Trial exams discussion

[hyperblade01]

hey just did chemology 2008 exam.....
) Question 3 (8 marks)
A typical Hall-Heroult cell used for the electrolytic extraction of aluminium from alumina, Al2O3,
operates at 5.00 volts with a current of 150 000 amperes.
(a) Write the half equation for the reduction of the aluminium ion in the alumina.

wouldn't the product be Al (s) but the answer says Al (l)

(l) indicates it's in molten form - at the temperatures the cell operates the aluminium will be molten

[jaja]

but how do you know?

[chem-nerd]

because of the temperature that it operates at (~1000C). You should know this because they talk about the use of cryolite to dissolve the alumina as it lowers the temperature by about 1000 degrees.

[jaja]

oh k. thanks

just done stav 2008 exam... im wondering for question 8,9,10. it required us to know in detail about the button cell...like the volts, equations and cathode and anode.. but is it necessary..??

And just out of curosity, why is K only effected when temp change? but not with pressure, volume, ect... rather than rote learning, i am interested to know why

[kendraaaaa]

From memory those questions could be completed without memorization. You can derive the half equations from the whole equation, and hence the cathode/anode.

[jaja]

ohh kk.. another question... from STAV publishing 2008
short answer Q5)

' blood refrigerated at 4C was found to have a ph of 7. from this information, blood at this temperature is....Acidic..... Basic......or ...Neutral (circle your response).... (ii) explain your response"

the answer says acidic refering to the self ionisastion of water... but what does blood got to do with water???

Q6c) explain two functions of ammonium salt solution.....
the answer says "its acidic and provides H+ions"..is this general for all salt bridges? and how so?

[NE2000]

ohh kk.. another question... from STAV publishing 2008
short answer Q5)

' blood refrigerated at 4C was found to have a ph of 7. from this information, blood at this temperature is....Acidic..... Basic......or ...Neutral (circle your response).... (ii) explain your response"

the answer says acidic refering to the self ionisastion of water... but what does blood got to do with water???

Q6c) explain two functions of ammonium salt solution.....
the answer says "its acidic and provides H+ions"..is this general for all salt bridges? and how so?

For the first one. Blood is (I forgot the proportions) part cellular and part plasma. Within the cells there is going to be a lot of solution as well. So we can say that blood is in solution. Because it is in solution it does relate to water. As water's self-ionization is endothermic, at 4C the neutral pH would be higher (lower concentrations) and hence pH 7 would be acidic

For the second one. No this is not general for all salt bridges, but where H+ is involved in a half equation it has to come from somewhere, and if your salt bridge has a known acid (normally a weak one) then you know where the source of H+ is. Salt bridge = completing the circuit and maintaining electrical neutrality is the safe stuff that's general for all bridges. The other stuff is dependent on the half-equations. Eg. KOH could be a salt bridge and the OH- could then partake in the reaction.

[jaja]

how did everyone find the neap o8 exam?

[kendraaaaa]

That was a bloody hard exam.

[jaja]

phew... i thought i was just dumb wat did u get?

[kendraaaaa]

69% (haha)

this was 2 and a half weeks ago, i'm gonna attempt it again on teh weekend and see how i go. what did you get?

[methodsboy]

has any1 done the 08 VCAA? i ended up with an A (quite happy with that)

[arthurk]

yeah i thought the 08 vcaa exam was kinda easy
nothing strange enough to make u think wth is going on

[methodsboy]

^some multi choice were killers

[jaja]

ahh.. worser than you! you need to know the material inside and out to answer the  the questions correctly.
currently correcting the exam as we type,
Neap 08 MC Q12
how would you know that the initial temperature was 273 K?....

Q13) MC