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 a worthy goal for students to aspire to.
The official examiners' comments and a marking scheme (of sorts) are available on the Board of Studies web site, www.boardofstudies.nsw.edu.au (Click on it to go there; find the required year (towards the bottom of the page if not the current one), select Chemistry in the alphabetical list, left or right click on the 'yes' in the exam report column (right column) to open or save it.)
The comments made at the beginning of Answers and comments for the 2006 HSC exam paper are equally relevant to 2007. You might like to review them by clicking on the link.
You need a copy of the exam paper to make sense of what follows. 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 exam papers', scroll down the page till you find '2007 exam papers', click on it, choose ' Chemistry' in the alphabetical list, click on the 'yes' in the exams column (middle one) then you have the option of opening or saving it. You can also get a copy of the examiners report while you are there: click on the 'yes' in the right hand column .
Section I part A (Multiple Choice questions)
Question & Answer Comment (and relevant page in CCHSC)
1. A Renewable means that we can use it up then re-make it. If we burn ethanol, it goes to carbon dioxide which plants can then make into glucose and starch which we can then convert back to ethanol. The other products come from materials we dig out of the ground. While we may find new deposits of the ores, we cannot re-make the ores once they are used up. (p 34-5) 2. D When glucose molecules combine to form cellulose, a molecule of water is released between each pair of glucose molecules. That by definition is a condensation reaction. An addition reaction involves two or more molecules joining together without the release of any atoms; polymerisation of ethylene to polyethylene is an addition reaction. (p. 22-3, 8-13) 3. B In a galvanic cell electrons flow through the external circuit (a wire) while ions flow through the salt bridge, so that eliminates A and C. The anode is the electrode where oxidation occurs so at the anode electrons are released to the external circuit and flow through it (the wire) to the cathode where the reduction reaction absorbs them. 4. None of the four answers is correct. The IUPAC name of the given compound is 3-heptanol. C is probably the answers the examiners want. It is the name used in an alternative naming system used in some important chemical journals but it is not the IUPAC name. (p 12 and CCPC p 267) 5. D You need to look up the standard electrode potentials and subtract the two values for each pair. The question is about the magnitude of the value not its sign. (p 68-70 particularly equation 2.14) 6. C You need to know that phenolphthalein changes from colourless to pink as alkalinity (pH) increases. The only alkaline solution here is NaOH. (p 109, 139) 7. D You need to know that neutralisation reactions are exothermic, meaning that DH for them is negative. D is the only graph with a negative DH. (p 157) 8. C From the information given both acids are weak. Only strong acids are completely ionised so A is incorrect. X will be ionised to a greater extent than Y so B is incorrect. If X is ioinised to a greater extent than Y, it produces a greater concentration of H+ ions which means that it has a lower pH, so C is correct. Both acids require the same amount of NaOH for neutralisation: strength does not affect this. (p 140-2) 9. A Citric acid is a weak acid so the citrate ion is a weak base, so a solution of citrate will hydrolyse (react with water) to a small extent to form citric acid and OH–. These OH– ions will make the pH > 7. (p 152-4) 10. C To change pH from pH 1.0 to 2.0 we need to change H+ concentration from 0.1 mol/L to 0.01 mol?=/L, that is dilute it by a factor of ten, so 90 mL needs to go to 900 mL so 810 mL needs to be added. Answer D was for students who knew what the question was about but did not read it carefully enough. (p 135) 11. B Something you need to know – the definition of hardness. (p 277) 12. A Fossil fuels are coal, oil and natural gas (and oil shales and tar sands). They all contain carbon and hydrogen so when burnt, normally produce carbon dioxide and water. Sulfur dioxide is only produced if there is sulfur in the fuel, often but not always the case. Carbon (soot) is produced during incomplete combustion which does not always happen. Hence B and C are wrong. But which of A and D is correct? The examiners' report gives A as the answer. I guess it is possible to burn a fossil fuel without producing carbon dioxide – by just producing carbon monoxide with or without some soot, but it is highly unlikely that no CO2 would be produced. If the fossil fuel contains hydrogen – and all of them do – it will end up as water so I guess that makes A the correct answer. Maybe the question is trying to combat the popular misconception that coal is carbon rather than being a mixture of compounds which are predominantly carbon but which also contain hydrogen (and often nitrogen and sulfur). If coal is heated in the absence of air it decomposes to volatile gases (formerly used as town gas) and a porous solid called coke that is virtually pure carbon. When coke is burnt it produces only carbon dioxide (no water), but I guess coke is not a fossil fuel but rather a product made from a fossil fuel. Frankly I do not like this question: It seems to be nitpicking rather than testing chemical understanding. (CCPC p 247-8, 268-9) 13. A By Le Chatelier's principle a decrease in pressure will cause the reaction to move to the left (to generate more moles of gas to try to resist the pressure drop) so B and C are incorrect. The reaction releases heat as it goes from left to right (DH negative), so it absorbs heat as it goes from right to left so A is the answer. D is awkwardly worded: does it mean the rate of the reverse reaction will decrease? (p 117-8) 14. D You need to know that in AAS the wavelength of light used is one that is absorbed by the element being analysed for. This means that it is one that is emitted when the element falls back to its ground state after being excited in a flame test. It is this choice of wavelength that makes AAS so particularly sensitive. AAS is quantitative; in fact it is not much good for qualitative analysis because a different lamp (light source) is needed for each element being detected. (p 224-7) 15. B The net result of the two given reactions is
O + O3 ® 2O2
The Cl is consumed in the first step but regenerated in the second so overall it is not used up. This is a requirement of a catalyst. It also brings about or speeds up the overall reaction, the other requirement of a catalyst. (p 253, CCPC p 291, 296-8)