CHEMISTRY Stage 2 Sample WACE Examination 2010 Marking Key · PDF file · 2016-08-01SAMPLE MARKING KEY 2 CHEMISTRY ... where large hydrocarbon molecules are broken ... A student was

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CHEMISTRY

Stage 2

Sample WACE Examination 2010

Marking Key

Marking keys are an explicit statement about what the examiner expects of candidates when they respond to a question. They are essential to fair assessment because their proper construction underpins reliability and validity. When examiners design an examination, they develop provisional marking keys that can be reviewed at markers’ meetings and modified as necessary in the light of student responses. This marking key has been developed by examiners in conjunction with the sample examination paper and, as is the case with any external examination developed by the Curriculum Council, is a provisional document that can be modified if necessary in the light of student responses.

SAMPLE MARKING KEY 2 CHEMISTRY STAGE 2

Section One: Multiple-choice 50 Marks

1. b 2. b 3. a 4. d 5. b 6. c 7. d 8. c 9. a

10. b 11. c 12. b 13. c 14. c 15. b 16. d 17. c 18. c 19. a 20. a 21. b 22. b 23. d 24. a 25. d

End of Section One


Section Two: Short answer 80 Marks Question 26 (3 marks) Write the chemical formula for each of the following:

Description Marks (a) sulfur trioxide SO3 (b) ammonium sulfide (NH4)2S (c) copper(II) sulfate-5-water CuSO4.5H2O

1 mark each

Total 3 Question 27 (3 marks) Name each of the following compounds

Description Marks (a) N2O dinitrogen monoxide (b) AlCl3 aluminium chloride (c) Fe(OH)3 iron(III) oxide

1 mark each

Total 3 Question 28 (4 marks) (a) Using examples, explain what is meant by the term ‘isotope’? (2 marks)

Description Marks Isotopes are atoms of the same element with different numbers of neutrons/Atoms with the same atomic number but different mass numbers or similar. (1 mark)

1 mark

E.g. carbon exists as a number of different isotopes such as C-12 and C-13 (any reasonable example and use of symbols)

1 mark

Total 2 (b) Would you expect two isotopes of the same element to have similar or different

chemical properties? Explain your answer. (2 marks)

Description Marks Similar chemical properties 1 mark Chemical properties are dependent on number of valence electrons in the atoms which are the same in different isotopes. 1 mark

Total 2


Question 29 (10 marks) (a) Complete the following table. (8 marks)

Symbol Atomic Number

Mass Number

Number of

Protons

Number of

Electrons

Number of

Neutrons Electron

Configuration

C126

6 12 6 6 6 2, 4

+241

20Ca

20 41 20 18 21 2, 8, 8

Description Marks Each entry one mark 8 ×1 mark Total 8 (b) Explain why lithium, sodium and potassium all produce ions with a single positive

charge and are placed in the same group on the Periodic Table. (2 marks)

Description Marks All of these elements have one valence electron and so will form a single positive ion. 1 mark

The elements in a column in the main block have the same number of valence electrons. 1 mark

Total 2

Or similar statements. Both points must be addressed for the 2 marks. Question 30 (11 marks) Litmus is an indicator that can be used to identify if an aqueous solution is acidic or basic. It is red in acidic solutions and blue in basic solutions. (a) Classify the following aqueous solutions as either acid or basic, and give colour of litmus

when added to those solutions. (b) Suggest a use around the home.

Solution Acidity Colour of Litmus Household Use NaOH (Caustic soda) Basic Blue Drain cleaner or

Oven cleaner HCl (Spirits of salt) Acidic Red Brick cleaner

CH3COOH (found in vinegar)

Acidic Red Flavouring or

Cleaning agent

NaHCO3 (Baking soda)

Basic

Blue Rising agent or

Cleaning agent


Description Marks

Each entry one mark 11 ×1 mark Total 11

Accept any valid use around the home, one per substance Question 31 (6 marks) Ozone is important in reducing the amount of ultraviolet radiation reaches the surface of the earth. There are a number of gases, called ozone depleting chemicals, which enter the upper atmosphere and reduce the concentration of ozone. Chlorofluorocarbons (CFCs) are a group of such chemicals. Yet, when chlorine is combined with sodium to form sodium chloride, the compound produced is used to enhance the flavour of foods and as a preservative. For each of the species listed on the table below draw an electron dot diagram showing all valence electrons. Use : or – to represent a pair of electrons.

Species Electron dot diagram Sodium chloride NaCl

Cl –[ ]- Na +[ ]

1-Chloro-2-fluoroethane (C2H4ClF)

Ozone (O3)

Description Marks Each structure two marks 3 × 2 marks -1 overall if non-bonding electrons missing -1 overall if brackets/charge missing Total 6

OOO

C

H

H

ClC

H

F

H


Question 32 (5 marks) In the refining of oil a number of chemical processes occur. Two of these processes are known as thermal cracking and catalytic cracking, where large hydrocarbon molecules are broken down into smaller hydrocarbon molecules. (a) Will the boiling points and vapour pressure of the large hydrocarbon molecules be

higher than, lower than or the same as the small hydrocarbon molecules? (2 marks)

Description Marks Boiling point: higher 1 mark Vapour pressure: higher 1 mark Total 2 (b) Explain your answer to part (a). (3 marks)

Hydrocarbons are covalent molecular substances. Boiling point is dependent upon the strength of the intermolecular forces. Larger molecules have larger intermolecular forces More energy is required to break the bonds, therefore boiling point is higher.

Description Marks

Three valid points 3 marks Two valid points 2 marks One valid point 1 mark Total 3 Question 33 (12 marks) Draw the structural formulae and name organic compounds that fit each of the following descriptions. (a) The product formed when propene reacts with bromine. (3 marks)

Structure Name

1,2-dibromopropane C

H

H

H C

H

Br

C

H

H

Br


(b) The geometric isomers for pent-2-ene. (6 marks)

Structure Name

CH

H

H

C

H

C

H

CH

HC

H

HH

Cis-pent-2-ene

CH

H

H

C H

CH

CH

H

CH

H

H

Trans-pent-2-ene

(c) A cyclic isomer for C6H12. (3 marks)

Structure Name

Cyclohexane (or any other valid structure)

C

HH

CH

H CHH

C

HH

CH

HC HH

Description Marks Structure (2 marks) plus name (1 mark) 3 marks × 4 = 12 Where a structure is incorrect but the name matches the structure 1 mark each = 4 -1 overall if hydrogens missing from full structures -1 overall if skeletal structures used with hydrogens present Total 12 Full structural formulas, condensed structural formulas are both acceptable


Question 34 (4 marks) Balance each of the following half-equations and identify them as either oxidation or reduction processes. (a) Sn2+ → Sn4+ + 2e- oxidation (b) 2 H2O → O2 + 4 H+ + 4e- oxidation

Description Marks each correctly balanced equation 1 mark × 2 = 2 identifying as oxidation or reduction 1 mark × 2 = 2 Total 4 Question 35 (7 marks) A student was given a sample of molten lead(II) bromide to electrolyse. (a) Draw a labelled diagram of the electrolytic cell. On the diagram show

(i) direction of electron movement (ii) anode and cathode (iii) direction of ion movement (4 marks)

e-

Br- Pb2+

anodecathode

Description Marks diagram 1 mark (i) shown 1 mark (ii) shown 1 mark (iii) shown 1 mark Total 4


(b) Write half-equations for the anode and cathode processes, and the overall equation. (3 marks) Anode equation

2 Br- → Br2 + 2e-

Cathode equation

Pb2+ + 2 e- → Pb

Overall equation

Pb2+ + 2Br- → Pb + Br2

Description Marks

Each equation 1 mark × 3 Total 3 Question 36 (8 marks) Sulfur dioxide is added to wine to prevent undesirable redox reactions. The gas can be produced in the laboratory by adding hydrochloric acid to solid sodium hydrogensulfite (NaHSO3) to produce sulfur dioxide, sodium chloride and water. (a) Write a balanced equation for this reaction. (2 marks) NaHSO3(s) + H+(aq) → Na+(aq) + SO2(g) + H2O(l)

Description Marks Correct formulae 1 mark balanced 1 mark Total 2 (b) Is the reaction in (a) a redox reaction? Explain your answer. (2 marks)

Description Marks No 1 mark There is no change in oxidation number (or similar statement) 1 mark Total 2 (c) What would be the colour of the final solution? (1 mark)

Description Marks Colourless 1 mark Total 1 (d) Write an equation to represent the dissolving of sulfur dioxide in water. (2 marks) SO2(g) + H2O(l) → H2SO3(aq) SO2(g) +H2O(l) → H+(aq) + HSO3

-(aq)

Description Marks Correct formulae (either one) 1 mark balanced 1 mark Total 2


(e) Describe how you could test to see if sulfur dioxide was being produced and give the expected observation. (2 marks)

Damp blue litmus – will turn red

Smell – pungent odour

Description Marks Any of the above tests (or any reasonable alternative) 1 mark Corresponding observation 1 mark Total 2 Question 37 (7 marks) A 4.322 g sample of baking soda (Na2CO3) was found to be contaminated with barium chloride (BaCl2). An excess of sulfuric acid was added to the mixture and the precipitate formed (BaSO4) was collected, washed, dried and weighed. The mass of barium sulfate was found to be 0.326 g. (a) Write an equation to represent the formation of barium sulfate. (2 marks) Ba2+(aq) + SO4

2-(aq) → BaSO4(s)

Description Marks Correct formulae 1 mark balanced 1 mark Total 2 (b) Calculate the percentage by mass of barium chloride in the contaminated sample. (4 marks)

Description Marks n(BaSO4) = 0.326

137.3 + 32.06 + 64 1

= 1.40 x 10-3 mol = n(BaCl2)

1

m(BaCl2) = (1.40 x 10-3) x (137.3 + (2 x 35.45)) = 2.91 x 10-1g

1

% BaCl2 = 0.2914.322

x 100%

= 6.73% 1

Total 4

End of Section Two


Section Three: Extended answer 70 Marks Question 38 (15 marks) Dilute solutions of hydrogen peroxide (H2O2) are used as hair bleach. Hydrogen peroxide will spontaneously decompose at room temperature to produce oxygen gas and water.

2H2O2 → 2H2O + O2 (b) It is recommended that hydrogen peroxide be stored in the refrigerator. Explain this

statement in terms of rates of reactions. (3 marks)

Description Marks Reducing the temperature reduces the rate of the decomposition of H2O2. 1 mark At lower temperatures fewer particles have sufficient energy to react. 1 mark At lower temperatures, the average speed of the particles is reduced and so the particles will collide less often. 1 mark

Total 3 (b) From the information provided in this question, is the activation energy for this reaction

most likely to be relatively large or small? (1 mark)

Description Marks Relatively small 1 mark Total 1 (f) Given that the change in enthalpy for this reaction is -390 kJ, draw an energy profile

diagram showing

(i) labelled axes (ii) activation energy (iii) ΔH (iv) transition state (5 marks)

H transition state catalyst pathway Ea ΔH Reaction progress


Description Marks

Exothermic curve 1 mark Axes labelled 1 mark Activation energy shown 1 mark ΔH shown 1 mark Transition state shown 1 mark Total 5 (g) Show and clearly label, on the reaction profile diagram in part (c), the effect of the

addition of the catalyst to the reaction. (1 mark)

Description Marks Line shown 1 mark Total 1 In the school laboratory, more concentrated solutions of hydrogen peroxide are used, with the addition of a manganese dioxide (MnO2) catalyst, to produce oxygen. (c) Explain why more concentrated solutions would be used in the laboratory. (2 marks)

Description Marks There are more particles per unit volume, therefore there will be more particles colliding per unit time. 1 mark

More collisions will result in a faster reaction rate which is required for a laboratory preparation. 1 mark

Total 2 (d) Explain why MnO2 is classified as a catalyst and how it increases the rate of reaction.

(3 marks)

Description Marks MnO2 is a catalyst because it is not consumed in the reaction (do not accept ‘does not take part in the reaction’) 1 mark

The MnO2 produces an alternative reaction pathway with a lower activiation energy. 1 mark

More particles now have sufficient energy to react. 1 mark Total 3 Question 39 (9 marks) The reaction between aluminium metal and concentrated sodium hydroxide is often represented to produce hydrogen gas to fill weather balloons. The reaction can be represented by the following equation:

2 Al(s) + 2 OH-(aq) + 3 H2O(l) → 2 (Al(OH)4)-(aq) + 3 H2(g) At a weather station 863 g of aluminium was added to 7.50 L of 4.55 mol L-1 solution of sodium hydroxide.


(a) Determine the limiting reagent of this reaction (show all working). (5 marks)

Description Marks Calculates moles of aluminium n(Al) = 863

26.98

= 32.0 mol

1 mark

Calculates moles of hydroxide n(OH-) = 7.50 x 4.55 = 34.1 mol

1 mark

Works out mole ratio 1 mole of Al requires 1 mol OH-

32 moles of Al requires 32.0 mol OH- 2 marks

Identifies Al as LR n(OH- required) > n(OH- available) therefore Al is LR

1 mark

Total 5 (b) Calculate the volume of hydrogen gas produced at S.T.P. (2 marks)

Description Marks Calculates moles of hydrogen n(H2) = 3

2 (32.0)

= 48.0 mol

1 mark

Calculates volume hydrogen v(H2) = 22.4 x 48.0 = 1.07 x 103 L

1 mark

Total 2 (c) Suggest how you might safely dispose of the excess reagent and why you would use

that method. (2 marks) Dilute with large quantities of water – dilute bases reduces rate of reaction. Neutralise with a weak acid such as acetic acid – will react with the base and is

not as dangerous as a strong acid.

Description Marks Suggested method (any reasonable – suggestions above) 1 mark Supporting reason 1 mark Total 2


Question 40 (11 marks) Caproic acid is the chemical responsible for the smell of dirty socks and contains carbon, hydrogen and oxygen only. It is a weak, monoprotic acid. A 0.531 g sample of the acid was combusted to produce 1.21 g of carbon dioxide and 0.493 g of water. (a) Determine the empirical formula of the acid. (6 marks)

Description Marks Calculates mass carbon

n(C02) = 1.2144.01

= 2.75 x 10-2 mol m(C) = (2.75 x 10-2) x 12.01 = 0.3303 g

1 mark

Calculates mass hydrogen

n(H2O) = 0.49318.016

= 2.74 x 10-2

m(H) = 2 x 1.008 x 2.74 x 10-2 = 5.52 x 10-2 g

1 mark

Calculates mass oxygen m(O) = 0.531 - (0.3303 + (5.52 x 10-2)) = 0.146 g

1 mark

Calculates moles oxygen

n(O) = 0.14616.00

= 9.12 x 10-3 mol

1 mark

Determines C: H: O ratio n 2.75 x 10-2 5.52 x 10-2 9.12 x 10-3 ratio 3 6 1

1 mark

Determines empirical formula EF = C3H6O 1 mark

Total 6


(b) Given that the molar mass of caproic acid is 116 g mol-1, determine the molecular

formula of the acid. (3 marks)

Description Marks Calculates empirical formula mass EFM = 58.058 1 mark

Calculates ratio MFM:EFM

MFMEFM

= 11658.058

1 mark

Determines molecular formula MF = C3H12O2

1 mark

Total 3 (c) Zinc oxide can be used as a foot powder to reduce the smell. Write an equation for the

reaction between zinc oxide and caproic acid solution. (2 marks)

Description Marks Correct formulae 2C6H12O2 + ZnO → 2C6H11O2

-+ Zn2+ + H2O 1 mark

balanced 1 mark Total 2 Accept alternative but correct formulas for caprioc acid. Question 41 (25 marks) When soils containing iron pyrite (FeS2) are exposed to air, the following reaction can occur.

2 FeS2(s) + 7 O2(g) + 2 H2O(l) → 2 Fe2+(aq) + 4 SO42-(aq) + 4 H+(aq)

These types of soil are called acid sulfate soils. The pH of groundwater in these soils will decrease. If this groundwater discharges into lakes and rivers it will also cause their pH to decrease. (a) Explain how this reaction causes the pH of groundwater to decrease. (3 marks)

Description Marks pH is a measure of the concentration of hydrogen ions. 1 mark The reaction produces hydrogen ions. 1 mark The greater the concentration of hydrogen ions, the lower the pH. 1 mark Total 3


(b) Determine the oxidation numbers of sulfur and oxygen in the sulfate ion. (2 marks)

Element Oxidation number in SO42-

S +6

O -2

Description Marks Each oxidation number 1 mark Total 2 (c) Water that comes into contact with acid sulfate soils will contain sulfuric acid which will

cause the corrosion of steel (mainly iron) and concrete (which contains calcium oxide). Write equations for the reaction of sulfuric acid with the following substances.

For each reaction describe what you would be expected to observe including any colour

changes, precipitate formation (giving colour) and gas formation (giving colour and odour). (6 marks)

(i) Iron metal is added to dilute sulfuric acid.

Equation: Fe(s) + 2H+(aq) → Fe2+(aq) + H2(g) (also accept the equation for the formation of Fe3+)

Observation: Colourless, odourless gas evolved; pale green solution formed.

(ii) Calcium oxide solid is added to dilute sulfuric acid.

Equation: CaO(s) + 2H+(aq) → Ca2+(aq) + H2O(l) Observation: white solid dissolves.

Description Marks Correctly balanced equation 2 marks × 2 observation 1 mark × 2 Total 6

-1 if not ionic, otherwise -1 for each error. Observation only accepted if equation has some merit

A series of trials were conducted to find the most effective way to neutralise a lake that had become acidic due to the effect of acid sulfate soils. Three samples of 1000 L of lake water were placed in tanks. Different chemicals were added to two of the tanks and the pH was measured each day for seven days. The water in Tank 1 has been left untreated. Solid calcium carbonate has been added to Tank 2 and a solution of potassium hydroxide has been added to Tank 3.


Table of pH Values Days Conditions 1 2 3 4 5 6 7 Tank 1 (Untreated water)

3.5 3.6 3.6 3.7 3.7 3.7 3.8

Tank 2 (Solid CaCO3)

3.5 4.1 4.6 5.2 5.7 5.9 6.0

Tank 3 (KOH solution)

3.5 6.5 7.8 8.0 8.4 7.9 7.8

(d) List three variables that should be controlled in this experiment. (3 marks)

Volume of water Size and type of tank Time of addition of chemicals

Description Marks

Any three reasonable answers (suggestions above) 1 mark × 3 Total 3


(e) Draw graphs for the three sets of data on the one pair of axes. (5 marks) Any reasonable title

I I I I I I I 1 2 3 4 5 6 7

Time (days)

7 –

8 –

3 –

2 –

1 – 0

4 –

5 –

6 –

9 –

pH

Description Marks correctly labelled axes 1 mark title 1 mark each graphed line. 1 mark × 3 Total 5 (f) Write equations to represent the reactions occurring in Tanks 2 and 3. (4 marks)

Tank Equation 2

CaCO3(s) + 2H+(aq) → Ca2+(aq) + CO2(g) + H2O(l)

3

H+(aq) + OH-(aq) → H2O(l)

Description Marks

Correct formulae 1 mark × 2 balanced 1 mark × 2 -1 if not ionic -1 for each error otherwise Total 4


(g) Based on safety, environmental impact and sustainability, list two extra pieces of

information that you would need in order to decide which chemical to use. (2 marks) Purchase cost Toxicity Corrosiveness Persistence in the environment

Description Marks Any two reasonable answers (suggestions above) 1 mark × 2 Total 2 Question 42 (10 marks) Pure water is a very poor conductor of electricity but presence of a solvent can alter the conductivity. A student tested the conductivity of the following 0.100 mol L-1 solutions:

Non-electrolyte Weak electrolyte Strong electrolyte

ethanol (C2H5OH), acetic acid (CH3COOH), sulfuric acid (H2SO4) sodium chloride (NaCl).

(a) Rank these solutions from poorest conductor to best conductor. (4 marks) Poorest conductor Ethanol Acetic acid Nitric acid Best conductor Magnesium chloride (1 mark for each correct answer)

Description Marks Each correct answer 1 mark × 4 Total 4 From your understanding of bonding, explain your ranking with the aid of equations. There are a number of ways students may approach this answer. The following points should be addressed: Ability of solutions to conduct is dependent upon the concentration of ions in solution. Ethanol is a covalent molecular substance and exists as molecules in solutions. There are no mobile charges and so the solution does not conduct. Acetic acid is a weak acid (or weak electrolyte). Some of the molecules ionise/react with water to produce ions. The concentration of ions is low and so conductivity is poor. Sulfuric acid is a strong acid (or strong electrolyte) and it is diprotic, so there will be between 0.200 and 0.300 mol of ions per litre of solution. H2SO4(aq) → 2 H+(aq) + SO4

2-(aq)


Sodium chloride is an ionic compound and therefore a strong electrolyte. It will produce 0.200 mol of ions per litre of solution. NaCl(aq) → Na+(aq) + Cl-(aq) The greater concentration of ions in the H2SO4 solution mean it will be a better conductor than NaCl.

Description Marks Full explanation (all four substances) plus two equations 1 mark × 6 -1 for each error or omission Total 6

End of questions


ACKNOWLEDGEMENTS Section Three Question 41 Adapted from:

WestOne Services. (2009). Chemistry Unit 2B: Chemistry and the environment: Teaching resource (SCIENCE1053). West Perth, WA: WestOne Services. WestOne Services. (2009). Chemistry Unit 2B: Chemistry and the environment: Learning resource (SCIENCE880). West Perth, WA: WestOne Services.

This examination paper – apart from any third party copyright material contained in it – may be freely copied, or communicated on an intranet, for non-commercial purposes in educational institutions, provided that it is not changed and that the Curriculum Council is acknowledged as the copyright owner. Teachers in schools offering the Western Australian Certificate of Education (WACE) may change the examination paper, provided that the Curriculum Council’s moral rights are not infringed. Copying or communication for any other purpose can be done only within the terms of the Copyright Act or with prior written permission of the Curriculum Council. Copying or communication of any third party copyright material can be done only within the terms of the Copyright Act or with permission of the copyright owners.

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CHEMISTRY Stage 2 Sample WACE Examination 2010 Marking Key · PDF file · 2016-08-01SAMPLE MARKING KEY 2 CHEMISTRY ... where large hydrocarbon molecules are broken ... A student was