chm315109 - chemistry - assessment report · pdf file•...

Chemistry Course Code: CHM315109

2013 Assessment Report

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EXAMINERS’ COMMENTS PART 1 -‐ CRITERION 5 GENERAL: • When answering questions, students should consider the number of marks allocated to each;

e.g. a 3 mark question usually requires 3 good points to be made in the answer. • Reactions involving nitrate ions were frequently used incorrectly throughout this booklet. • In redox questions students should provide half-‐equations in their answers more frequently, even

if the question does not ask for them explicitly. Question 1 a)

• Generally well done • Some students:

o Were not explicit in showing which oxidation number belonged to which vanadium species.

o Incorrectly assigned a negative oxidation state to V3+ and V2+. o Did not include + or – sign with their answer. This did not get full marks. o Incorrectly reported the oxidation state in the form of ions.

b) • Most students picked the correct VO4

3-‐ species but many students neglected to explain why this was the most powerful oxidiser.

• Commenting on “highest oxidation state” was not enough in the explanation. Students needed to relate to ability to gain electrons or potential to be reduced.

c) • This question was marked as “all or nothing”. • The biggest mistake was assigning 4H2O rather than 3H2O.

Queston 2 • Many students missed the second part of the question “Identify the reducer and the oxidiser, if

relevant”.

a) • A number of students incorrectly found changes of oxidation state in this question.


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b) • Some students didn’t say it was a redox reaction. • Frequently Ca2+ or Ca were incorrectly identified as the oxidiser or reducer.

Question 3 • Some students had their oxidation and reduction ½ equations the wrong way around. Oxidation ½ equation • Students could present either the H2C2O4 or C2O4

2-‐ half equation • Some students neglected 2CO2 in the products which made the balancing incorrect. Reduction ½ equation • Many students incorrectly gave the Br2/Br-‐ half equation • Often the charge was left off BrO3

-‐ which made the balancing of electrons incorrect. • Some students included Na with NaBrO3 but did not have Na+ on the product side. Net Redox equation • Errors were carried forward into this part of the question and full marks given if the right

technique was used. Question 4 • Many students were incorrectly using nitrate ions throughout this question. • Some students incorrectly thought Sn(s) was present in the Sn(NO)3 ½ cell.

a)

• Often students labelled the anode/cathode and their polarity, however, did not label which electrode was Pt(s) or Cu(s).

• The anode and cathode was frequently identified the wrong way around.

b) – d) • Errors were carried forward in these questions from how the cell was labelled in (a). • Students often assigned Sn → Sn2+ + 2e-‐ rather than Sn2+ → Sn4+ + 2e-‐

d)

• The position of the Pt(s) in the shorthand representation was often incorrect. e)

• Students need to be careful when discussing oxidisers. They need to refer to the metal ion e.g. Ag+ as the oxidiser, not just Ag, as this is a reducer.

• Students should be referring to the ‘half-‐cell’ rather than the ‘cell’ when talking about what’s happening at the anode and cathode; e.g. “silver half-‐cell” or “copper half-‐cell”, not “silver cell”.


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• Despite the tin half-‐cell being removed from the system, a number of students were still including it in their answer.

• For full marks, students needed to discuss what happened at both the anode and cathode. • Often Ag2+ was reported rather than Ag+.

Question 5 a)

• Many students only gave generic corrosion answers here rather than relating it to differential aeration.

• Some students talked about stress points which also gained some marks • Generally, there was a lack of half-‐equations in these answers.

b)

• The diagram needed to be completed and labelled to gain full marks on this question. It was often left blank.

• The direction of the battery was frequently the wrong way around. • There were a number of students who incorrectly connected the impressed current to both

ends of the pipe rather than to the pipe cathode and another piece of metal as the anode. • It was common for students to start talking about sacrificial anodes as part of this question

too. The anode can be inert or any reactive metal, not necessarily stronger reducers. Question 6 • Many students were incorrectly using nitrate ions throughout this question.

a)

• Again, students are confusing Mg/Mg2+ when discussing oxidisers. Mg2+ is an oxidiser, Mg is a reducer, these must not be used interchangeably.

• Some students incorrectly said there was no reaction as there was no oxidising agent present as the electrodes were inert.

• Half-‐equations were lacking in this question.

b) • Most students accurately said to use molten Mg2+compounds. • Some people followed this with the inaccurate ½ equation using Mg2+(aq) rather than Mg2+(l). • Some students incorrectly talked about:

o Using more reactive electrodes o Using molten Mg rather than a molten Mg2+ compound. o Increasing the concentration of the magnesium nitrate solution. o Although molten magnesium nitrate would decompose, no marks were deducted for

using this electrolyte.


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Question 7 • Many students were incorrectly using nitrate ions throughout this question. a)

• Students needed to state what was happening at both the anode and cathode. • Half-‐equations need to be labelled to show which is the anode and cathode.

b)

• Students needed to indicate the sequence of products that would form over time at the cathode.

• Frequently students didn’t write about the reduction of water (after Cu2+ and Pb2+). • Many students incorrectly said Na(s) would form! • There was a group of students also talking about the copper then oxidising again back to

Cu2+ rather than moving to the next strongest oxidiser. • Some discussed the formation of anode ‘mud’ which had no relevance to this question.

Question 8 a)

• The reasoning to go with the order of reducers was often unclear. • Students should be referring to weaker/stronger oxidisers/reducers rather than “above” or

“below” on the ECS or SRP • There was confusion in explanations regarding reducers. It is important students refer to

the metal rather than the metal ion when talking about reducers. • Many students has the sequence correct, but reported it backwards or in the form of metal

ions.

b) • This needed to be a metal not a metal ion. • Many students did not pick up on the ‘colourless’ product with the reaction of A with acid,

which discounts many metal ions (such as copper) from this answer. • Many students inaccurately picked Ag or Cu. • This question was often left unattempted. • Some students even had non-‐metals as their answer.

PART 2 -‐ CRITERION 6 In general, this section of the exam was handled competently by students. Question 9 a) Almost all students got this correct. b) Most performed very well in this question, with some getting the process involved confused and

occasionally ‘inverted’.


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c) Whilst the general shape of the endothermic reaction potential diagram was competently done with ΔH correctly identified, many students incorrectly labelled the activation energy.

d) Students provided a variety of valid answers to this question, with many receiving full marks. Question 10 a) Students performed well on this question. Some incorrect answers confused ‘matches’ with

catalysts. b) The key to success to this question was identifying that the catalytic convertor was acting as a

catalyst! When this was identified, the rest of the question proceeded well. c) Almost all students got this correct, identifying the large surface area provided by the catalyst

within the catalytic convertor. Question 11 This was a challenging question with many students getting some way towards a solution. The exam markers would advise that students give clear processing instructions for each step in future Hess’ Law questions, as many ‘part marks’ can be awarded in such questions. Question 12 a) Many students performed well on this question. Care needs to be taken when calculating the

activation energy of an exothermic reaction given the ‘reverse’ exothermic data. b) Almost all students were able to indicate a decreased Ea* pathway for the catalysed reaction.

Question 13 a) A number of students were unable to write the formula for ammonia, NH3. (The formula was

given in question 14). A number of unfortunately incorrect formulae for ammonia were given, which created subsequent calculation issues.

b) In general, this question was carried out very competently, albeit with many ‘errors carried forward’.

Question 14 Many students were able to correctly apply LCP to this question. There was some confusion about the role that the high temperature (900oC) played here, where a reaction kinetics solution was required for full marks. As there was an element of ‘real world extension’ in this question marks were given for answers that included ‘removal of product (NO)’ and ‘economic considerations’, etc. Exam markers would recommend that a separate treatment of each dot point would assist in similar questions.


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Question 15

a) Most students correctly identified the ICl3(g) partial pressure increase at t = 7 minutes and applied LCP to it gaining 2 marks. The more subtle, more ‘gradual’, temperature change where the reverse exothermic reaction was favoured was not handled as well.

b) Many students correctly identified the pressure increase at t = 10 minutes and applied LCP to it. The graphical representations were variable in quality. Full marks were awarded for recognition of a pressure/concentration increase, the correct analysis of this, including the changed ratios of reactants and products as a new (fourth) equilibrium was established.

PART 3 -‐ CRITERION 7 Question 16 a) This was either answered poorly or not at all by a significant number of candidates who were not

able to apply their knowledge of the kinetic theory. b) Knowledge of ideal and real gases and intermolecular attraction between different molecules was

not clearly understood by many candidates. Question 17 Most candidates demonstrated an understanding of ionisation energies, but not all explained that the first ionisation energy for Al with removal of a 3p electron required less energy than removal of a 3s electron from Mg, as the 3p electron was at a higher energy level. Question 18 This question was only answered well by a small number of candidates. The formation of sulfurous acid, H2SO3, by dissolving of SO2 in water was generally not identified and sulfuric acid was frequently being shown as the acid instead. The dissociation of the acid to provide H+

(aq) ions was often not provided. Question 19 a) This part was answered correctly by the majority of candidates.

b) In this part there was significant confusion between atomic and ionic radii demonstrated. Question 20 This was generally quite well done. Some common errors however, included not naming with alkyl side groups in alphabetical order e.g. 3-‐methyl-‐3-‐ethylpentanal, instead of 3-‐ethyl-‐3-‐methylpentanal. The writing of the empirical formula for the final structure demonstrated that the formula for benzene is generally not well understood.


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Question 21 This question was either well answered or poorly answered by candidates. Question 22 Most candidates answered this question well. A number of possible isomers were acceptable. Question 23 Some errors for this question included the naming of the organic product in (a) and (b). The product in (b) is butanone. Butan-‐2-‐one was provided by many students, which is not correct but was accepted in this exam. There was confusion about what is an observation. In part (a) hydrogen gas is produced, but this can only be confirmed after testing. The observation would be that a gas was produced. Question 24 This was the most challenging question for candidates with very few students gaining full marks. The majority of students did not recognise that compound A was an unsaturated primary alcohol. Another difficulty was that it was generally not identified that the unsaturated part of compound A became saturated with the addition of steam, the H2O adding on across the double bond with another –OH group resulting on the chain. This reaction is included on the information sheet. In part (b) the catalyst (concentrated sulfuric acid) needed to be included for full marks. A significant number of candidates were not able to correctly write the formula for the pentanoate ester, frequently providing an ester with 5 carbons in total. PART 4 -‐ CRITERION 8 This section was long as indicated by several scripts which were left blank towards the end, despite the ‘harder’ earlier questions being dealt with correctly. Students should note that illegible information does not earn marks. The examiners try very hard but some words and numbers were indecipherable! Students should be proficient with the calculator they bring to the examination. There were many instances of everything being correct, all working was shown but the final calculated answer was incorrect. It’s sad to note that bad algebra is still present. Several students had trouble rearranging equations, e.g. c = nV !!


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Question 25 No problems, mostly done correctly. Almost all students recognized the molar relationship that n(HCl) = 2 x n(Pb(NO3)2). Question 26 a) Mostly done well.

A few students divided the masses by atomic numbers instead of the relative atomic mass of the element. A common mistake was reading Fe as F and using atomic mass of 19.00 instead of 55.85. Another error was not rounding off 13.9 to 14. Several students appeared to not know what ‘empirical formula’ meant.

b) This question was generally found to be difficult by many students. Many who figured that one formula unit has 126 g mol-‐1 of water and hence 7 H2O, incorrectly wrote the formula as FeSO11H14.7H2O or FeS.7H2O. Comparatively few students saw that 7H2O still leaves 4O, hence it must have been FeSO4.7H2O.

Question 27 This question was badly done with very few getting this totally correct. Even fewer remembered that ∆H is (–)ve, which cost them ½ mark. The most common mistake was to us E = mc∆T, where m was 5.00 g and c was the calibration factor. Another mistake was the addition of 273 to the value of ∆T, and hence an incorrect value for the calibration factor. This is disappointing considering both ∆T and the calibration factor (cf) are defined on the information sheet! For the work and emotion involved, this question probably should have been worth 4 marks. Question 28 This question was answered well. Most remembered to halve n(e-‐) to get n(Cl2). Common mistake was to use Cl instead of Cl2. Calculation of volume of Cl2 using the gereral gas equation was answered correctly by nearly everyone. Question 29


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a) Most, but not all recognized that one of the reactants was limited. One mark was lost for not acknowledging that. Once that was figured, there was no problem.

A surprising number calculated the mass of CO2 rather than its volume. A few forgot that CO2 is a gas and used n = cV to calculate volume (of a solution of CO2?)

b) This part was not answered well. Mostly students calculated pH based on the concentration of HCl(aq), forgetting that some of it had reacted.

Question 30 This was an ‘all or nothing’ question. Those who attempted it did it correctly. Many seemed to put it in the ‘too hard’ category and did not even try. A common mistake was to calculate the moles of silver chloride instead of moles of silver, and then get stuck because that was greater than the mass of the original sample! Question 31 a) This was well done although a few did not put in a two way arrow, which cost half a mark. b) This was not done well. The equilibrium concentrations of CF4 and CO2 were generally used as

(0.308/20) rather than the correct (0.154/20). c) Similarly, this was not done well. Many went down the path of (x – 0.3) and got into unnecessary

strife with the quadratic formula. Recognizing that Kc remained unchanged, earned 1 mark. Common mistakes with students who used the correct equilibrium ratio formula, was to forget to calculate the square root.

Question 32 a) This question was confusing and difficult to mark too, since errors carried forward still earned

some marks. The most common mistake was to miss the fact that n(H2O2) = 5/2 x n(MnO4

-‐). The other common mistake was to calculate the mass of H2O2 in 20.00 mL instead of 25.00 mL as asked.

b) Very few students managed to get this part correct. Two different but correct calculation methods were used. Some calculated the mass of H2O2 in one mL using mass = 34.02 x cV; and then since 1 mL of bleach has mass of 1.12g, the % by mass could be calculated. An alternative method was to calculate the mass of 25.00 mL of bleach as (25.00 x 1.120) g = 28.00 g and hence the % by mass could be calculated as this 25.00 mL contains the mass of H2O2 found in part (a).


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GENERAL COMMENTS As in previous years, there are a number of key points that need to be re-‐emphasised, especially for this 'calculation' section where many students lost marks due to poor answering technique or errors in expressing their answers. - 'Significant figure' misuse was frequently encountered and usually penalised by a ½ mark. As a

general rule, expressing final answers to 3 significant figures is usually sufficient. Many gave answers to 1 or sometimes 9 sig figs!

- Students 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.

- Students 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.

- Students should 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.

- Students need to be reminded to consider whether their answer is within the limits of possibility;

i.e. “does my answer sound reasonable?”

TASMANIAN QUALIFICATIONS AUTHORITY

ASSESSMENT PANEL REPORT

CHM315109 Chemistry

20% (105) 24% (124) 20% (101) 36% (186) 516

24% (137) 30% (169) 24% (135) 23% (131) 572

11 % 19 % 39 % 31 %

22 % 26 % 27 % 25 %

11 % 19 % 39 % 30 %

51% (265) 49% (251) 0% (1) 100% (515)

55% (312) 45% (260) 0% (2) 100% (569)

53% 47% 0% 100%

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Previous 5 years (all examined subjects)

Last year (all examined subjects)

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