PEM 2019 Chemistry Trial HSC Answers PDF

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2019 Year 12 Chemistry Trial examination. Marking Guidelines and model Answers. Section I 1 D

Multiple Choice 2 D

3 B

4 A

5 C

6 B

7 D

8 A

9 C

10 D

11 B

12 B

13 C

14 B

15 C

16 A

17 D

18 B

19 C

20 C

Section II 21. Marking guidelines ● Correctly calculates the value of Q and states that the system is not at equilibrium ● Uses equilibrium expression in some form but gets incorrect answer ● States the equilibrium expression in some form

Marks 3 2 1

Sample answer T od e t e r mi nei ft hes y s t e mi sate qu i l i br i umi ti sne c e s s a r yt oc al c u l a t eQ, t h er e a c t i onq uot i e nt , ands e ei fi ti st hes ameasK.

Th e r e f or e ,t hes y s t e mi sn ota te q ui l i b r i u m.

22. a. Marking guidelines ● Wr i t e st h ec or r e c tba l a nc e de q ua t i o nu s i n gc he mi c a ls y mb ol s ● Wr i t e sac or r e c twor de qu a t i on OR ● Gi v e st hec or r e c tf or mu l af o rc a r b onmono xi dea ndf o rc h l o r i neg a s

Marks 2 1

Sample answer CO(g) + Cl2(g) ⬄ COCl2(l) △H = -ve 22. b Marking guidelines ● Correctly identifies one factor which influences the equilibrium and describes the way in which this factor can be used to maximise production of phosgene, using their answer in part (a) ● Co r r e c t l yi d e nt i fie sonef a c t orwhi c hi n flu e nc e st hee q ui l i br i u m OR ● Re l a t e sLeCha t e l i e r ’ spr i n c i pl et oma x i mi s i n gpr o duc t i onb yi n di c a t i n gc or r e c te q ui l i br i u ms h i f t

Marks 2 1

Sample answer Increasing the pressure/decreasing the volume of the reaction vessel will shift the equilibrium in the forward direction. According to Le Chatelier, a shift in the forward direction will counteract the change which results in more product being formed.

2 3. Marking guidelines ● Correct solubility expression and applies this value to the solubility AND ● Correctly explains shift in equilibrium AND ● Correctly deduces the continued use of hydrogen sulphide in solution. Correctly addresses TWO of the criteria above Correctly addresses ONE of the criteria above

Marks 3

2 1

Sample answer AgCl(s) ⬄ Ag+(aq) + Cl-(aq) Silver chloride is very insoluble with a Ksp of 1.77 x 10-10. This means very little reactant will be converted to their ions in water at equilibrium. The reaction between silver ions and hydrogen sulphide will reduce the silver ion concentration. This decreases the silver ion concentration shifting the equilibrium in the forward direction to produce more silver ions. This more silver chloride will dissolve as the equilibrium is counteracted. If hydrogen sulphide continues to be bubbled through the solution, eventually all silver chloride will dissolve and be replaced by black silver sulphide.

24. a. Marking guidelines ● Provides correct balanced equation ● Correctly calculates the value of the unknown using the Ksp expression ● Calculates the concentration of Pb2+ in water ● Calculates the concentration Cl- in water Correctly addresses THREE of the criteria above Correctly addresses TWO of the criteria above Correctly addresses ONE of the criteria above

Marks 4

3 2 1

Sample answer

24. b. Marking guidelines ● Provides assumption of [Cl-] ● Correctly calculates the solubility of PbCl2 ● Qualitatively and quantitively compares the differences in solubility Correctly addresses TWO of the criteria above Correctly addresses ONE of the criteria above

Marks 3

2 1

Sample answer

2 5.a . Marking guidelines

Marks 1

Marking guidelines

Marks 1

● Correct equation Sample answer OH- (aq) + HA(aq) → H2O(l) + A (aq) Blue 2 5.b. ● Correct equation Sample answer H3O+ (aq) + A- (aq) → H2O(l) + HA(aq) yellow 2 5.c . Marking guidelines ● Explains why the solution can act as an indicator, as the colour changes with changes in pH / changes in concentration of H3O+ ● AND ● Refers to the reactions in parts (a) and (b) ● Explains why the solution can act as an indicator, as the colour changes with changes in pH / changes in concentration of H3O+

Marks 2

1

Sample answer -

The solution is a green equilibrium mixture of HA (yellow) and A (blue). The equilibrium in the solution can be represented: a q) a q)+ H3O+ ( HA ( aq)+ H2O ( l )⬄ A-( yellow blue When a strong base is added, the reaction in (a) occurs and the indicator solution turns blue. Considering the equilibrium reaction above, the green solution turns blue, as the concentration of H3O+ decreases and the indicator equilibrium shifts to the right. When a strong acid is added, the reaction in (b) occurs. Considering the equilibrium reaction above, the green solution turns yellow, as the equilibrium shifts to the left as the concentration of H3O+ increases. Hence the green solution can act as an indicator as it is a different colour in acidic and basic solutions; i.e. in solutions of different pH.

26. Marking Guidelines ● outlines the Arrhenius and Lowry-Bronsted definitions of acids and bases ● explains why the Arrhenius definition cannot account for NaHCO or being a base ● identifies that the Arrhenius definition cannot account for NaHCO being amphiprotic ● explains how the Lowry-Bronsted definition accounts for its amphiprotic behaviour ● includes two or more correct, relevant chemical equations Correctly addresses FOUR of the criteria above Correctly addresses THREE of the criteria above Correctly addresses TWO of the criteria above Addresses ONE of the criteria above

Marks 5

3

3

4 3 2 1

Sample answer The Arrhenius definition of acids/bases is that they ionise in water to give H+ ions, and bases ionise in water to produce OH- ions. This definition does not account for the behaviour of NaHCO3: ● it gives basic aqueous solutions even though it does not contain OH- ions ● it reacts with both acids and bases The Lowry-Bronsted definition is that acids are proton donors and that bases are proton acceptors. NaHCO3 can be classified as an acid or a base using this definition, depending on reaction conditions. Thus, it produces basic aqueous solutions because when it dissolves it ionises to give Na+ ions and HCO3- ions. The hydrogencarbonate ions accept a proton from water, producing OHions and hence a pH greater than 7: HCO3- + H2O � H2CO3 + OH- . In this case it is behaving like a Lowry-Bronsted base. However, in solutions of weak bases it can act as an acid, donating a proton: HCO3- + PO42- � HPO4- + CO32-. Thus the LowryBronsted definition can account for the basic and amphiprotic behaviour of NaHCO3. -

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27. a. Marking Guidelines Outlines the steps in a method used to standardise sodium hydroxide solution by reacting it with an acid of known concentration

Marks 1

Sample answer The sodium hydroxide solution must be standardised by titrating it with an acid of exactly known concentration. This can be a primary standard (can be accurately weighed out) such as oxalic acid or a solution of, for example, hydrochloric acid which has been previously standardised against a primary standard such as anhydrous sodium carbonate. 27. b. Marking Guidelines Correct answer to 3 significant figures Correct answer to incorrect number of sig. figs. Sample answer

Marks 2 1

27. c. Marking Guidelines ● Identifies an appropriate indicator and justifies the choice in terms of the indicator colour change ● the pH at the equivalence point of the titration Correctly addresses ONE of the above criteria

Marks 2 1

Sample answer The titration involves a strong acid and a strong base. As a result, at the equivalence point, the pH will be at 7. Litmus solution and bromothymol blue are the best indicators as they change colour at pH = 7. (However, because of the rapid change in pH within a drop of the equivalence point, any of methyl orange, phenolphthalein, litmus and bromothymol blue can be used as answers as long as they are correctly justified.)

28. Marking Guidelines Correct balanced equation Correctly calculates pH Correctly calculates concentrations of species at equilibrium with assumption made Correctly calculates pOH Correctly addresses THREE of the criteria above OR ● Correctly addresses all criteria without making an assumption Correctly addresses TWO of the criteria above Addresses ONE of the criteria above ● ● ● ● ●

Marks 4

3

2 1

Sample answer

29. Marking Guidelines ● Calculates average titrant used AND ● Correctly writes the balanced chemical equation AND ● Calculates moles and mass of citric acid AND ● Calculates percentage of citric acid in orange juice AND ● Assessment is made Correctly addresses FOUR of the criteria above Correctly addresses THREE of the criteria above Correctly addresses TWO of the criteria above Addresses ONE of the criteria above

Marks 5

4 3 2 1

Sample answer

30. ● ● ● ● ● ● ● ● ●

Mar ki ngGui de l i ne s Thoroughly explains the intermolecular forces and polarity of Butyl acetate Thoroughly explains the intermolecular forces and polarity of Butan-1-ol Thoroughly explains the intermolecular forces and polarity of Acetic acid Compares the structure of all three compounds. Compares and contrast all three compounds with regards to their boiling point and molar masses. Correctly addresses FOUR of the criteria Correctly addresses THREE of the criteria Provides a limited understanding of intermolecular bonding and structure of all three compounds Identifies the types of intermolecular forces present with all three compounds

Marks 5

4 3 2 1

Sample answer Despite having different molar masses, all three molecules have similar boiling points due to the different structures and resulting intermolecular forces. Butyl acetate has the largest molar mass and therefore greatest dispersion forces but is only slightly polar and has no hydrogen bonding. Butan-1-ol has lower molar mass than butyl acetate and therefor smaller dispersion forces but it is polar and contains a hydrogen bound to an oxygen. Therefore, it exhibits hydrogen bonding resulting in strong intermolecular forces. Acetic acid has the lowest molar mass and so the weakest dispersion force, but it is polar and contains a hydrogen bound to an oxygen allowing the formation of hydrogen bonds between molecules. The presence of a second oxygen in acetic acid increases the hydrogen bonding compared with butan-1-ol. Therefore all three molecules have similar total strength of intermolecular forces and similar boiling points.

31.a. Mar ki ngGui de l i ne s ● Co r r e c tba l a nc e de q ua t i o nwi t hs t a t e s

Marks 1

Sample answer C2H5OH(l) + 3O2(g) � 2CO2(g) + 3H2O(l) 31.b. Mar ki ngGui de l i ne s ● Co r r e c the a tr e l e a s e dwi t hun i t sa nd2s i gni fic a ntfigu r e s

Marks 1

Sample answer

31.c. Mar ki ngGui de l i ne s ● Ca l c ul a t e se n e r gyp r odu c e df r o mc ombu s t i ons AND ● Ca l c ul a t e sma s sofe t h a no l ● Ca l c ul a t e se n e r gyf r o mr e l e a s e df r omt hec a mp i n gs t o v e

Marks 2

1

Sample answer

32. a Marking Guidelines ● Sho wss t r u c t ur a lf or mul a sf or1pe nt a no lANDa c e t i ca c i d ● Sho wsas t r uc t ur a lf or mu l af or1 pe nt a nolORa c e t i ca c i d Sample answer 1-pentanol: CH3(CH2)3CH2OH

Marks 2 1

acetic acid: CH3COOH

32. b. Marking Guidelines ● Explains TWO reasons for using the reflux technique ● Explains ONE reason for using the reflux technique

Marks 2 1

Sample answer The reaction mixture is boiled under reflux, using an open condenser. This enables the reaction to be carried out at the boiling point, to maximise rate, without loss of reactants by vaporisation. The use of an open condenser avoids any hazardous build-up of pressure in the system.

33. a. Marking Guidelines ● Correct structural formula Sample answer

Marks 1

33. b. Marking Guidelines ● Explains how to distinguish between saturated an unsaturated hydrocarbons ● Fails to mention that Bromine water reacts with double bond

Marks 2 1

Sample answer Bromine water is decolourised instantly by the monomer reacting with the double bond. The polymer has single bonds only and reacts very slowly or not at all. 33. c. Marking Guidelines ● Correctly identifies it as Phenyl Benzene OR Ethenyl Benzene

Marks 1

34. a. Marking Guidelines

Marks 1

Marking Guidelines

Marks 1

● Correctly identifies Pb2+ 34. b. ● Correct ionic equation with states Sample answer Ba2+(aq) + SO42-(aq) � BaSO4(s) 34. c. Marking Guidelines ● Justifies the order of adding HCl, and H2SO4, and the addition of excess HCl to prevent precipitation of PbSO4. ● Justifies the order of adding HCl and H2SO4, and the addition of excess HCl to remove Pb2+ ● Justifies the order of adding HCl and H2SO4 OR gives a justification of adding excess HCl.

Marks 3 2 1

Sample answer The student knew there were two cations. The HCl was added first because from the list of possible cations, it precipitates only Pb2+. If he had added H2SO4 first, it would have precipitated both the Pb2+ and the Ba2+ and there would have been no way of telling that both cations were in the ppt. HCl was added until ppt of the Pb2+ ceased in order to prevent remaining Pb2+ from precipitating with SO42-, which would have provided a false positive in the sulfate precipitation test. The Pb2+ had to be removed to ensure that any precipitate in the sulfate test was not Pb2+, but rather Ba2+.

35. a. Marking Guidelines ● Correct scale AND ● Labels axes with units AND ● Correct plots ● Correctly addresses TWO of the criteria above ● Correctly addresses ONE of the criteria above

Marks 3

2 1

Sample answer

35. b. Marking Guidelines ● Correct upriver plots AND down river plots ● Correctly addresses ONE of the criteria above

Marks 2 1

Sample answer See above in 35. a. 35. c. Marking Guidelines ● Identifies sodium concentration is within safe limits AND ● Correctly explains the change in sodium concentration ● Correctly addresses ONE of the criteria above OR ● Describes the change in sodium concentration

Marks 2

1

Sample answer The concentration of sodium ions downriver are within safe limits for fresh water organisms. During low rainfall periods, the river flow will decrease, reducing the dilution effect. As a result, the sodium ion concentration down river will increase, possibly above the maximum level.

36. a. Marking Guidelines ● Correctly identifies TWO inconsistencies in the mass spectrum and explains why the sample is contaminated ● Identifies One inconsistency and explains why the sample is contaminated

Marks 2 1

Sample answer Results show an additional peak at 32, and a larger peak at 29 which is not in the standard mass spectrum of ethanol. Therefore, the sample is contaminated. 36. b. Marking Guidelines ● Relates both peaks to the mass of possible fragments of methanol ● Relates one peak to a fragment of methanol

Marks 2 1

37. a. Marking Guidelines

Marks 1

Marking Guidelines ● Completes balanced equation with state symbols

Marks 1

● Correct identifies monomer glucose 37. b.

Sample answer C6H12O6 (aq) � 2CO2 (g) + 2H2O(l) 37. c. Marking Guidelines ● Completes balanced equation with state symbols

Marks 1

Sample answer C6H6 (g) � C2H6 (g) + H2 (g) 37. d. Marking Guidelines ● Describes the development and use of a named biopolymer and makes a judgement on its use based on described advantages and limitations of the biopolymer. ● Outlines a recent biopolymer AND identifies an advantage and limitation of the biopolymer ● Outlines the recent development of a named biopolymer ● Identifies a recently developed biopolymer or states an advantage or limitation of the biopolymers in general.

Marks 4 3 2 1

Sample answer Fr o mt hef r a c t i on ald i s t i l l at i o no fc r udeo i l ,f ol l owe db yt he r malandc a t a l y t i cc r ac k i n g ,weo bt ai ne t hy l e newhi c hc anbe p ol y me r i s e di nt oanumb e rofs y nt he t i cpol y me r s .Th ep ol y me r sp r odu c e df r omt h e s emo nome r sh av ev e r yus e f ulpr o pe r t i e s ,an d h av et r ans f or me dt hemode r nwo r l d. Pr odu c t ss uc ha sf oo dwr appi n g , pol y s t y r e nei n s ul at i on,PVCd r ai np i p e s , anda l loft h e u bi qu i t ou spl as t i car o undus , ar er e l at i v e l yr e c e ntmo de r nphe no me na.Theb i g g e s ti s s uewi t ht he s epl as t i c si st hatt he yar enon b i o de g r ada bl e ,a ndc o nt r i but et ot hev a s tamoun tofl a ndfi l lweg e ne r at e ,a ndal s od e s t r o yhab i t a t ,bo t ht e r r e s t r i ala ndaqu at i c , k i l l i n gl a r g enumb e r sofma r i nean i ma l sf ore x ampl e .Fo s s i lf ue l sar eal s ono nr e ne wabl er e s our c e sanda r et h e r e f o r edwi ndl i n g a ndbe c omi n gmor ee x pe ns i v easar e s ul t . Bi op ol y me r sa r eana l t e r n at i v es o ur c eoft h e s emat e r i a l s ,ando nes u c hbi op ol y me r ,po l y hy dr o x y bu t a noa t e( PHB)a l r e a dyfinds c omme r c i a lapp l i c at i ons . PHBi sp r od uc e db yf e r me n t a t i on ,us i n gt heba c t e r i aAl c a l i g e ne se u t r op hus , byf e e d i n gi to nanut r i e n t r i c hd i e tt oi n c r e as ei t sp opul at i on, andt he nr e s t r i c t i n gt hes upp l yo fn i t r o g e ni ni t sf o od.Th i sc au s e si tt op r o du c ePHBasan e ne r gys t o r a g ec ompou nd. Onc et h epr odu c t i ono fPHBi sc ompl e t ei ti sh ar v e s t e dbys o l v e nte x t r a c t i on .PHBhasp r ope r t i e s

s i mi l art opol y pr op e ne ,a ndhasf oun dc omme r c i a lus easpl as t i cb ot t l e sandc on t a i n e r s ,f o re x a mp l ef ors hampoop ac k a gi n g .I t h asahi g hme l t i n gpo i n t ,i ss t r o n g ,wat e rr e s i s t a nta ndr e l at i v e l yr i g i d,mak i n gi ti de alf ort he s es o r t sofappl i c at i o ns .Ho we v e ri t i sa l s obi ode g r ada bl e , whi c hme anst ha ton c edi s p os e dof , ande x po s e dt oba c t e r i ai nt hee n v i r o nme n t ,i ti sbr o k e ndo wn c ompl e t e l y , e l i mi nat i n gt h ei s s u eo fl anda nds e ap ol l u t i on .I ti sal s oar e ne wa bl ema t e r i als i n c ei ti smadeb yl i v i n gor g a ni s ms , e l i mi nat i n gt h ei s s u eofd wi n dl i n gf os s i lf ue lr e s o ur c e s . Ont h eot he rh an d, c u r r e n t l yb i o pol y me r sar emor ee x pe n s i v et h an p ol y me r so bt ai ne df r o mf os s i lf u e l s ,b utt h i swi l lc ha n g easf o s s i lf u e l sc o nt i n uet odwi ndl eandt h e i rpr i c ei n c r e a s e s .Asar e s ul t t h i sb i o pol y me ri sane x c e l l e ntal t e r n at i v et os o meoft hemat e r i al sc u r r e nt l yobt ai ne df r o mf os s i lf u e l s ....