3.3 Questions and Answers - Chemteach - University of Canterbury - New Zealand

3.3 Questions and Answers Archive

  1. Can the students expect calculations based on titrations?
  2. Confusion about the use of the terms “galvanic cells” and “electrochemical cells”.
  3. Prefer that applications will include electrochemical cells
  4. Why are calculations included both here and in AS 3.2?
  5. Electrochemical cell conventions
  6. Follow up question - there is still the issue of half cells at the anode where the two species are both aqueous
  7. Balancing of redox equations in alkaline solution

Q1. Can the students expect calculations based on titrations? (ie: similar to 3.2.)

A. Although this is covered extensively in the Internal Standard 3.2,the wording of the special notes for 3.3 ("generic processes involve reactions and calculations") does allow for the possibility of a titration type question being asked in the external standard as well.

So the conclusion would be to revise this skill at end of year just in case.

Q2. This achievement standard involves identifying and describing oxidation-reduction reactions including galvanic cells.

There is some confusion among teachers about the use of the terms “galvanic cells” and “electrochemical cells”. As both terms are used interchangeably in the standard we would suggest that galvanic is replaced by electrochemical as this seems to be more commonly understood to encompass all the cells. Many equate galvanic cells with the specialised cells such as the lead-acid battery.

A. Agree that we should use the term electrochemical cell consistently. This simply defines a system made up of two electrodes in contact with an electrolyte and presumably includes both galvanic cells (an electrochemical cell that produces electricity from a spontaneous chemical reaction) and electrolytic cells (an electrochemical cell for electrolysis i.e. non-spontaneous reactions).

Q3. Prefer that applications will include electrochemical cells

A. Can't use the term will in assessment documents as it forces examiners to include a question on this outcome even if `they would prefer not to in any particular year.

Q4. Why are calculations included both here and in AS 3.2? It is suggested that they be dropped from here and the number of credits for this standard reduced to 2.

A. No problem repeating a particular outcome in two different standards – in this case however there are also the possibility of questions other than just redox titrations e.g those determining unknown oxidation states from mole ratios.

Q5. Electrochemical cell conventions. I have a query regarding the schematic drawing of electrochemical cells. In cell diagrams I always teach that the LHE is shown as oxidation eg:

Cu(s)/Cu2+(aq)// … or Pt/Fe2+, Fe3+ // … etc

But, in the mark schedule for two recent NZIC tests they show: Pt/Fe3+,Fe2+ // … and again: Pt/I2,I-// … (meaning the LHE is shown as reduction). The first time I assumed it was a mistake, but have since seen it several times in various sources.

I have checked textbooks (Packer) and the web (Bestchoice included) but there is nothing specific except that the electrodes must always be on the outside. This means the only time there appears to be a discrepancy is when there are 2 species in solution plus a Pt or C electrode in the LHE.

What do you know about the rules? The NCEA Assessment specifications show nothing of relevance. I guess my main concern is that if a student showed the LHE as oxidation (which they must do, anyway, if one of the species is a solid metal) when a Pt or C electrode is involved is that correct?

A. My understanding is that by convention the cell diagram should be drawn with the anode on the left and the cathode on the right which means for a spontaneous reaction that the reduction half cell will always be on the right. If the order of the two species in each half cell is then written in order from reactants to products any species that is the actual electrode will automatically always appear on the outside of the diagram. Back to top.

Q6. Follow up question: Thanks – I agree with what you have said but there is still the issue of half cells at the anode where the 2 species are both aqueous.  The question I was concerned about comes from a NZIC paper (2004 I think) asking about the cell diagram for a cell made up of Fe2+, Fe3+ and I2, I- half cells. The marking schedule gives

Pt / I2, I- // Fe3+, Fe2+ / Pt (which, at the anode, is contrary to what we think). For achieved: “Both species shown in each side of //.  Could be reversed. Pt omitted” but for Merit: “Correct diagram. Both half-cells shown as reduction ” (my emphasis). A similar question in the following year's paper has a similar answer. Bestchoice also shows a similar cell with Pt and the anode written as reduction.  It would add another layer of difficulty to our teaching if the requirement is reduction to be shown at the anode when an inert electrode is used, which is why I asked my original question.  I have not been able to find any definitive ruling.

A. Unfortunately there is no agreed convention for this so either form is acceptable. Sources such as BestChoice use the Fe3+/Fe2+ form even when oxidation is occurring as this is the order that the species appear in the redox potentials i.e. order is always oxidant, reductant regardless of whether it is at the cathode or anode. Personally I prefer to have the order given in each half cell so that it is reactant,product as I believe this gives a visual reinforcement (particular to weaker students) of he actual process occurring at each electrode.

But at the end of the day it is personal choice and either format should be accepted by external assessors.

Q7. I see the 2006 NZIC practice exam has questions requiring the balancing of redox equations in alkaline solution. I have reread the two most recent versions of the standard and it does not mention this point anywhere.

It used to be that only acidic conditions were required for "normal" Year 13 students but anything went for Scholarship. Can you shed any light on this?  It isn't a big deal but I must have missed something somewhere.

A. As balanced redox equations in alkaline conditions are not specifically excluded in either the Achievement standard itself or in the Assessment specifications (unlike the previous Bursary prescription where, as you point out it was specifically mentioned), my interpretation is that an examiner could ask a question on this so students should be suitably prepared and aware of the possibility.

I guess it's a timely reminder for all of us old timers that we can't necessarily take old “understandings” for granted as the Achievement Standards are new, standalone statements that do not owe any formal allegiance to the previous prescriptions.

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