Bishops Physical Science Department

Matric Mock Examinations

2011

Physical Science

Paper 2

Chemistry

Examiner: DL Marks: 150Moderator: KW Time: 3 hours

SECTION A

QUESTION 1: ONE-WORD ITEMS

Give ONE word/term for each of the following descriptions in the questions numbered (1.1 – 1.5).

1.1 A class of compounds that can be represented by a general formula. (1)

1.2 The group of compounds containing the following functional group. OC (1) H

1.3 The name of the energy required to get a reaction underway. (1)

1.4 The general name given to the type of electrochemical cell in which chemical energy is converted to electrical energy. (1)

1.5 The name of the process by which nitrogen is obtained from air. (1)[5]

QUESTION 2: MULTIPLE-CHOICE QUESTIONS

Four options are provided as possible answers to the following questions. Each question has only ONE correct answer. Choose the answer and make a cross (X) in the block (A – D) next to the question number (2.1 – 2.10) on the ANSWER SHEET.

2.1 Which of the following factors will always influence the rate of a reaction without having any effect on the position of the equilibrium?

A temperature.

B concentration of reactants.

C catalyst.

D pressure of gaseous reactants.

2.2 The forces that hold iodine molecules in their crystalline structure in solid iodine

A covalent bonds

B hydrogen bonds

C London forces

D ion-dipole attractions

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2.3 Consider the following statements about the following chemical equilibrium:

2SO2(g) + O2(g) ⇌ 2SO3(g) (ΔH < 0)

I the reverse reaction is exothermic

II the value of the equilibrium constant will decrease with a decrease in temperature

III the concentration of SO3 increases when the temperature is decreased

IV when the pressure in the reaction vessel is decreased the equilibrium shifts to the left

Which statements are correct?

A I and II

B II and III

C II and IV

D III and IV

2.4 The N:P:K ratio in a 100 kg bag of fertilizer is given as 3:1:1. The mass in kg of nitrogen (N) in the bag has a value:

A Equal to 20

B Less than 20

C Greater than 20

D That cannot be determined

2.5 In an endothermic reaction the difference between the energy taken in by the reactants to form the activated complex and the energy released in forming products is known as;

A the enthalpy of reaction

B the temperature of the reaction

C the activation energy

D the activated complex

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2.6 The Standard Hydrogen electrode is a reference electrode which is used to…….

A determine the standard electrode potential of all other half cells.

B determine the cell potential of a battery.

C manufacture electrochemical cells of various potential.

D produce hydrogen by electrolysis.

2.7 Which of the following Kc expressions are valid for the following reaction?

CH2CH2(g) + Br2(l) ⇌ CH2BrCH2Br(g)

A [CH2CH2] [CH2BrCH2Br]

B [CH2CH2] ∙[CH2BrCH2Br]

C [CH2BrCH2Br][CH2CH2] ∙ [Br2]

D [CH2BrCH2Br][CH2CH2]

2.8 Which ONE of the following compounds has structural isomers?

A Ethane

B 1,2-dibromoethane

C 1-bromoethane

D 1-bromoethene

2.9 The ampere-hour is the unit of measurement for:

A Current

B Charge rate

C Cell capacity

D Battery power

2.10 Which ONE of the following is incorrect about a galvanic cell?

3

A Oxidation occurs at the anode..

B Electrons flow from the cathode to the anode

C Reduction occurs at the cathode.

D A salt-bridge is always necessary for it to operate.

One Word Answers 5

Multiple Choice 10 x 2 20

TOTAL SECTION A: 25

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SECTION B: Long Questions – Please start each question on a new page

QUESTION 3 RATE AND EQUILIBRIUM.

3.1 In an experiment, the equilibrium established between three substances; reactants A and B and product C was investigated. Certain initial concentrations of each substance were mixed together and then allowed to reach equilibrium in a container with a volume of 1 dm3.

The reaction between these three substances may be represented by the equation:

A (g) + m B (g) ⇌ n C (g)

(where m and n are coefficients required to balance the equation and are greater than 1)

The changes in concentration (in mol.dm-3) are shown in the graph below:

3.1.1 Identify which line (i, ii and iii) belongs to which substance (A, B and C).(3)

3.1.2 How does the rate of the forward reaction compare to the rate of the reverse reaction during the first 30 s? (1)

3.1.3 After 30 s, how does the rate of the forward reaction compare to the rate of the reverse reaction? Motivate your answer. (2)

3.1.4 From the graphs, calculate the reacting ratios of A and B (the values m and n) in the balanced reaction equation. (2)

3.1.5 Calculate the equilibrium constant (Kc) for this reaction using values from the graph. (You will not be penalised here for incorrect values for m and n. If you have not managed to calculate m and n in question 3.1.4 substitute any values here.) (3)

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i

ii

iii

2,5

1,5

3.2 Methanol can be prepared commercially from carbon monoxide and hydrogen using a suitable catalyst, according to the reaction:

CO (g) + 2 H2 (g) ⇌ CH3OH (g) ; ΔH = -92 kJ.mol-1

3.2.1 Is this reaction exothermic or endothermic? Explain your answer. (2)

3.2.2 How could the pressure under which the reaction is performed be adjusted to maximise the yield of methanol? (1)

3.2.3 How could the temperature at which the process is performed be adjusted to produce more methanol at equilibrium? (1)

3.2.4 If extra catalyst is added at equilibrium, how will this affect the total amount of methanol produced? (1)

3.2.5 If more carbon monoxide is added, how will this affect the yield of methanol? (1)

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QUESTION 4 ELECTROCHEMICAL CELLS AND BATTERIES

4.1 An electrochemical cell based on the reaction below may be set up using the apparatus shown.

5 Fe2+ + MnO4- + 8 H+ → 5 Fe3+ + Mn2+ + 4 H2O

Two Platinum Acidified KMnO4

U-tube cotton wool Solution ofElectrodes solution KNO3

Voltmeter Solution of Leads FeSO4

4.1.1 Draw a sketch of the cell set up. Label all the apparatus you used in the table above. Show clearly where the various solutions are used. (5)

4.1.2 Show on the diagram the direction electron flow through the circuit and voltmeter.(1)

4.1.3 Write down the half-reaction that occurs at the cathode. (2)

4.1.4 Identify the reducing agent in the overall reaction? (1)

4.1.5 Calculate the cell emf assuming standard conditions. (4)

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4.2 The Nickel-Cadmium cell is an example of a small and lightweight secondary cell. A schematic drawing of the cell is shown below:

The plate to be used as the anode is impregnated with nickel salts and the plate to be used as the cathode is impregnated with cadmium salts. Both are treated with potassium hydroxide prior to use. The active anode material is cadmium and the active cathode material is nickel(III) hydroxide with potassium hydroxide as the electrolyte.

The half-reactions occurring at each electrode are:

Anode: Cd(s) + 2 OH- (aq) → Cd(OH)2 (s) + 2 e-

Cathode: Ni(OH)3 (s) + e- → Ni(OH)2 (s) + OH- (aq)

The cell develops a potential difference of 1,25 V and it is reversible. No gases are produced during its operation.

4.2.1 Write down the overall reaction for the cell. (2)

4.2.2 Explain the following underlined words from the text above.

(a) electrolyte

(b) secondary cell (4)

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QUESTION 5 ENERGY AND CHEMICAL BONDING5.1 The following information relates to the use of ethanol as a fuel. The Upside of Ethanol

Ethanol is a renewable fuel that comes from agricultural feedstock, and thus can be produced domestically.

Using ethanol (particularly E85, an 85-percent blend of ethanol to petrol) also results in less pollution, reducing smog-forming emissions by as much as 50 percent relative to petrol. E85-powered vehicles also contribute to global warming, although experts disagree about just how much greenhouse gas is emitted when using ethanol.

One might expect that by using E85, net carbon dioxide emissions would be almost zero. The sugar cane in Kwa-Zulu Natal used to make the ethanol absorbs CO2 from the atmosphere during its growth, and then this CO2 is put back into the atmosphere when the ethanol is burned in an automobile engine. Distilling ethanol is however, also an energy-intensive process that often uses electricity generated from coal, another source of greenhouse emissions.

A University study also contradicts a common criticism of ethanol: that it takes more energy to produce it than it delivers as a motor fuel. The study concludes that ethanol made from sugar cane does indeed have a positive "net energy balance," particularly if you consider that other valuable products, such as sugar and acetone, are byproducts of the ethanol-making process.

The DownsideE85 may be better for the environment and the South African farmer, but it has some drawbacks.

The first is price: ethanol prices fluctuate on a different cycle from petrol, meaning that ethanol is sometimes cheaper but sometimes more expensive.

Ethanol, regardless of the price you pay for it, contains less energy than petrol. This means that your car won't go as far on a litre of E85, and your fuel economy will decrease by 20-30 percent.

Another issue is that E85 is not widely available only in the South Africa. The Department of Energy Affairs lists several hundred E85 stations in the country, but nearly half of those are in two regions: Gauteng the financial hub of South Africa with the most number of motorists and motor cars in the country, and Durban.

To put things in perspective, there are more than 180,000 stations nationwide selling fuel. While all of them may not need to offer E85, it is clear that wider distribution is needed before E85 can begin to displace petrol sales.

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Using ethanol made from sugar cane instead of crude oil would lead to a moderate 13 percent reduction in greenhouse emissions. Using cellulosic ethanol from feedstock such as sugar cane pictured here could result in 88 percent less greenhouse gas emissions.

A table of bond energies for various bonds involving the elements carbon, hydrogen and oxygen is provided.

Bond Energy Bond Energy

C C 347 O=O 494

H H 436 H O 460

C O 343 C=O 707

H C 414

5.1.1 Using the bond energies (given in kJ.mol-1) in the table, calculate theenergy of reaction (ΔHr) for the combustion of ethanol. The value for the combustion of C6H14 - hexane is given too.

2CH3CH2OH + 7O2 4CO2 + 6H2O

2C6H14 + 19O2 12CO2 + 14H2O ∆Hr = -5 400 kJ∙mol-1

(Assume that all reactants and products are in the gaseous phase.) (5)

5.1.2 Explain by referring to bond energies why the enthalpy of reaction for hexane a constituent of petrol is large and negative. (2)

5.1.3 Given that the energy output per mol for hexane is 5 400kJ, calculate the energy output per kilogram for hexane. (3)

5.2 The upside and downside of the use of ethanol is discussed in the article above.

5.2.1 Use the calculations you have done in 5.1 above as well as information supplied in the article to explain why it is that greater use of E85 has not been made in South Africa. (4)

5.2.2 Why do you think that the greatest usage of E85 is in the Gauteng and Durban regions? Support your answer with at least two reasons from the article.

(4)[18]

QUESTION 6 THE CHEMICAL INDUSTRY AND SCIENTIFIC METHOD

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The reaction of the industrial process for the production of ammonia gas is

Reactants and product are in the gaseous phase during the reaction. The data for the yield of ammonia gas under various temperature - pressure conditionsobtained in an experimental plant are given in the table below.

PERCENTAGE YIELD OF NH3 AT EQUILIBRIUMPressure (atmospheres)

Temp (C) 10 30 50 100 300 600 1000200 5.0300 14.7 30.3 39.4 52.0 71.0 84.2 92.6400 3.9 10.2 15.3 25.1 47.0 65.2 79.8500 1.2 3.5 5.6 10.6 26.4 42.2 57.5600 0.5 1.4 2.3 4.5 13.8 23.1 31.4700 0.2 0.7 1.1 2.2 7.3 12.6 12.9

6.1 In this experimental plant, a chemical consortium would like to establish the most economically viable way of producing their ammonia. They have done it via the scientific method of investigation.

6.1.1 Write a suitable hypothesis for this investigation? (2)

6.1.2 If each run of the above table is done at a particular temperature, what is the independent variable? (1)

6.1.3 What is the dependant variable? (1)

6.1.4 What would be a control variable in this investigation? (1)

6.1.5 Draw a graph of percentage yield vs. pressure at 5000C (4)

6.1.6 Explain why the conditions resulting in a percentage yield of 42,2% is chosen as the most economical conditions for the production of NH3 (3)

6.2 3 moles of nitrogen gas is mixed with 6 moles of hydrogen gas in a reaction vessel with a volume of 2 dm3. Equilibrium is established at 300 K and an analysis shows that 3 moles of NH3 are present at this equilibrium.

6.2.1 Calculate the equilibrium constant Kc at 300 K. (6)

6.2.2 How the value of Kc will change when the temperature is raised to 500 K? Explain. (3)

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QUESTION 7 ORGANIC CHEMISTRY.

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N2 + 3H2 ⇌ 2NH3 (ΔH < 0)

7.1 Esters are organic compounds that all have distinctly fruity smells. They can be prepared in the laboratory by reacting an alcohol with a carboxylic acid in the presence of concentrated sulphuric acid.

7.1.1 What function does the sulphuric acid have in the reaction? (1)

7.1.2 Give the correct IUPAC name for each of the following alcohols and carboxylic acids:

H

a) H b) H H O

H C O H H C C C H

H H H H

c) O d) H H H H O H

H C O H H C C C C C O

H H H H(8)

7.1.3 All of these molecules are in the liquid phase at 250C. Even (a) which is the lightest of them all. Using sound scientific reasoning – explain why this is so. (3)

7.1.4 Give the correct word equation for the production of methyl methanoate from its parent molecules. (3)

7.2 Consider a compound with a molecular formula C4H8O.

7.2.1 Draw the structural formula of a ketone that has the molecular formula C4H8O. (2)

7.2.2 Draw the structural formula of two aldehydes that both have the formula C4H8O. (4)

7.2.3 C4H8O is also the formula of an unsaturated alcohol. Draw a suitable structural formula for this molecule. (2)

7.2.4 Referring to inter-molecular forces, briefly explain why you would expect the molecule in 7.2.3 above to be soluble in water. (3)

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7.3 Chlorofluorocarbons (CFC’s) are well known to have a harmful effect on the earth’s ozone layer leading to so-called holes in the ozone layer. Ozone absorbs a large percentage of the high energy ultra-violet radiation from the sun. CFC’s were commonly used as refrigerants and as aerosol propellants. Alternatives to CFC’s are now being developed.

7.3.1 Briefly explain the risks to humans associated with the release of CFC’s into the upper atmosphere and consequent ‘holes in the ozone layer’. (3)

7.3.2 Give the correct IUPAC name for the product of the addition reaction between propene and hydrogen chloride. (2)

7.4 One simple homologous series in organic chemistry is the alkanes.

7.4.1 What is the general formula for alkanes? (1)

7.4.2 Give the correct IUPAC name of the alkene shown below. (4)

H

H H H H C H H H

H C C C C C C H

H H H H C H

H

7.5 The reaction of alkanes with halogens is only spontaneous in the case of fluorine. Chlorine and bromine both require light or heat for reaction to occur with alkanes.

7.5.1 Complete the following reaction.

CH3CH3 + Br2 (2)

7.5.2 What type of reaction is this? (1)

7.5.3 Why do we need to expose this reaction to light to get it going? (1)

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QUESTION 8 THE CHEMICAL INDUSTRY.

8.1 Chlorine gas is produced commercially by the electrolysis of brine. Two other products are hydrogen gas and sodium hydroxide. A simplified diagram of a membrane cell is shown below.

B C

A

NaOH

+ -

8.1.1 What is the name given to this industry? (1)

8.1.2 Name the substances that are represented by each arrow labelled A – C. Write the letters A to C in your script with the name of the substance next to it. (3)

8.1.3 Give the half reaction that occurs at the anode. (1)

8.1.4 State the function of the membrane in the membrane cell. (1)

8.1.5 The membrane cell is now the preferred choice for this industry as it is more environmentally friendly than the two other types of cell still found in operation today. Name the two other types of cell and briefly explain the main environmental hazard that each one presents. (4)

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TOTAL SECTION B: 125

GRAND TOTAL: 150

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