ CH2 ─(CH2 ─ CH2)─n

B C5H12 + O2 CO2 + H2O

C CH2(Br) - CH(Br) - CH3 CH2 = CH - CH3 + Br2

D Butene + HBr

E HCOOH + C4H9OH + H2O

product 1

product 2

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3.1 Identify the type of reaction depicted in A, B ,C and D (4)

3.2 Consider the molecule C5H12 used in the unbalanced reaction B as shown in the list above. There are three isomers which have the formula C5H12, one of them being pentane as seen below.

H H H H H

H C C C C C H H H H H H

3.2.1 Define the term isomer. (2)

3.2.2 Draw the structural formulae and name the other two isomers of pentane (6)

3.2.3 Which one of these two isomers drawn in 3.2.2 above, will have the highest boiling point? Explain your answer fully. (3)

3.3 Consider the reaction B once again. 108 g of C5H12 reacts completely with excess oxygen to produce CO2 and H2O. B C5H12 + O2 CO2 + H2O 3.3.1 Balance the equation for reaction B. (3)

3.3.2 Calculate the volume of oxygen (at STP) that is required to react completely with 108 g of C5H12. (4)

3.3.3 Calculate the mass of CO2 released. (3)

3.3.4 An extra 67,2 dm3 of oxygen is now added to the reaction container. How much more pentane will need to be added to ensure complete combustion of all the oxygen ? (3) 3.3.5 C5H12 is now combusted in limited supply of oxygen. Explain, with the aid of a balanced chemical equation, show how this reaction would differ to when C5H12 was in excess oxygen. (3)

3.4 Reaction D, which shows butene reacting with hydrogen bromide, is shown below.

D Butene + HBr

product 1

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3.4.1 Butene is classified as an unsaturated organic compound. Explain what is meant by the term – “unsaturated”. (2)

3.4.2 Draw and name the molecule formed from this reaction identified as product 1. (3) This reaction is also able to continue as seen in the reaction pathway below, to form the alcohol, butanol (C4H9OH)

butene + HBr

3.4.3 Name the type of reaction that will convert product 1 to butanol. (1)

3.4.4 Identify what other reactant would be used in this conversion. (2)

Butanol can now be reacted according to reaction E as seen below

E HCOOH + C4H9OH + H2O

3.4.5 Identify this type of reaction. (1)

3.4.6 Name the product formed (2)

3.4.7 Concentrated sulphuric acid (H2SO4) is added to the reaction mixture. State TWO functions of the sulphuric acid in this type of reaction. (2)

3.4.8 Identify TWO industrial or commercial uses for this type of reaction. (2)

QUESTION 4

Carefully read the passage below and answer the questions that follow :

In 1938, a scientist at chemical industrial giant DuPont named Roy Plunkett made a rather

curious observation: A tank of gaseous tetrafluoroethene (CF2=CF2) that was supposed to be full,

seemed to have no gas in it. Rather than discarding the tank, Plunkett decided to explore further

by cutting the tank open. He found the inside of the tank was coated with a waxy white substance

that was remarkably unreactive toward even the most corrosive chemical agents. Upon further

testing and investigation, the waxy white substance turned out to be a macromolecule. The

polymer he accidentally discovered was the addition polymer known chemically as (poly)

tetrafluoroethene. Plunkett immediately saw huge potential for this particular substance and

product 1 butanol

product 2

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4.1. Explain the following terms associated with addition polymerisation

4.1.1 Free radical. (2)

4.1.2 Propagation (2)

4.2 Why is a free radical (represented by the symbol R ) so important in an addition polymerisation reaction ? Support your answer by using a monomer of Teflon to show the reaction between the free radical and the monomer. (4) 4.3 During the final stage of polymerisation, the Teflon molecule is represented as seen below:

4.3.1 Name the final stage of addition polymerisation. (1)

4.3.2 Using structural formulae, suggest TWO ways in which the reaction can be halted during the final stage. (5)

4.3.3 Teflon has been found to be very useful in the following situations :

List THREE properties that this polymer must have if it can be used for these purposes. (6)

DuPont took out a patent on this chemical substance which they called “Teflon”. DuPont

immediately set about making this polymer on a huge industrial scale and Teflon is still a

substance that is used widely today.

F F F F

R C C C C

F F F F n

.

.

It is ideal for laboratory environments such as containers, magnetic stirrers and tubing for highly corrosive chemicals which would dissolve/ react with glass containers.

It is used to make body piercings.

It can be used to prevent insects climbing up surfaces painted with the material.

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QUESTION 5

A group of students is trying to investigate the rate of a chemical reaction. The reaction between

hydrochloric acid and calcium carbonate, producing calcium chloride, carbon dioxide and water, is

being investigated. The apparatus (pictured below) is set up so that the mass of the reaction

vessel and contents can be measured at regular time intervals. The same experiment is repeated

FIVE times under different conditions.

The reaction equation is: CaCO3 + 2HCl CaCl2 + CO2 + 2H2O

The table below summarises the various conditions under which each investigation was carried

out.

Conditions 1 2 3 4 5

Mass of CaCO3 6g 6g 6g 6g 6g

State of CaCO3 Chips Powder Chips Powder Powder

[HCl] 2 mol.dm-3 2 mol.dm-3 1 mol.dm-3 1 mol.dm-3 1 mol.dm-3

Calcium carbonate

Reaction flask

Hydrochloric acid

Mass meter

20 marks

Cotton wool plug

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Volume of HCl 0,05 dm3 0,05 dm3 0,05 dm3 0,1 dm3 0,05 dm3

Temperature 25oC 25oC 25oC 25oC 25oC

5.1 Consider the table above. Based on the table, what factors are affecting the rate of this reaction? (2) 5.2 The rate of a chemical reaction is explained using the Molecular Collision Theory

5.2.1 What are the TWO main criteria of the Molecular Collision Theory ? (4) 5.2.2 How would the reaction rate of conditions 1 and 3 compare ? Explain your answer using the Molecular Collision Theory . (2)

5.3 Under which TWO conditions listed above will the reaction proceed at the same rate ? Explain your answer. (3)

The following results were obtained from the reaction under Condition 1.

Time(minutes) 0 1 2 3 4 5 6 7 8

Mass (g) 256,00 255,08 254,60 254,28 254,08 253,92 253,84 253,80 253,80

Total loss of

mass (g)

5.4 According to the Law of Conservation of Mass, the total mass of reactants must be equal to the mass of the products. In this reaction, however, the mass is decreasing. Explain why do you think this is happening. (2)

5.5 Why do you think that a cotton wool plug is inserted into the opening of the reaction flask ? (2) 5.6 The above table has been recreated on the special answer sheet provided. On the special answer sheet, complete the table by filling in the total loss of mass after each minute. (2)

5.7 Time is considered to be the independent variable for the experiment while the total loss of mass is the dependent variable. 5.7.1 Explain why this is so in terms of your understanding of the meaning of these two variables. (4)

5.7.2 Using the graph paper provided plot a graph to show the relationship between the total loss of mass of CaCO3 and time. Ensure that you draw in a trend line to show how the reaction has progressed over time. (6)

5.8 Based on the graph you have drawn, explain, in terms of Molecular Collision theory, the shape of the graph between the following time intervals.

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E p (

kJ.m

ol-1

)

B

877,8

795,0

0

Reaction co-ordinate

5.8.1 t = 0s and t = 3s

5.8.2 t = 4s and t = 7s

5.8.3 t = 7s and t = 8s (6)

QUESTION 6

A sportsman has sprained his ankle and needs to apply a cold compress pack in order to keep the swelling down. He has a hot and a cold pack available, but the labels are missing. The only information that can be obtained from the two packs is that one pack contains the chemical calcium chloride and the other contains ammonium nitrate. Each pack consists of an inner bag containing the respective chemical, and an outer bag containing water. When squeezed the inner bag is broken, the chemicals inside dissolve in the surrounding water. You have been tasked to investigate whether the chemicals will dissolve endothermically or exothermically.

6.1 Explain the difference between an endothermic and an exothermic reaction? (2)

6.2 On researching the dissolution of calcium chloride, you find the following energy profile

(not to scale):

33 marks

A

C D

E

X

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6.2.1 Identify the quantities is represented by the letters A – E on the energy profile ? (5)

6.2.2 What is the name associated with position X on the energy profile ? (1)

6.2.3 According to this energy profile, is the dissolution of calcium chloride an endothermic or an exothermic reaction ? Explain your answer. (3)

6.3 On further research, you find that the dissolution of ammonium nitrate in water is given by the following reaction equation:

NH4NO3 (s) + 25,1 kJ NH4+ (aq) + NO3

- (aq)

6.3.1 What does the abbreviation (aq) represent in the above reaction equation ? (2)

6.3.2 In the reaction there appears an energy value of +21,1 kJ. What does the position and magnitude of this value indicate about this reaction ? (2)

6.3.3 In your answer book, draw a neat sketch of the energy profile for this type of dissolution reaction. (2)

6.3.4 If this reaction was catalysed, indicate on your sketched energy profile, how a catalyst would affect the dissolution reaction. (2)

QUESTION 7

A fertiliser company produces ammonia on a large scale at a temperature of 450 °C. The balanced equation below represents the reaction that takes place in a sealed 2dm3 container.

N2(g) + 3H2(g) ⇌ 2NH3(g) ∆H < 0

To meet an increased demand for fertiliser, the management of the company instructs their engineers to make the necessary adjustments to the dynamic chemical equilibrium of the reaction to increase the yield of ammonia.

In a trial run on a small scale in the laboratory, the engineer makes adjustments to the equilibrium mixture. The graphs below represent the results obtained.

C

on

cen

trat

ion

(m

ol∙

dm

-3)

N2

H2

NH3

19 marks

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7.1 Explain what is meant by the term – dynamic chemical equilibrium? (3)

7.2 During which time intervals would the reaction have been in dynamic chemical equilibrium ? (2) 7.3 Using le Chatelier‟s Principle, identify and explain the changes made to the equilibrium mixture between each of the following times:

7.3.1 t1 and t2 (3)

7.3.2 t5 and t6 (3) 7.4 At t3, both the reactants and product showed a sudden increase in concentration. 7.4.1 What do you think happened at this point to cause the increase in concentration? (2)

7.4.2 Using le Chatelier‟s Principle, explain how the equilibrium now re-establishes itself between t3 and t4 . (3) 7.5 At t7, the following amounts of reactant and product were present at equilibrium at a temperature of 4500C :

1,28mol of H2,

49,6g of N2

12,31dm3 of NH3 gas at STP

The volume of the reaction container is 2dm3

7.5.1 Write down an expression for the equilibrium constant (Kc) for this reaction. (2)

7.5.2 Calculate the value of Kc for this reaction at 4500C (4)

7.5.3 What does the magnitude of Kc indicate about the equilibrium position of the reaction? (2)

7.5.4 What would happen to the value of the equilibrium constant (Kc) calculated above, under the following conditions ? Simply answer INCREASE, DECREASE or STAY THE SAME)

(a) N2 (g) is removed from the system

(b) The temperature is decreased to 3500C

(c) The size of the reaction vessel is increased (6)

QUESTION 8

t2 t6 t4 t0 t7

30 marks

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8.1 Define the following electrochemical terms :

8.1.1 Oxidation (1)

8.1.2 Reduction (1)

8.1.3 Reducing agent (2)

8.2 Magnesium metal is added to a container filled with a solution of copper nitrate

8.2.1 What would be observed happening in the container? (2)

8.2.2 Explain these observations using a balanced electrochemical reaction to assist your answer (4)

8.3 For many years, dentists used an combination of silver and mercury as the basis of dental filling material for cavities in teeth.

8.3.1 Why do you think dentists preferred to used these substances rather than zinc or iron as fillings for dental cavities? (2)

8.3.2 Often when a person, who has silver/mercury dental fillings, accidentally bites into a piece of food wrapping that contains aluminium foil, that person experiences a sudden sharp pain in his teeth. Using electrochemical principles, explain why this pain would occur. (3)

15 marks

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