Answers To Difficult Questions (3 Viewers)

[yoakim]

I got ~18,000ppm for Cl- conc, and 0.003L for S (3mL).

 

[Shrikar]

2/3 marks............think it was 3

Yeah, it was worth 3 marks, don't worry, i screwed up alot of 3 markers.

hello~

the wikipedia site is very unreliable!
i think for question15: the answers c) initiator
but everywhere i read it seems like most people agree with b) catalyst.

man... i tripped in all of the calculations questions..
i hate calculations....T_T;;

Join the club. I feel the same way.

I got ~18,000ppm for Cl- conc, and 0.003L for S (3mL).

I think thats what most people got. I don't think I even got anywhere near that.

 

[Fish Sauce]

I think thats what most people got. I don't think I even got anywhere near that.

Same. I got something like 72000

 

[claudia b]

BTW: multiple choice answer th 7 is D... for answer see preliminary conquering chemistry page 285- REMEMBER: neutralization is an exothermic reaction

 

[jayel89]

does anyone at all remember the actual question or like the measurements given in the KOH and sulfuric acid question?? i noe the answer, but just need the actual question given.

 

[joshuajspence]

does anyone at all remember the actual question or like the measurements given in the KOH and sulfuric acid question?? i noe the answer, but just need the actual question given.

im pretty sure it was

what volume of 0.005 mol/L KOH solution is needed to neutralise a sulfuric acid solution which was originally 0.005 mol/L solution, but had been diluted by a factor of 10 (pH increase by 1)

thats basically it

 

[mzduxx2006]

Q15 Multi Choice
Yes, Cl radical IS A CATALYST :wave:

http://en.wikipedia.org/wiki/Catalyst

"Examples of homogeneous catalysts are H+(aq) which acts as a catalyst in esterification, and chlorine free radicals in the break down of ozone."

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Chemical Reason for lower Heat of Combustion: Incomplete Combustion (took me some time to figure out lol)

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Anyone have anything on why you can't flame test? Edit : Lead poisoning apparently

Edit: For industrial chem ppl: Hope nobody put down Solid Sulfur in their Equilibrium K constant ;P

OMG YES. SAID IT WAS A CATALYST......um for the flame test my explanation was that flame tests identify cations not anions. for the love of god please tell me im right

 

[joshuajspence]

OMG YES. SAID IT WAS A CATALYST......um for the flame test my explanation was that flame tests identify cations not anions. for the love of god please tell me im right

yeah i said the same for flame tests...

flame tests could distinguish the solutions... but i dont think a flame test can be used to identify a solution of sodium carbonate as it would give the same colour as sodium chloride

 

[Armenikum]

OMG YES. SAID IT WAS A CATALYST......um for the flame test my explanation was that flame tests identify cations not anions. for the love of god please tell me im right

I am pretty sure it's an initiator...and since when was reading wikipedia taken with more than a grain of salt besides last minute assignments? lol

 

[La Divina]

Same. I got something like 72000

yes me too! its way off though ><

 

[dwatt]

yeah i said the same for flame tests...

flame tests could distinguish the solutions... but i dont think a flame test can be used to identify a solution of sodium carbonate as it would give the same colour as sodium chloride

You had me feeling good for a second. What you say is true, but I think it doesn't apply to our exam. Because whoever was trying to find the solutions knew what which species were present, just not in which bottle. Ie. he knew that sodium carbonate was present, not sodium chloride, but just didn't know which solution it was.

While I wish you were right, I think the only possible answer to this one is that lead vapour is toxic.

 

[Fish Sauce]

yes me too! its way off though ><

Bugger. Oh well at least if someone else got it that means we followed some sort of logic haha

 

[joshuajspence]

You had me feeling good for a second. What you say is true, but I think it doesn't apply to our exam. Because whoever was trying to find the solutions knew what which species were present, just not in which bottle. Ie. he knew that sodium carbonate was present, not sodium chloride, but just didn't know which solution it was.

While I wish you were right, I think the only possible answer to this one is that lead vapour is toxic.

but surely you can do a flame test under a fume hood? theres no reason why you cant?

 

[Kmara2nv]

thats wat i got 60 ml DDDDDD

 

[Kmara2nv]

well im fked..hopefully ill gett marks for working..

For the KOH diluting with H2S04 i considered it was diprotic..
everyone got 3 ml...i got 6x 10-3...but i convert that to 60ml -___-"

For the ppm question. i got 0.51 mol/L i converted it to 18050...but then i thought, that number is too big..so i moved it to 18 ppm -____-"

and industrial chem.
misread my 7 for a 2..got a k value of 4 instead of 1.33

yay thats wati got finally someone like me lol

 

[jtyler]

For 15, I answered Initiator but in retrospect can see the strong argument for Chlorine radicals acting as catalysts. The thing that gets me is that when considering the composition and decomposition of ozone wholly, the presence of Chlorine radicals changes the position of equilibrium. Catalysts do not, they only decrease the time required to reach equilibrium. Also, based solely on the equations detailed (going by memory here, forgot to take the paper), if you removed Chlorine radicals from the equation the reaction could not occur, whilst if you removed the catalyst from the equation of any other reaction, it can still occur. Yes, I know ozone does break down on its own (or via reaction with O radicals), but if I remember correctly there was something in the question that referred solely to the equations shown. Another argument for Chlorine as a catalyst is the fact that its reformed. If I could do the paper again, I'd probably say catalyst, but I think there's a very strong argument for initiator :/ All in all a rather poor question that relies too heavily on trivial semantics.

For the question regarding AAS, a lot of people here seem to think it was the same principle as a flame test which is untrue. For a start, AAS can identify metals flame tests cannot. Secondly, the lamp does not, as someone suggested, 'excite' the gaseous atoms, the photons are fired through the sample in order to detect which wavelengths are absorbed. Atomic absorption spectrometry. A flame test excites electrons in order to (crudely) detect the wavelengths emitted as they return to their ground state. It is an application of atomic emission spectrometry. Although my teacher and I argued about whether AAS is suitable qualitatively (I argued it was possible, he didn't award the mark), I answered C (answer regarding white light), after ruling out all the alternatives...

 

[tanguyen]

I chose the two answers you said were wrong. I figured the role of chlorine in those reactions was the same with the role of an organic peroxide catalyst which starts the polymerisation of polyethylene. The AAS one was just a guesstimated choice.

 

[nooj]

For 15, I answered Initiator but in retrospect can see the strong argument for Chlorine radicals acting as catalysts. The thing that gets me is that when considering the composition and decomposition of ozone wholly, the presence of Chlorine radicals changes the position of equilibrium. Catalysts do not, they only decrease the time required to reach equilibrium. Also, based solely on the equations detailed (going by memory here, forgot to take the paper), if you removed Chlorine radicals from the equation the reaction could not occur, whilst if you removed the catalyst from the equation of any other reaction, it can still occur. Yes, I know ozone does break down on its own (or via reaction with O radicals), but if I remember correctly there was something in the question that referred solely to the equations shown. Another argument for Chlorine as a catalyst is the fact that its reformed. If I could do the paper again, I'd probably say catalyst, but I think there's a very strong argument for initiator :/ All in all a rather poor question that relies too heavily on trivial semantics.

For the question regarding AAS, a lot of people here seem to think it was the same principle as a flame test which is untrue. For a start, AAS can identify metals flame tests cannot. Secondly, the lamp does not, as someone suggested, 'excite' the gaseous atoms, the photons are fired through the sample in order to detect which wavelengths are absorbed. Atomic absorption spectrometry. A flame test excites electrons in order to (crudely) detect the wavelengths emitted as they return to their ground state. It is an application of atomic emission spectrometry. Although my teacher and I argued about whether AAS is suitable qualitatively (I argued it was possible, he didn't award the mark), I answered C (answer regarding white light), after ruling out all the alternatives...

where do you think the absorbed wave length goes, it goes to excite the vaporised metal atoms and thus the photon is absorbed, it cant mysteriously disappear, that energy must be used, those who do quanta to quark option is physics would know that d is definitely the answer.

the explanation as to how this works is that the cathode is made of the metal to be tested, the cathode is heated and the electrons excite and fall back down and give out a photon which corresponds to the energy difference between the excited state and the stationary state and the vaporised metal ions which do not hav sufficient energy to become excited absorb this phton of light as it has the exact amount of energy to go from stationary state to its excited state hence it absorbs the photon of light. so that means the spectral lines observed in the flame test corresponds to the wavelength absorbed by the metal ions hence d is the answer.

also for question 15 i wouldn't really consider it an equilibrium, whilst the forward and backward reaction are occurring i think the fact there is continuous energy (UV rays) being used to break the bonds kind of means there really isn't a natural equilibrium and its not really a closed system.

 

[Armenikum]

For 15, I answered Initiator but in retrospect can see the strong argument for Chlorine radicals acting as catalysts. The thing that gets me is that when considering the composition and decomposition of ozone wholly, the presence of Chlorine radicals changes the position of equilibrium. Catalysts do not, they only decrease the time required to reach equilibrium. Also, based solely on the equations detailed (going by memory here, forgot to take the paper), if you removed Chlorine radicals from the equation the reaction could not occur, whilst if you removed the catalyst from the equation of any other reaction, it can still occur. Yes, I know ozone does break down on its own (or via reaction with O radicals), but if I remember correctly there was something in the question that referred solely to the equations shown. Another argument for Chlorine as a catalyst is the fact that its reformed. If I could do the paper again, I'd probably say catalyst, but I think there's a very strong argument for initiator :/ All in all a rather poor question that relies too heavily on trivial semantics.

For the question regarding AAS, a lot of people here seem to think it was the same principle as a flame test which is untrue. For a start, AAS can identify metals flame tests cannot. Secondly, the lamp does not, as someone suggested, 'excite' the gaseous atoms, the photons are fired through the sample in order to detect which wavelengths are absorbed. Atomic absorption spectrometry. A flame test excites electrons in order to (crudely) detect the wavelengths emitted as they return to their ground state. It is an application of atomic emission spectrometry. Although my teacher and I argued about whether AAS is suitable qualitatively (I argued it was possible, he didn't award the mark), I answered C (answer regarding white light), after ruling out all the alternatives...

Sounds good to me on both counts - I put initiator AND white light ----bos will have to take them solely on your explanation

 

[Armenikum]

where do you think the absorbed wave length goes, it goes to excite the vaporised metal atoms and thus the photon is absorbed, it cant mysteriously disappear, that energy must be used, those who do quanta to quark option is physics would know that d is definitely the answer.

the explanation as to how this works is that the cathode is made of the metal to be tested, the cathode is heated and the electrons excite and fall back down and give out a photon which corresponds to the energy difference between the excited state and the stationary state and the vaporised metal ions which do not hav sufficient energy to become excited absorb this phton of light as it has the exact amount of energy to go from stationary state to its excited state hence it absorbs the photon of light. so that means the spectral lines observed in the flame test corresponds to the wavelength absorbed by the metal ions hence d is the answer.

also for question 15 i wouldn't really consider it an equilibrium, whilst the forward and backward reaction are occurring i think the fact there is continuous energy (UV rays) being used to break the bonds kind of means there really isn't a natural equilibrium and its not really a closed system.

The way I learnt it was that when electrons go from normal to excited state, they need to ABSORB energy, and when they fall back, they EMIT energy.

Therefore, I don't know why you talk about electrons falling back into normal states. The point of the flame is to excite the electrons. When these elctrons are excited they move up from normal state to excited state, ABSORBING energy, in the form of light. The wavelengths to which these atoms absorb, are characteristic of a particular element, and the more they absorb, the greater the concentration of them.

And maybe this is a stretch, but C is a more OVERALL statement about AAS, whereas D may be true, but is only one part of AAS. C is the general idea of AAS, and if you read the question again, maybe it is asking about AAS as a whole process....if that mae sense:wave: