Chemistry
Course Code: CHM315114
2014 Assessment Report
Page 1 of 10
As in previous years, there are a number of key points that need to be re-‐emphasised, especially for the 'calculation' section where candidates lost marks due to poor answering technique or errors in expressing their answers. - 'Significant figure' misuse was frequently encountered. As a general rule expressing final answers
to 3 significant figures is usually sufficient. Many gave answers to 1 or sometimes 9 significant figures!
- Candidates must take more care with setting out calculations. All calculations should be labelled (e.g. n(HCl) = ... rather than n = ...) and calculation steps should follow sequentially.
- Candidates should avoid rounding off numerical answers prematurely; e.g. in 250.0 mL of 0.400 mol L–1 X(aq) the n(X(aq)) = 0.100 mol not 0.1 mol - For most chemical equations, the reactant and product states should be indicated as subscripts. - Candidates would be advised to write in pen (not pencil) and not to cross out any answer part
until they are sure it has been replaced by a preferred answer. In some cases we saw correct answers that had been crossed out but not replaced by anything else.
- Candidates need to be reminded to consider whether their answer is within the limits of possibility; i.e. ‘does my answer sound reasonable?’
PART 1 – CRITERION 5 Question 1 (a) Most candidates correctly identified that Cd(s) was being oxidised but many then said that Ni was
being reduced rather than more correctly saying that ox(Ni) = 4+ changes to ox(Ni) = 2+. A number of candidates gave the correct answers but lost part marks because they did not include reasons for their selections.
(b) Those who had Ni as the oxidising agent rather than NiO2 lost half a mark. Question 2 A pleasing number of candidates established the equation:
Br2(g) + 2NaOH(aq) → NaBr(aq) + NaOBr(aq) + H2O(l) and by correctly using oxidation numbers established that bromine was being reduced from ox(Br) = 0 to ox(Br) = –1. This was necessary to gain 1.5 marks. A very significant number of candidates then assumed that NaOH was undergoing oxidation and came up with extraordinary products such as H2+, Na2+ and Na3+.
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Few realised that Br2 was being oxidised from ox(Br) = 0 to ox(Br) = +1 in NaOBr. A few recognised that this type of redox reaction is called ‘disproportionation’ although the examiners were not expecting this in the answer for full marks. Question 3 (a) Well done with most establishing ox(Ti) = +4. (b) Even though this half-‐equation is on the information sheet, a surprising number of candidates had
errors which were often caused by the inclusion of the spectator K+ ion. A number had MnO2 as the oxidising agent rather than acidified MnO4
–. (c) This was not well done with a number confusing TiO2+ with TiO2
+. Some had parts (b) and (c) interchanged.
(d) In the majority of cases, those who had parts (b) and (c) correct managed to get this part correct.
The use of ‘error carried forward’ meant that a mark was awarded for an incorrect answer as long as it was an appropriate redox combination of parts (b) and (c).
(e) This part was surprisingly poorly done. To gain one mark, candidates had to mention that the
colour change was associated with the reduction of purple MnO4– ions to form pale pink or near
colourless Mn2+(aq) ions. Because the permanganate solution was ‘acidified’, a number assumed that this was an acid/base reaction occurring.
Question 4 (a) A generally easy question and yet serious weaknesses in the understanding of corrosion were
encountered. Examples of common errors included: − incorrect statements such as ‘copper is a stronger oxidiser than iron’. − steel is the element Sn or Al or Cr. − the cathode half-‐equation is Cu2+(aq) + 2e– → Cu(s) or − the cathode half-‐equation is 2H2O(l) + 2e– → 2OH–
(aq) + H2(g)
It was acceptable to show the anode half-‐equation producing Fe2+ or Fe3+ so long as the electron number was consistent.
The main inadequacies in the corrosion answers were the failure to use the correct half-‐equations and the omission of reasons for the pipe to corrode ‘so quickly’. Those who gave good answers included comments about the corrosion system being an electrochemical cell and that the EMF thus generated accelerated the corrosion process. Also, the fact that the copper became the cathode site meant that there was a greater surface area and rate for the cathode half reaction:
2H2O(l) + O2(g) + 4e – → 4OH–
(aq)
A pleasing number discussed the ‘hot’ water causing and increased corrosion rate too.
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(b) There were many possible answers accepted here and even though the corrosion problem is most likely on the inside of the pipe, painting, greasing and polymer coatings were awarded marks. Some unlikely answers included: − noble coatings (these will be easily scratched/damaged) − enclosing the system in a vacuum to exclude air − excluding water from the pipe − using gold, silver or even lead pipes − coating the Fe pipe with lithium as a sacrificial anode The preferred answers involved: − a non-‐electrically conducting junction (polymer) between the Cu and Fe pipes − Zn (or similar reactivity) sacrificial anodes − galvanising the Fe pipe internally and externally − applied EMF − having the whole pipeline constructed from the same metal (e.g. Cu)
Question 5 (a) and (b)
This question was marred by the fact that at least 75% of candidates assumed that the cathode half-‐reaction would be the reduction of sulfate ions to form SO2 gas rather than:
2H+(aq) + 2e– → H2(s)
Candidates lost marks where they had the wrong direction for electron flow in the external wire and incorrect ion movement in the salt bridge. Electrode polarities were sometimes omitted as were the labelling of the electrodes and the ions present in each electrolyte.
A considerable number of candidates showed the sulfate ion as having a charge of 1– rather than 2–.
(c) Very few candidates realised that the cell would cease functioning due to the formation of PbSO4(s)
precipitate in the salt bridge. Those who described the new system correctly as if the salt bridge was KNO3(aq) or NaNO3(aq) were given some credit.
Question 6 (a) No reaction occurs under standard state conditions. For determining that the Eo net was negative
earned 1.5 marks but to get full marks, candidates had to compare the oxidising strengths of Hg2+ and Cu2+ or the reducing strengths of Hg and Cu. These comparisons were frequently presented in a confused way. Magnesium sometimes appeared in place of Hg.
(b) A spontaneous reaction does occur in this case and to get two marks, a balanced equation (net
ionic or total) was expected. A disappointing number of candidates had a reaction between potassium and chlorine. This scored zero.
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In both parts of this question, no marks were awarded to candidates who determined spontaneity of reaction by discussing the position (above or below) of reactants in the ECS.
Question 7 This question was not well done and proved to be one where ‘trial and error’ was the best pathway to success. Those who deduced the correct answer were expected to present the standard reduction potential in the form of a half-‐equation:
i.e. In3+(aq) + 3e– → In(s) Eo = –0.34 V
Marks were lost where candidates had no half equation, the wrong Eo sign or omitted the units for Eo (V). Valid attempts were awarded at least one mark. Question 8 This was well done and is a lab experiment that practically all Chemistry candidates should have undertaken. Many gained the full mark of 4/4. The errors that occurred were surprising and showed a lack of familiarity with common laboratory chemicals. Some thought the brown colour associated with I2(aq) (or more correctly the complex ion I3– (aq)) was due to Fe3+(aq) even though there was no mention of iron in the question. Some said Na+(aq) ions are brown and a few even had sodium metal being formed at the cathode! The phenolphthalein colour was usually well explained but again some thought the pink colour was due to Mn2+(aq) or Co2+(aq) as these are shown on the information sheet. A few candidates reasoned that the pink colour was an indication of acidity. It is important that candidates correctly use ‘iodide’ and ‘iodine’ in the correct context as this was a common error. Question 9 This was well done by the majority of candidates. The biggest difficulty encountered was where candidates didn’t have a nickel anode in their electrolytic cell. Although not expected, this would have been a logical choice. A Ni(NO3)2(aq) electrolyte would have been preferable to the insoluble Ni(OH)2 or molten nickel compounds that some had. Other errors included: − failure to include a DC power supply − electrode polarities reversed − no indication of the direction of electron flow − the incorrect spelling of nickel − having an electrode outside the electrolytic cell
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A number of candidates had C (graphite) or Pt as the metal to be nickel plated! PART 2 – CRITERION 6 Question 10 (a) Very poorly answered in general. Candidates did not recognise that covalent bonds in the gas
molecules needed to be broken, or that ions in solution did not require bonds broken. Many candidates thought that the gases would react, perhaps because the question states ‘at SLC’. Candidates needed to address both reactions to get full marks.
(b) Generally answered OK, and attempted better than part (a). However, many candidates just gave
very generic responses not linking their answer to the question. Some candidates were confused making statements about energy released when breaking bonds, and energy required to form bonds. Some also thought that the increase in energy was due to more bonds forming than being broken.
Question 11 (a) Very poorly attempted and answered. This question created a great deal of confusion with
candidate responses including many errors errors. The units of mol min–1 should give candidates the hint that the answer involved a quotient with moles of hydrogen on the numerator and minutes on the denominator.
(b) Candidates were able to draw lines on the graph, but many graphs were not labelled. Experiment
2 was not well represented with many candidates drawing the line to meet the original experiment and not applying the ratio for the differing concentration. Experiment 3 was generally well drawn in comparison.
(c) This question was generally well done. Many candidates produced the right type of distribution
curves, but did not label them appropriately, and in some cases were too generous with proportions past Ea. The written explanation was much better, most candidates linking the change in reaction rate to kinetic molecular theory.
(d) Again, generally very well attempted and answered. As the candidates were to use a method not
mentioned previously, they should have spoken about increasing surface area of the zinc (which most managed), or increasing the concentration of HCl. Some candidate responses indicated pressure needed to be changed, or included a previously mentioned method.
Question 12 (a) Very well done. (b) Mostly well done. Most candidates scored marks here, some lost marks for incorrect labelling and
a very few candidates had an endothermic reaction profile or an alternate graph type.
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(c) This was well attempted, but often poorly executed, especially in the second part where they were required to find the value for the C–Cl bond energy. There was a common error with candidates getting –171 kJ, this was due to using +157 kJ rather than –157 kJ; or incorrectly manipulating the algebraic relationship.
Question 13 This question was very well attempted, and very well answered with candidates showing a very good understanding of Hess’ Law. Question 14 (a) Very well done. (b) This was a mixed response where most candidates could state the equilibrium position, but
wanted to increase or decrease K. (c) Well attempted. Many candidates again could explain the effect on the equilibrium position, and
some correctly stated that Kc would decrease. For full marks candidates were required to explain that the Kc decrease was due to the increase in reactant concentrations.
(d) Fairly well answered. Candidates did need to make a link with increasing volume and decreasing
pressure (some used concentration); but there was some candidates wanting to decrease the volume. Candidates needed to consider the ratio of gaseous molecules to receive full marks.
Question 15 (a) Very poorly done. Many candidates were able to talk about equilibrium being reached, but not
how/why it was obtained. Many candidates did not consider that initially there was no methanol. (b) (i) Most candidates attempted this question. Some did not show the x/2x relationship of methanol
to hydrogen on their graph, so ½ mark was deducted. Some candidates also did not start their methanol concentration graph at the zero point. Many did not label the diagram. (ii) Very well attempted. The most common issue was that candidates did not mention the effect on the yield and thus could not achieve full marks.
PART 3 – CRITERION 7 This section was generally well answered. Candidates are advised to become more fluent in the use of the information sheet, as this section relies heavily on the information presented there. Candidates are advised that they must define acronyms before using them in this section. e.g. effective nuclear charge (ENC), intermolecular forces, (IMF).
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Question 16 Well answered by the majority of candidates who correctly named the compounds and identified the functional groups. Question 17 (a) Most candidates explained the diffusion process well and recognised that gas molecules were
involved. Many did not recognise that an explanation of the evaporation process where liquid molecules gained energy to form gases was required.
(b) (i) Most candidates attempted this question and recognised that hydrogen bonding was involved
but few explained how differences in electronegativity lead to this phenomenon. Many candidates correctly related differences in intermolecular forces to boiling points.
(b) (ii) This was a more difficult question and related to success in (b)(i). There was a poor
understanding of polarity with many candidates thinking that the solvent was responsible for breaking covalent bonds in the polar group. A few candidates used pentane instead of pentanal. Common errors included: ‘C=O bonds are non-‐polar and insoluble’, ‘pentanal is non-‐polar’.
Question 18 (a) This question was well answered by most candidates. (b) This question was generally well answered although some did not recognise that it was a
substitution reaction. (c) Whilst most candidates recognised that an alkene was required, many struggled with correct
nomenclature for the monomer involved. (d) There were a large number of possible correct answers to this question. It was surprising that
many overlooked the ‘standard’ sodium test for alcohols. Those that used a redox reaction did not recognise that a carboxylic acid could be produced for both the alcohol and the aldehyde.
Question 19 Most candidates answered this question well identifying possible compounds and naming them correctly. It was pleasing that candidates correctly recognised the hydrolysis products of the chosen ester. The most common omission was not to interpret the result of the acidified permanganate test.
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Question 20 (a) A minority of candidates recognised that a tertiary alcohol was involved. (b) Most candidates correctly recognised that an alcohol was involved. Of concern was that many
thought that secondary alcohols cannot be oxidised. Question 21 (a) Electron configurations were well answered. Some candidates struggled with the transition metal,
Ti. (b) (i) This question was very well answered.
(ii) Most candidates were able to explain the general trend of ionisation in Period 3, many did not give details for Al and Cl. Candidates are advised not to attribute human feelings/ emotions to atoms; e.g ‘sodium wants to lose an electron’.
(iii) Generally well answered with candidates recognising that Na+ is isoelectronic with Ne and thus E2 would remove the 2nd electron from an inner (2p) sub-‐shell.
Question 22 (a) Candidates who received full marks in this question often used a shielding analogy. Many
confused oxidation with reduction. (b) Most candidates correctly identified Bi as the most metallic element in Group V (15). Common
errors included mistaking Group 5 with Group V. Perhaps future questions should consider a ‘double listing’ as per the information sheet V, 15.
(c) Most candidates could explain why hydrogen could be assigned to group VII (17), however, many
were unable to give an adequate explanation for its possible placement in Group I. The use of suitable chemical equations showing the formation of H+ and H– ions would have seemed appropriate.
Question 23 (a) Well answered by many candidates who recognised the contribution of nuclear charge and
decreasing atomic radius to a general increase in electronegativity. (b) Well answered by most candidates. A bonus mark was awarded for recognising the existence of
the amphoteric oxide, B2O3. (c) Few candidates were able to correctly identify of lithium oxide, Li2O and consequently struggled to
balance the equation correctly.
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PART 4 – CRITERION 8 Question 24 a) Generally well answered. Common errors were not giving answers to correct significant figures,
using oC rather than K and using poor algebraic manipulation skills. b) Generally well answered. It was possible to use the answer from part a) or the information given
in the question. Most candidates used N = n.NA but many did not calculate n correctly, using n = m/M rather than the gas laws. Some candidates cannot calculate percentages.
Question 25 Generally well answered. A common error was to assume that the empirical formula was the same as the molecular formula. Question 26 a) Some candidates failed to recognise that this was a limiting reagent question. Others incorrectly
identified the limiting reagent or failed to provide enough evidence to support answers. b) Poorly answered by many candidates. Many candidates did not recognise that the concentration
of the iron(II) chloride is zero because it is the limiting reagent. Candidates seemed confused about the definition of ‘filtrate’.
Question 27 a) Generally well done. The most common errors were the miscalculation of the M(glucose). b) Well answered. Question 28 Generally well answered. The most common mistake was failing to recognise that the number of moles of the metal was a third of the number of moles of electrons. Rounding off answers early could give a different answer for the identity of metal M which was gold.. Question 29 a) Some failed to recognise that this question was a limiting reagent question, causing similar errors
to Q26. Note that it would have been simpler to find the volume of chlorine using the fact that 1.00 mole of gas at STP occupies 22.4 L rather than applying PV = nRT.
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b) Some confusion arose with regards the identity of the excess reagent even if it was correct in part a). Some candidates failed to use mole ratios from the given equation.
Question 30 a) Generally well done. Note that the data in this question are given to 4 significant figures. b) Many assumed that [HLa] at equilibrium was 0.1000 and did not construct an ICE table. Rounding
off early made an impact on the accuracy of the answer, which should have been given to 4 significant figures. Some were not able to give the equilibrium expression, or identify that [H3O+
(aq)] = [La–(aq)]. Question 31 Many candidates used the sulfite ion rather than the sodium sulfite to calculate mass and percentage purity. Some confusion arose between the sulfite and sulfate ions, and some attempted to turn the question into a limiting reagent question. A few candidates solved by finding the concentration of the sulfite ion in the original solution and carried on with value from titration.
TASMANIAN QUALIFICATIONS AUTHORITY
ASSESSMENT PANEL REPORT
CHM315114 Chemistry
19% (106) 27% (149) 20% (108) 34% (188) 551
20% (105) 24% (124) 20% (101) 36% (186) 516
10 % 19 % 39 % 32 %
22 % 28 % 22 % 28 %
11 % 19 % 39 % 30 %
50% (276) 50% (275) 0% (1) 100% (550)
51% (265) 49% (251) 0% (1) 100% (515)
54% 46% 0% 100%
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