2014 Chemistry Exam Solutions

[RKTR]

wouldnt 10a. not have the OH bit in there and just be Zn(s)->Zn2+(s)+2e-, since if it had the OH- bit in there, what would the reduction equation be? Since there is no other products apart from Zn(OH)2 what would the O2 and H2O become?

I did the same as you

[Chalkhous]

and isnt the role of potassium hydroxide for 10b to be the electrolyte?

[suberman123]

and isnt the role of potassium hydroxide for 10b to be the electrolyte?

I wrote to provide OH- ions for the reaction since the question assumes acidic electrolyte but idk

[thushan]

Oh yeah, that's an entirely valid answer.

[Aman476]

I do t get why 30 is b, if you have more stuff left over (un reacted mass) that would make it seem like you have less cuo initially present.......

[DSubShell]

I also don't really get the difference between Q30 Multi answers B and D. I think they would both have the same (correct) effect? o.o

[Aman476]

In last s.a did everyone say change in heat would be same since heat is dependent on how much reactant actually reacts which isn't affected by catalyst?

[RKTR]

I do t get why 30 is b, if you have more stuff left over (un reacted mass) that would make it seem like you have less cuo initially present.......

For 30 did you choose C?

[Aman476]

For 30 did you choose C?

Yeah but I think I'm wrong since I kind of guessed between c and d

[thushan]

I went with B, but I can't see why A isn't true either. If CuO was contaminated with carbon, heating it will do nothing (there's no oxygen in the tube) to the carbon, so it's similar to having unreacted CuO.

[KayKay]

SAQ 10a.

Zn(s) + 2OH- (aq) --> Zn(OH)2 + 2e-

Hmm.. I sorta interpreted it as
anode: Zn(s) -> Zn2+(aq) + 2e-
cathode: H20(l) + O2(g) + 4e- -> 4OH-(aq)
And the fact  that Zn2+ and OH- precipitated was a separate thing so I used that anode equation.... Might've lost a mark for that :/

[KayKay]

I went with B, but I can't see why A isn't true either. If CuO was contaminated with carbon, heating it will do nothing (there's no oxygen in the tube) to the carbon, so it's similar to having unreacted CuO.

Wouldn't the oxygen produced from CuO -> Cu + O2 be able to react with the carbon contamination?

[ammmy]

What will the A+ cutoff be?

[Mujteba]

Shouldn't both the reactions in 11a produce a solid product since the cell is rechargeable?

[acinod]

I just had a look at MCQ 16, my opinion:

I'm going with C for the following reason:
- you can extrapolate down, yes, but it wont be as accurate (not 100%, maybe 99%) as between 1-4 ppm; you cannot be fully certain of its accuracy
- in 2011 VCAA there was a similar question and the position was that you can't extrapolate "beyond the range of the graph" (i.e between the points)

I'm gonna go with you here Thushan.

Found this from the textbook solutions:
'Techniques such as AA, GLC, HPLC, UV, etc. do not directly produce measures of concentration. Standards must be used. More than one standard should be used and the unknown sample should lie between these standards. This is because a zero standard may be contaminated with trace amounts of the chemical being tested . Calibration graphs are often non-linear and multiple standards increase the chance of detecting incorrectly prepared solutions'

So MCQ 16 is likely to be C