HSC Chemistry Questions, Feedback, Links

HSC STUDY GUIDES, FEEDBACK LINKS & REVISION VIDEOS

HSC Chem Study Guides here 

HSC Physics Study Guides here

9.1 - SKILLS    Video 1/1

9.2.1 – Polymers      Video 1/2         Video 2/2

9.2.2 – Cellulose and Biopolymers       Video 1/1

9.2.3 – Ethanol         Video 1/1

9.2.4 – Electrochemistry   Video 1/1

9.2.5 – Nuclear Chemistry           Video 1/2      Video 2/2

9.3.1 – Indicators    Video 1/1

9.3.2 – Acidic Oxides & LCP   Video 1/2   Video 2/2

9.3.3 – Acids & pH   Video 1/1

9.3.4 – Acid Theory & Titrations   Video 1/2   Video 2/2

9.3.5 – Esterification     Video 1/1

 

9.4.1 – Collaboration & Monitoring   Video 1/1

9.4.2 – The Haber Process   Video 1/3   Video 2/3   Video 3/3

9.4.3 – Ion Analysis   Video 1/1

9.4.4 – CFC’s & The Ozone Layer     Video 1/1

9.4.5 – Water Analysis & Treatment   Video 1/1

9.6.1 –History of Electrochemistry   Video 1/1

9.6.2 –Steels & Rusting   Video 1/1

9.6.3 – Electrolysis   Video 1/2  Video 2/2

9.6.4 –Corrosion Protection  Video 1/1

9.6.5/6 –Deep Ocean Corrosion   Video 1/1

9.6.7 – Conservation & Restoration   Video 1/1

Remember this - All it takes is "Just Work"!

And watch this before your study/exams etc - How could you not want to do well and dominate the state after watching this ?

9.3.3 HSC Questions Feedback

This set of questions stood out as something that we can work on. You all knew hat pH was and how it related to strength and concentration but the way that these concepts are expressed is crucial – you have to be very careful with terminology (eg weak & strong) and you have to clearly make the link between strength, concentration [H+] and pH.

WARNING – AVOID THE TRAP we really have to stop falling for the tricks where Q ask about increasing or decreasing pH. HIGHER [H+] = LOWER pH

2001

Q20b)  You need to correctly use the terms strong and weak, You need to link them to %I and thus the final hydrogen ion CONCENTRATION and thus to pH via the formula. ‘Even though both acids are at the same concentration (0.1 mol/L) , HCl is a strong acid with 100% ionisation and thus will have a [H+] of 0.1 mol/L. Citric acid is weak and thus has a %I < 100 &  will have [H+] of less than 0.1mol/L. Thus the pH of the hydrochloric acid is lower than citric acid as pH = -log[H+] (ie pH HCl = -log[0.1] = 1) and thus as [H+] increases, pH decreases’ Note you could figure out the citric and HCl %I form the data – this is a GREAT way to answer these questions and reinforce the concept of strng and weak. But it is also something to watch for because according to the data in this question %I for citric is 25% which means that for THIS QUESTION saying the %I of citric is 8% is technically wrong.

2002

Q22b &c) for b) you needed to know that sulfuric acid was diprotic, thus can produce a higher [H+] then HCl when at the same concentration, so sulfuric acids pH will be lower. For c) ditto Q20b) 2001e above, without the calculations.

2003

Q8) We have been over the diprotic nature of sulfuric acid SO many times…

2004

Q7) WARNING – AVOID THE TRAP we really have to stop falling for the tricks where Q ask about increasing or decreasing pH. HIGHER [H+] = LOWER pH

Q24a) This was dilution so use the dilution equation. In addition the #SF in the ‘normal’ numbers in the data is used to calculate the # decimal places in a pH answer (& the #decimal places in pH data = the #SF you need when calculating the answer)

Q24b)  If you are given two chemicals and asked about their uses you should give a different use for each. Also Citric acid is NOT ascorbic acid (vitamin C). Citrus fruits have both these acids in them but it does not mean that they are the same thing. Just like, Pepsi has water and sugar and caffeine in it but they are different chemicals.

Q24c)  You have to give the pH equation. You have to say that higher [H+] = lower pH then you have to say why the [H+] – ie hydrogen ion CONCENTRATION - is different in each and thus why the pH is different in each. Many fell into the trap of talking about the ‘amount’ or moles of H+ in solution – THAT DOES NOT MATTER. pH depends on hydrogen ion CONCENTRATION. Also – NEVER talk about things being ‘more’ or ‘most’ or ‘less’ acidic. This means nothing in Chemistry and reflects badly on you as a Chemistry student. If you mean ‘stronger’ say ‘stronger’. If you mean ‘more concentrated’ say ‘more concentrated’. Finally you need to ay that HCl was strong a t100% ionised and that citric and acetic were weak but had different [H+] as citric is 8% ionised and acetic 1% ionised(giving citric a higher hydrogen ion concentration and a lower pH than acetic).

2005

Q8)  We have been over the diprotic nature of sulfuric acid SO many times…

2007

Q10)  “...must be added…” I’ve thrown this trick at you a few times. It is cruel but you need to be aware and looking for it in ALL dilution questions.  

Q21a&b)  We have been over the diprotic nature of sulfuric acid SO many times…

Q21c)  The same inexplicable numerical error showed up in approximately half of the papers. Again – work together to discuss solutions to the questions but go and work independently. IF you are just copying answers without thinking: a) it is not helping  b) you are not learning, c) you are not showing the person you are copying from respect (because if you were thinking you might have found their mistake and helped them) & d) it means that I can’t fully diagnose problems you are having.

2008

Q14)  This was basically identical to 2006 Q17b and 2012 Q28 but we did well in those and poorly in this. The main error was using the moles of hydrogen ions rather than the concentration in the pH calculation. A silly, avoidable, mistake.

2010

Q21) You need to distinguish yourself from the crowd so if you are given data USE IT! Just like 2001 Q20b – you have the data to calculate the %I so do so. Basically if you ever get a compare the pH question and you can calculate %I then do it. (remember in our first assessment the rule that if you were provided with a stimulus you had to use it in your answer – this is the same – if provided with numbers and you can do a calculation to support your answer then do the calculation)

2012

Q11)  WARNING – AVOID THE TRAP we really have to stop falling for the tricks where Q ask about increasing or decreasing pH. HIGHER [H+] = LOWER pH. You shoud know the different %ionisation of hydrochloric, acetic an citric acids.

Q18) 1) Weak acids are in EQUILIBRIUM with their conjugate base, water and hydronium. Changing any of these chemicals will shift the equilibrium. Eg CH₃COOH _(aq) + H₂O_(l)  ↔ CH₃COO^-_(aq)  + H₃O⁺_(aq)  Diluting is adding water, thus by LCP the reaction will shift to remove water, thus reducing the acid conc and increasing the conc of conjugate and hydronium – thus raising the %I.

2)  WARNING – AVOID THE TRAP we really have to stop falling for the tricks where Q ask about increasing or decreasing pH. HIGHER [H+] = LOWER pH

9.3.2 HSC Questions Feedback

Generally good results but disappointing that half the class did not read the instructions to leave 2002-Q21, 2006-Q22 or 2009-Q18 AND in addition, did not do the extra 2010 and 2011 questions that were sent with the instructions.

2001

Q5) Read the question – UNPOLLUTED rainwater

Q9) It is expected that you know that LCP is about position of equilibrium (yield) NOT about rates of reaction. Also you should know that catalysts only affect the rate NOT the yield of a reaction.

Q10) Read the equations very carefully. IF something is not equilibrium then LCP does not apply.

2003

Q7) Read the WHOLE question carefully ‘reacts with’ is very different to ‘acts as’

2004

Q9) Be careful – this was a limiting reagent Q and like all these Q the limiting reagent is usually written second to trick those who don’t realise it is a limiting reagent question.

Q10) BE careful  - decreasing the pressure causes the equilibrium to shift to the side with more moles of gas (to counteract the decrease in pressure)

Q18) Analyse is a high level verb. For this Q you needed to ID that metal (left side) oxides were generally basic, non-metal (right side) acidic and that there were some metals near the semimetals that are amphoteric. THEN to fully answer the Q you should have included:

• Example of acidic (eg CO₂), basic (eg CaO) and amphoteric (eg Al₂O₃)
• An example reaction showing at least one of th above
• NB that some oxides with low ox# are neutral eg CO, N₂O
• That the far right side of the PT – Noble gases do not form oxides at all

2005

Q22a) Whenever you are asked to plot experimental data be aware of outliers. If you THOUGHT about the reaction you would realise that it would have been impossible for less H2 to be formed at 0.56 g than at 0.33g. The graph should have been two straight lines with intersection corresponding to the stoichiometric amount of Zn.

Q22b) After 1g of Zn the HCl was the LIMITING reagent thus any Zn beyond 1g would not increase the amount of H₂ produced. It is important that you use the term/s limiting and/or excess in your answer. How to know that 1g was the stoichiometric mass of Zn – either from the intersection of the two straight lines on the graph OR by calculating the maximum moles of H₂ and thus the stoichiometric moles of HCl/Zn  

Q22c) In this Q the molar mass has 4SF, the molar volume is 4SF, the mass of An is 2SF and the volume of HCl is 1SF – BUT the volume of HCl is not used in the calculation so the correct # SF is…

2006

Q22) you were not asked to answer this Q but NB that the Q only asked about sulfur dioxide and it said discuss the evidence – thus you had to look at the evidence that SO₂ was increasing (acid rain damage, ice cores etc) and the difficulties (low conc, highly soluble, only measured in recent times) AND you have to use equations in your answer (ie 2+)

Q23bii) This was poorly answered - as soon as you have an equilibrium sign and are asked to explain the effect of –anything- then refer to LCP. Adding NaOCl increase the conc of OCl^-, thus via LCP the equilibrium shifts… AND please don’t make the embarrassing mistake that decreasing acidity = decreasing pH.

2007

Q22b) you should have mentioned that S combusts to form SO₂, that SO₂ is a respiratory irritant, that SO₂ causes acid rain (with equation) and the effects of acid rain on the environment (built and natural). Then you just had to say that reducing S would have a positive impact. Evaluate was a poor verb to choose for a 4 mark Q. I’m not sure how critical it was to get full marks.

2008

Q20a) the unit of Molar mass is g/mol  (ie M = m/n = grams/mol)

2009

Q2) see 2001 Q5

Q15) This is a tricky but important question. There was no excuse for leaving it blank. This is a question in your assessment task so those who didn’t do it I suggest you work to catch up.

Q18) Again you did not need to do this Q – but NB that it was very similar to 2006 Q22) but this time about N rather then S. So the same strategy applies except that NO_X also causes ground level ozone/smog so it needed to be mentioned.

2010

Q18) This linked to 9.3.4 – none of the ins in the options were in the equilibrium reaction – so at first glance it might seem that none had any effect. But NB that there was H+ in the equilibrium so anything that affects [H+] will effect this equilibrium. I wonder if any of those salts will add or remove H+ from solution.

Q26) Another excess/limiting Q – just be aware that 0.50 and 0.20 are 2 SF.

2012

Q15) I wonder what acid I would get if I reacted SO₃ with H₂O?

Exciting news: While procrastinating about this marking I discovered my first ever emerald ore in Minecraft & I got an achievement for killing a ghast with its own fireball! It is amazing that such a ridiculous-looking, pixelated, monster could be so terrifying in-game. The sounds of one approaching puts me on the edge-of-my seat. (pictures from here and here)

& there is an awesome Minecraft desktop wallpaper here

9.2.5 HSC Questions Feedback

2001 - Q16 – whenever discussing an isotope you MUST say what sort of radiation it emits AND preferably give a decay equation (remember that you are trying to demonstrate how much Chemistry you know in your answers).

2002 - Q19 – you have to say too say unstable n/p ratio AND Z>82 (ie >82 protons - the atomic number is the # protons NOT the number of protons + neutrons). In addition when it says to use examples from the graph USE SPECIFIC EXAMPLES WITH EQUATIONS

2003 - Q18 – You need to revisit this video. The key idea is that a nuclear reactor is a source of neutrons (from fission*) and that a particle accelerator is a source of high velocity charged particles (NB you cannot accelerate neutrons in a PA as they are uncharged – hence the need for nuclear reactors to obtain neutrons). Radioisotopes can be made in TWO ways in a nuclear accelerator (from n-bombardment to make high n/p beta minus emitters AND as a direct result of fission as some fission daughter atoms are radioisotopes). Transuranics up to Z=95 can be made by n-bombardment of U, Np, Pu in a nuclear reactor which makes them beta-minus emitters which transmutate into Np, Pu, Am respectively.
Particle accelerators can be used to bombard protons into nuclei to decrease the n/p ratio and make beta plus emitters (eg O-18 + proton => F-18 + neutron), or can collide small ions into heavy nuclei to make heavy transuranics.
*in the reactor the fuel is a fissionable isotope - usually U-235. When U-235 absorbs neurons it undergoes fission = splits into two smaller nuclei and releases neutrons and gamma rays. Other (non-fissionable) isotopes do not undergo fission. They just absorb neutrons, eg U-238 + n => U-239.

2004 - Q26 -Benefits – must mention NDT for industry = increased productivity, safety, product control etc (NB that testing can be done ‘in situ’ ie while products are being made without stopping the process).

Must mention minimisation of invasive surgery for medical isotopes (lowering surgery risks eg infection) and diagnosing and treating cancer = healthier society

Risks – many – BUT make it clear that it is the RADIATION that the radioisotopes emit that is the danger. Risks include exposure to healthy people and healthy tissue = cellular DNA damage and cancer, terrorism risks, danger of radioactive waste to the environment, problems with long term storage etc

2006 – Q16 - see 2003 Q18 above. But make sure you DEFINE transuranic as Z > 92 (ie beyond uranium)

2007 - Q19 – You have to refer to all the things in 2004 Q26 AND detail how a medical and industrial isotope are used. You must give the decay equation, radiation emitted, you should give the half life and most importantly you have to specify what this isotope does that could not be done in other ways (or could not be done as efficiently/safely etc). ALWAYS say why the radiation it emits is useful - ie gamma is highly penetrating so it can penetrate thick steel AND it can exit the body to be detected with a gamma camera so diagnosis can be made. 
But if you read the question carefully you will see that you have to specify CHEMICAL properties. You need to know that that half life and radiation emitted are nuclear properties so you have to also include things like reactivity, state etc. Eg Co-60 being a relatively inert solid metal can be easily regathered if spilled and is unlikely to contaminate the environment. Tc-99m can be tagged to many bioactive molecules so their path through the body can be traced AND our body had no chemical need for Tc so it is readily excreted. The judgement should evaluate their impact on society ie what can be achieved with these radioisotopes that could not otherwise be achieved and why that is good for society (ie health, economy, industry etc).

KEY misconception to avoid: It is VERY important that there is no doubt in the examiners mind that you understand that the diagnosis, detection, treatment, sterilisation is done BY THE RADIATION NOT THE RADIOISOTOPE. The radioisotope emits the radiation that is detected so don’t fall into the trap of saying ‘the isotope is detected by the gamma camera’ or the isotope kills the cancer cells’ because that is literally what some people think and if you write that it will be assumed that you are one of ‘those’ people.

2011 - Q23b – as for 2006 Q16

2012 - Q27a – it is vital that you emphasise the need for gamma rays to pass through he body for diagnosis (high penetration and low ionising power means they can be detected outside the body and are less likely to cause damage to healthy tissue on their way out). Beta because it is highly ionising can cause local damage to tissues  - ie tumours. But again make sure you emphasis that this radiation is EMITTED by the radioisotopes – the radioisotopes themselves are not the radiation.
Misconception - many seemed to think that the 'ability to pass through biological tissue' referred to the time it took for the radiation/radioisotope to pass through the body - NO!. Beta is ionising so it cannot penetrate very far (ie low ability to pass through biological tissue) once emitted as it collides with molecules, ionises them and loses its energy (good for killing tumours of the isotope is placed in the body at the site of the tumour). Gamma is highly penetrating so it can pass through tissue and exit the body once emitted and CAN be detected outside the body with a gamma camera so diagnosis can be done.

9.2.4 HSC Question Feedback

2001 Q18a – always label your salt bridges as KNO_{3 (aq)} and don’t forget the (aq)!

2001 – Q19 – NEVER discuss your dry cell and silver cell without writing the oxidation and reduction half equations (especially for a 7 MARK questions which asks you TO INCLUDE RELEVANT CHEMICAL EQUATIONS!)

2002 Q5 – oops should not be there

2004 – Q14 – be careful to read the question VERY carefully if they ask you to ‘list in order of…’. In this case they wanted it in order or Increasing ease of oxidation. Ie going from less easily oxidised to most easly oxidised – ie going from least to most reactive metals or from ‘most cathodic’ to ‘most anodic’.

2004 – Q15 – Even though we didn’t study the lead-acid cell the picture did show a dry cell abd you should know where the cathode and anode are (ie reduction and oxidation reactions occur there respectively). In addition, you know the –ve terminal is the node so you should be able to label that too

2004 – Q19 – If you lost marks in this question you really need to do it again. For part b you need to i) realise that #moles Cu = #moles CuSO4, ii) calculate the #moles Cu at the start, iii) calculate the #moles that have been LOST from the solution as the ppt on the cathode, iv) calculate the remaining #moles Cu, v) calculate the final conc of Cu in mol/L.

2005 – Q5 – this is a bit picky but you cannot actually observe electrons moving through the voltmeter (in fact voltmeters are designed to have negligible current flowing through them anyway). Also when metal ions produce a coloured solution(eg Cu= blue, Co = pink, Ni = green, Fe = green/brown) adding more ions intensifies the colour and removing the ions fades the colour.

2006 – Q19d – To measure a correct E^o value all temperatures must be at 25^oC, gases at 100kPa, solutions at 1 mol/L (and  notcontaminated) and the surface of the electrodes must be clean.

2007-Q20 – This Q made me a teeny bit cranky as only half the state studies the lead-acid battery. Anyway = basd on the info provided you should be able to figure out the oxidation reaction (anode reaction and thus the lead anode. For the overall equation you just had to add the two half equations together. Batteries lasting several years is good as lead is a toxic heavy metal and dangerous in the environment. Lasting several years meas less disposal/dumping of lead batteries and less pollution.

2008- Q25d – see tips for 2005 Q5

2009 Q9 – For oxidation number questions – you have to figure out the oxidation #s then put them in a balanced half equation so you can see what is going on. Eg if in the reactants it was Pb⁴⁺ and the products Pb²⁺, I would write Pb⁴⁺ ® Pb²⁺, then I would make the charges balance by writing Pb⁴⁺ +2e^- ® Pb²⁺, so I know it is GAINING electrons and being reduced (or you could just remember that reduction means a decrease in Ox# and reduction means GAIN…)

2009 Q26c & d – Woah – not sure what happened with this Q but for the record if a question has parts a, b, c ,etc then when they refer to a chemical eg ‘the metal’  in part c they usually mean the same metal that they introduced in parts a and b. and again for part d – don’t forget the (aq).

2010 – Q13 – Trick Q – this one got me because I was rushing and not reading the question. Yes connecting it with Fe would give the lowest E^o, but the Q asked for it to be attached to an anode – it the metal had to be more reactive (lower reduction potential) than Zn.

2010 – Q27 – Be careful, the anode is the species being oxidised ie the Zn. The cathode is the species being reduced, ie the HgO

2011 – Q12 – read Q carefully – see 2004 Q14

2011 - Q24 – one of the greatest HSC Q. Most were on the right track but got confused about what was going on. Hint – draw it on the diagram  - draw the Copper leaving solution and depositing on the cathode. Draw the Ni dissolving into solution at the anode –that will remind you when to add and when to subtract mass from the electrodes. Some mentioned that it was the Ni that deposited on the Cu - but– if you drew the picture and looked at the half reactions and what was going on you would see that this is clearly impossible.

2012 – Q13 & 14 – re read the Q and check your data sheet very carefully. Take care when calculating oxidation #s and carefully consider the relationship between oxidation, reduction, oxidant, reductant and change in oxidation #’s

2012 – Q33 – The BOS has not released the marking guidelines but you can find a sample answer I the pdf here and below are their general comments:

Question 33 (from here)

In better responses, candidates provided the correct name for an electrochemical cell and also provided a suitable description of the possible environmental impacts of the cell. Valid reasons for the need for collaboration between chemists were also stated, as were the consequences and effects of a lack of collaboration.

In mid-range responses, candidates often provided the roles of chemists and gave the effect of a correctly name electrochemical cell on the environment. In these responses, candidates provided a valid reason for the need for collaboration.

In low-range responses, candidates identified a role of a chemist and showed a basic understanding of an environmental impact of an electrochemical cell. In these responses, candidates often stated that chemists collaborate, but did not provide reasons for the collaboration.