Section I part A

The views expressed in this Answers and Comments document are those of the author who had nothing to do with setting or marking the paper. The answers are not in any way 'official'. They are simply the responses of a competent chemist familiar with the syllabus and the textbooks in common use. The answers are probably more detailed than would be required to gain full marks from HSC examiners but are worthy goals for students to aim at.

You need a copy of the exam paper to make sense of the answers given here. If you do not already have one you can get a copy from the Board of Studies web site www.boardofstudies.nsw.edu.au . Click on 'HSC exams' in the left hand column, select '2013 HSC exam papers' then find 'Chemistry' in the alphabetical list. Click on 'Examination Paper' to open it or right click on it to save it.

Marking Guidelines with Sample Answers and Notes from the Marking Centre are also available there. You might like to look at these. Some of the samples are not complete answers and some do not seem to answer the precise question asked, but overall they are of considerable help in giving you an idea of what the examiners were expecting – often a much lower level of sophistication than perhaps the question implied.

Some general comments

The 2013 exam paper continues the practice of putting heavy emphasis upon recall of factual knowledge (often with some interpretation of the recalled material being required) as opposed to problem-solving. As in 2012, this year recall of experimental procedures only occurs in the options. The table below gives a breakdown of marks versus type of exam question over several years.

Type of question		Marks
Type of question		2005	2006	2010	2012	2013
1	numerical calculation^{a b}	19 to 23	22	18 to 21	19 to 22	14 to 17
2	non-numerical problem-solving^{b c}	11 to 14	14 to 16	17 to 21	12 to 16	18 to 23
3	straight recall of factual material (describe, outline, summarise ...)	32 to 34	30 to 36	34 to 48	42 to 47	47 to 48
4	recall of factual material with some interpretation (evaluate, assess, analyse, discuss, compare ...)	28 to 31	29 to 35	8 to 20	18	10 to 15
5	descriptions of experiments (including risk assessments)	5	6	5 to 9	0 to 5	3 to 5
6	number of extended response questions^d	5	6	3	2	1
7	marks for extended response questions (included in categories 3 and 4 above)	30	22	17	13	7

^a Includes drawing graph if required
^b There are ranges because different options distribute marks slightly differently
^c Includes writing an equation or drawing a structure not given in a textbook
^d Parts (a), (b), (c), (d) and (e) in options questions count as separate questions

Averaged over these five years, about 35% of the marks have been for problem-solving (qualitative and quantitative) and 66% for recall of information (including description of experiments) with some interpretation of it.

Extended response questions

Extended response questions are the ones that require the most careful attention to make sure that you give the information asked for. With no guidance about how much emphasis to place on each aspect of the question, it is easy to give too much information about one aspect and neglect another equally important one. In tackling extended-response questions (questions that are just one or two sentences but worth 5 to 7 marks) you should start from the rule of thumb that the question requires one significant piece of information per mark, so a six mark question will require six pieces of relevant information; that probably means at least six sentences, often more. And it is going to require about 1.8 minutes per mark to answer as fully as the examiners want (11 minutes for a 6 mark question). Analyse the question carefully before you start writing to make sure you understand exactly what the question is asking for. If it contains two verbs such as saying 'describe' something and 'discuss' its importance or role in ..., then you need to split the marks between the two parts, say 3 for 'describe' and 3 for 'discuss'. If you feel that it is difficult to give 3 significant facts for the 'discuss' part, perhaps you could do a 4/2 split but a 5/1or 6/0 split would not be answering the question as asked. Even if you could easily give 6 significant facts for the 'describe' part, you must give some for the 'discuss' part even though that might be a struggle. Note that verbs such as evaluate and assess require you to make some judgment, so even if you cannot give a deep and insightful one, at least give some judgment even if it seems trivial, such as it's a good thing, it's really important or it's of no great significance, because the examiners are looking for some sort of judgment and they do not have to agree with your judgment to give you the mark (but they cannot give you the mark if no judgment is given).

The only questions on the 2013 paper that fit into this extended-response category are questions (e) of the options questions. Although Questions 23(a), 30 and 31(a) each carry 5 or more marks, they are not 'extended-response' questions, because they request very specific bits of information.

If you finish a question with specific instructions ahead of time, then you have extra time to give to questions that are causing you difficulties. However if you seem to have finished an extended-response question in much less time than was allocated to it, and have used only a small proportion of the space allocated to the question in the exam book, then look back over it very carefully; you may have omitted an aspect of the question. If not, then it is very likely that you have not given as much detail as the examiner is really expecting.

With extended-response questions be very careful that you answer the question asked and not answer a related question that you know a good answer for. Try to answer the question exactly as asked. If you cannot give enough detail about that question to fill the time allocation, then perhaps you could provide related but not strictly asked-for information, but do that only as a last resort. One reason for suggesting this is that sometimes it is not possible to spend the allocated time on the question asked; it was not a good question.

There has been a gradual decline in the number of marks for extended response questions over recent years; this probably reflects the difficulty students have in answering these questions as asked, and the difficulty markers have in assigning marks to answers in an objective way.

Answers and comments

Section I part A (Multiple Choice questions)

With one mark per question, these multiple-choice questions should require 1.8 minutes per question. However the last few questions are more difficult (or at least more time-consuming) than the first few, so you need to proceed as quickly as you can through the early ones and not relax if you seem to be ahead of time, because you will need that saved time for the last few questions. Plan on spending 36 minutes in total on these multiple-choice questions rather than the same amount of time (1.8 minutes) for each question.

Question & Answer		Comment (and relevant page in CCHSC)
1.	B	Something you need to know. (page 200)
2.	C	Again something you need to know. In AAS a solution of the sample is sprayed into the flame which vaporises it and then decomposes it into atoms. (pages 225-8) Note that the chemical meaning of atomise, to break into atoms, is different from the word's everyday meaning which is often just to produce a fine spray (mist) as in a perfume atomiser or in an insect spray.
3.	D	While chlorofluorocarbons (CFCs) are minor contributors to global warming, their main environmental impact is ozone depletion. (page 253-6)
4.	C	Just a matter of reading the equation: one mole butan-1-ol produces 4 moles CO₂ so 2 moles produce 8. (page 36, 170-1)
5.	A	Use the given Periodic Table to count along the last period (row) from left to right from 112 to 116. 116 falls in the column (group) containing O (atomic number 8), S (16) ... so A is the answer. (page 80-2 and CCPC page 47-8)
6.	A	Elements on the left of the Periodic Table have basic oxides which react with acids (P and T), elements near the right have acidic oxides which react with bases (R and U), so even if you don't know that Q and S are amphoteric (react with both acids and bases), you have enough information to get the correct answer. (page 115-6)
7.	C	B is complete combustion (all going to CO₂). A could not occur because hydrogen is so flammable that it would immediately react to form water. For incomplete combustion some CO and/or C must be formed. This is so for C and D. The question asks for a correct equation; D is not balanced so is not a correct equation, so C is the answer. (page 238 and CCPC page 286)
8.	A	B is a trihydroxy compound, D is a dihydroxy one. C is a (mono) hydroxy compound but it would be called 1-hydroxy....Only A is a 2-hydroxy compound. (page 170-2)
9.	A	This is basically the polymerisation of an ethylene-based compound of the type CH₂=CHX. These compounds polymerise by opening up the double bond to join to other molecules without the loss of any atoms, so the reaction is addition. Condensation in this context means loss of a small molecule (such as water) when reaction occurs. (page 13-6)
10.	C	Catalysts affect the speed of reactions, not the position of the equilibrium. Adding argon would not affect the position of equilibrium: you have to alter the pressure of at least one reactant or product to produce any pressure effect. By Le Chatelier's principle decreasing the volume would move the reaction to the left (2 volumes ® 1 volume) so decrease the yield of NO₂. Again by Le Chatelier's principle raising the temperature would push the equilibrium in the endothermic direction (from right to left) so C is the correct answer. (page 117-9, )
11.	B	Heat of combustion is heat released per mole of compound burnt so to get heat released per gram divide by the molar mass – 28 for CO, 16 for CH₄, 26 for C₂H₂ and 30 for C₂H₆. This gives heat per gram of 8.3, 55.6, 50, 52 kJ/g for CO, CH₄, C₂H₂ and C₂H₆ respectively, so the answer is B. (page 36-8 and CCPC page 280-1)
12.	B	The provided table of standard electrode potentials shows that the chlorine half reaction has the higher electrode potential so it will go as written (in the reduction direction) and force the zinc half reaction to go in the reverse direction; that is, Zn will be oxidised to Zn²⁺. By definition the anode is the electrode where oxidation is occurring so Zn is the anode; hence B. Note that most of the galvanic cells you deal with in the HSC have a salt bridge, it is not essential: when the one electrolyte solution can serve both electrodes as here no salt bridge is needed. (page 46-9, 70)
13.	D	A, B and C are ethylene-based polymers – all have a –C–C–C–C–C–C– backbone. The raw materials for ethylene-based polymers come from petroleum. D has a string of glucose units arranged as in cellulose which comes from plants, so D is the answer. (page 13-6, 25-6)
14.	B	You need the equation for the reaction: 2Na(s) + 2H₂O(l) ® 2NaOH(aq) + H₂(g) 2 moles Na produce 1 mole H₂. We are given 1 mole of Na (22.99 g) so it produces 0.5 mole H₂ gas. At the given temperature and pressure, from the Data Sheet 0.5 mole of any gas has a volume of 11.4 L, so B is the answer. A uses the molar volume at the wrong temperature. C and D result from saying 1 mole Na gives 1 mole H₂ – that is, from not working out the balanced equation. (page 128-31)
15.	C	The H⁺ ion concentrations in the original and diluted solutions are 10^–3 and 10^–5 mol/L respectively (from definition of pH), so the dilution factor is 1:100 or 10 mL going to 1000 mL (or 10 mL plus 990 mL) which is C. (page 134-5
16.	B	The water molecule provides a pair of electrons to form a coordinate covalent bond with the H⁺ ion to form the hydronium ion H₃O⁺. A forms hydrogen gas and a solution containing Ca²⁺ ions, a redox reaction; C forms a precipitate of AgCl while D is a weak acid plus a weak base – no significant reaction occurs. The NH₄⁺ ion is made when an NH₃ molecule forms a coordinate covalent bond with an H⁺ ion but that is not the question asked. (page 242-4)
17.	B	It does not matter whether the acid is strong (hydrochloric) or weak (acetic), the same quantity of strong base (sodium hydroxide) is required to react completely with the same molar amounts of the two acids. We need to distinguish between concentration (the number of moles per litre) and strength (the extent of ionisation of the acid). (page 140-2)
18.	D	Compound X could be ethylene or propene (so C is out), and hence Y could be ethanol or propan-1-ol, so B is out (dilute sulfuric acid on propene would not form ethanol). Compound Z has to be an ester of propanoic acid. Propan-1-ol and propanoic acid would form the ester propyl propanoate so A is out. If Y is ethanol then Z would be ethyl propanoate so D is the correct answer. (page 5-6,8-9, 173-6)
19.	A	This is the answer that the marking guide gives, though I believe that B is also a correct answer. From the data table the solution (indicator) changes colour around pH 3 (somewhere between 2 and 7). D is definitely wrong because it is a weak acid, strong base titration for which the pH at the equivalence point will be greater than 7. C is also wrong because being a weak acid, weak base titration, the pH at its equivalence point is around 7 but the pH change is quite small so when the solution has a pH of about 3 (when the indicator changes colour) the titration is a significant distance from the equivalence point. A is a strong acid, weak base titration so the pH at its equivalence point is less than 7. Our indicator changes colour at a pH less than 7 so it will correctly signal the equivalence point. B is a strong acid, strong base titration so the pH at the equivalence point is about 7, but there is a very big change in pH at this equivalence point, say from about 3 to 11 (or maybe 4 to 10) so when the indicator changes colour (at pH around 3) it will be for all practical purposes at the equivalence point, so B is an acceptable answer. (page 164-5)
20.	D	In ozone the average bond energy is between that of a single and a double bond which makes it weaker than for a double bond (as in oxygen). This means that it is easier to break a bond in ozone than in oxygen so ozone will be more reactive. The ozone molecule is polar, it is bent and it does have a greater molecular mass, but these factors are of little importance in determining reactivity for a substance that usually reacts in the gas phase. (page 238-43)