Worksheet 16 and Compulsory Activities
A lot of misconceptions and ugly Chemistry appeared in this set of activities. Most can be squashed pretty easily – but it is going to mean that you have to do some practise to get incorrect habits/thoughts out of your minds – if you made the same mistake 20 times in this activity it is going to take about that many correct attempts to purge your brain of that incorrect approach. You want to make sure that your exam-subconscious-autopilot-brain mode has the correct instructions hardwired in or else…
But before I look at the most troublesome questions here is my Rant of the Week
1) Electrode potentials MUST include the unit VOLTS. Without Volts (or at least a capital V) the answer is meaningless. Don’t let all that hard work calculating the answer be worth nought because you forgot to add a V at the end of your answer.
2) Cu2+ and Cu+ are NOT the same thing. Use the correct cell potential!
Cu2+ + 2e- ® Cu (s) Eo = 0.34V
Cu+ + e- ® Cu (s) Eo = 0.52V
If you are ever doing a copper cell then it is mostly likely to be a 2+ solution and the reduction potential is 0.34V. Learn to love that number and learn to be wary of 0.52V.
3) We learn these things for a reason: OIL RIG, RED CAT, ANGRY OX, ABC’s. At the very least check what you have written against these rules. e.g. you have written a half equation where electrons are being lost then (OIL RIG) that means it is oxidation and (ANGRY OX) that means it is at the anode which is –ve.
4) USE THE REDUCTION POTENTIAL SHEET!. E.g., If you are told that there is a chlorine half cell then look at the data sheet to, not just figure out ABC’s but to also to figure out what reactions are possible. All of these examples below are wrong for many reasons, but that didn’t seem to stop anyone – you should be able to explain what is wrong with each one of these:
Cl2 ® Cl- + e- FAIL
½Cl2 ® Cl2 + e- WT FAIL
Cl- + e- ® Cl2 SO FAIL
Cl- + e- ® Cl+ SO VERY FAIL
Cl+ + e- ® ½ Cl2 FAIL
½ Cl2 + e- ® Cl+ EPIC FAIL
½ Cl2 ® Cl+ + e- FAIL
Q1) The dry cell is also known as the Leclanche cell (after the inventor of one of the first dry cell batteries). It is non rechargeable. The Lead acid battery used in cars IS rechargeable
Q2) Just do it – people must have thought this was part of Q3 as many skipped it (and it was easy)
Q4) When converting from half equations to a full cell equation you must make the electrons balance before you add the half equations together. But when converting a full equation into half equations you should simplify the half cell equations at the end.
Q5,6, 7, 8) Main issue is reading the question and doing all the things the question asked.
Q6) So very bad – see Rant point #4 above. Also in shorthand notation it should go: ion|related gas,Pt
E.g. ||Cl- (aq)|Cl2(g),Pt
Q9) All pretty good except the overall cell equations. Just like any galvanic cell overall equation you: a) make the # of electrons the same in both half equations (i.e. for the dry cell you need to double that whole monkey-nuts equation)
b) simplify at the end (i.e. for the silver cell, the fact that electrolyte concentration stays the same means you should be able to cancel out the OH and H2O on both sides)
Q10) Just be careful - ‘ decreasing tendency’ means gong from the most likely to least likely
Q11) Similar to rant point 2 above. You need to be aware that there is a Fe2+/Fe half equation and a Fe2+/Fe3+ equations. Read the question carefully to know which one you need to use
Q13) To prove a reaction occurs ’as written’ means you need to prove the reaction is spontaneous. Spontaneous redox reactions give a positive Eo value. If you look at the equation provided, one reaction taking place is the oxidation of Iron(II) ions: Fe2+ ® Fe3+ + e-. Take out the Fe2+ and Fe3+ terms from the equation and you are left with the other half equation. That is on the data sheet too. Just add the Fe2+ oxidation Eo to the Eo of the reduction process and see if it is spontaneous.
Q14) First - figure out which is oxidised and which is reduced from the equation, then just add the two potentials together (remembering to reverse the sign of the oxidation Eo)
Compulsory activities – need to be done. What was done was done well except for-
Q2i) Draw the cell diagram needed to measure the Ti|Ti2+ cell potential. To measure a standard cell potential you need to connect it to the standard reference electrode. This is in your notes - you need to look up what the standard electrode is AND its conditions. Yes, technically you can figure out the Ti-cell potential by connecting it to Mg or Ni etc etc as some people drew– but you need to be able to explain the use (and draw a picture of) the standard reference electrode.
Summary feedback
A) We didn’t sand lead, we used a multimeter set to read voltage (not a voltmeter) & we used a new salt bridge for each new galvanic cell.
B) Explain means give a reason – so in the Chemistry context, that means give a reason based on some Chemistry concept. E.g. it is not good enough to say ‘The cell needed a salt bridge’. You need to say ‘The cell needed a salt bridge because...’ where you go on to give a reason based on chemistry.
C) While it is true that the greater difference in standard reduction potential, the greater the voltage (and you should say this) the Q refers you to the activity series so you should also relate the voltage to the relative reactivity of electrodes. Basically, the bigger difference in reactivity, the greater the difference in reduction potential and thus the greater the cell voltage.
D) Standard conditions are 25oC and 1mol/L solutions (and gases at 100kPa). If these conditions are not met the Eo values for the cells will be different to the data sheet and thus the cell potential (and possibly anode and cathode as different cells are effected differently by changing conditions) will be different to that predicted by the sheet. Corrosion or damage to the electrode surface can also change the transport of ions to the surface and effect Eo values.
E) You must mention storage (for proper disposal) all heavy metal solutions as part of your safety (i.e. do not tip down the sink)
F) Beware of the Cu+ vs Cu2+ problem identified above
G) Extracting data from graphs and tables is not just vital to extract data from standard potential tables to assess the validity of our experiment, it is also vital as that is how most numerical data is reported and scientists need to use this data to make calculations, judgements, conclusions and generalisations (i.e. about tends and patterns).
H) You need to calculate the theoretical voltages for your table, but take care with cell voltages to use the correct values. Also, the theoretical value should always be positive (i.e. the theoretical value is the voltage for the spontaneous reaction that should occur. Spontaneous reactions happen automatically without net energy input so their Eo will always be positive).
I) If you are gong to say ‘the salt bridge completes the circuit’ make sure you follow up with ‘by maintaining charge neutrality in the half cells’.
J) KISS – Keep it simple students. Answer the question and don’t over explain. Some of the explanations we use in class are gross-oversimplifications of very complex processes. I use these so that you get a better understanding of the basic process (so you will more easily comprehend & remember the theory) – but they aren’t so good in written reports. Eg for Q3c – just say ‘Anions migrate to the …. through the …. while cations….
K) In your results table, at minimum, you must include all the cells asterisked on the Excel table of result. Cu/Zn, Cu/Fe, Zn/Al, Al/Ni, Ni/Mg, Mg/Pb, Pb/Fe
L) Your cell diagram needs to have the full cell reaction, both half cell reactions, salt bridge, flow of electrons and ions labelled, +, -, all chemical species labelled AND label the anolyte and catholyte.
While I was marking the Experiment summaries I was imagining what an awesome game it would be if they made a Game of Thrones version of Kingdom Rush (a similar concept to the 'Rio' version of 'Angry Birds' but bout a trillion times cooler).
I then had to have a soundtrack for my marking - cue the Youtube hits below. Some people are very talented and have too much time on their hands.
If this does not send shiver down your spine then you need to a) stop what ever it is you are doing b) read 'A Game of Thrones' and/or c) Watch season 1 of the TV series d) come back here and watch this clip again to feel that sense of the awesome that sends a shiver down your spine
And to think that when I was younger I used to think that playing the violin was uncool. I was so naive in 2012
Hmmm, despite the fact that this performer has more talent than I can possibly imagine and despite the awesome choice of music, I'm still not convinced that the flute is cool - but seriously are these things even flutes?...
While the Piano Guys have the definitive Harry Potter and Pirates of the Caribbean performance, I'm still searching for the ultimate Piano of Thrones. This is one of the best I've heard so far
These next two blew me away. I wish I was cool enough to play a guitar.
I admire cellists ability to persevere with their instrument even while knowing that nothing played on a cello will ever surpass Cello Wars.
Well, these two instruments aren't really to my taste but it takes all kinds...
And the award for the most creepily awkward performance goes to...
And now to some Game of Thrones songs for your next birthday party, bucks or hens night or wedding
And then to the most coldest black-hearted song ever
And if you have seen the TV series you will probably find this funny (I did). If you haven't watched it- well, this clip won't make any sense and you will spoil the ending of season 1.
Actually I removed this because I thought about the content and then thought about who the audience is.
If you are determined to see it look up (and watch in this order, noting that they are major spoilers)
Hitler and Ned
Hitler and Khal Drogo
Hitler and Blackwater
The last one - even the name Hitler and... might be a spoiler
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