2001 Q18a – always label your salt bridges as KNO3 (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 Eo value all temperatures must be at 25oC, 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 Eo, 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:
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.
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