a level chemistry qns help replied by arigatoast @ Thu, 26 Jun 2008 00:25:05 +0800

arigatoast — Thu, 26 Jun 2008 00:25:05 +0800

u shld tell me that 2 years ago as i'm going year 3 once sch starts in aug...

for the NIE campus... i've visited it for the past 2 years though... lol..

Thanks aniwae!

a level chemistry qns help replied by UltimaOnline @ Wed, 25 Jun 2008 20:58:21 +0800

UltimaOnline — Wed, 25 Jun 2008 20:58:21 +0800

Hi Aigatoast,

Ahh yes the good ol NTU campus at Boon Lay (bus 179 or 199, anyone?). If you have the chance, visit the NIE campus in there, the architecture design layout of the NIE blocks is set at an laughingly AWKWARD angle. You have to see it for yourself to understand what I mean.

So it turns out the ACJC question is asking for the formula of this particular allotrope of oxygen. O4 or tetraoxygen, like O5, O6, etc, is awfully unstable, so in practice, you probably won't be able to isolate pure O4 and use it as the question described... but it's a theoretical question, afterall.

I still maintain that the question is still flawed with insufficient info given (for instance, you could argue that organic chemistry, equilibria and gas law calculations need to be involved, not just simple stoichiometry; this ACJC question is based on too many oversimplified assumptions), but of course I could be mistaken. Regardless, I'd be interested if you could share the ACJC answer with us here (perhaps after your cousin has discussed the question with his ACJC lecturer).

You just enjoyed 2 years of NS, now enjoy the rest of your undergrad days before you're irrevocably thrust into the cutthroat working world for the rest of your life, Arigatoast!

a level chemistry qns help replied by arigatoast @ Wed, 25 Jun 2008 15:12:49 +0800

arigatoast — Wed, 25 Jun 2008 15:12:49 +0800

hi there, i'm an undergrad at NTU. As i got an A in A level chem , my JC2 ACJC cousin often ask me difficult chem question..its H2 chem. I took a photo of this question from his homework...

The question was asking for the formula of that ozonised oxygen, so the answer should be 'O' with an unknown number in the subscript.

according to him, the answer is O4....

i think the key to this question is

an expansion occurs, which is half the contraction observed in the previous treatment .

Are you able to derive the answer from this info?

Thanks anyway.

a level chemistry qns help replied by UltimaOnline @ Wed, 25 Jun 2008 00:43:04 +0800

UltimaOnline — Wed, 25 Jun 2008 00:43:04 +0800

[Disclaimer : If you disagree with the contents of this post (which makes no claim whatsoever to be accurate or correct, or the final say on this question), then please disregard the contents herein and proceed as you would with the question. Perhaps you could interpret the question in the (strange) way the setter had originally intended, in which case, good for you.]

The question is surely flawed in its iteration, or at the very least, misleading. As such, the only offer of (limited, if at all) assistance will be of brief comments and/or hyperlinks (somewhat) relevant to the topic being discussed (by the question). Check with your teacher (or whomever gave you this dubious question) what he, or the question, was actually trying to get you to calculate or work out.

1) The term 'ozonized oxygen' means to (partially or usually fully) convert oxygen from it's most common allotrope of dioxygen O2, to it's ozone allotrope of O3. Hence, by definition, the formula of ozone is O3 (see http://en.wikipedia.org/wiki/Ozone ). The question is therefore not asking for "what is the formula of ozone", but is asking for something else (thus the iteration of the question is flawed and misleading).

2) Students of H2/H1 'A' level Chemistry are familiar with the oxidative cleavage of an alkene double bond by hot, concentrated KMnO4, but many students may not be aware that a similar (but not identical) oxidative cleavage occurs with ozone, known as ozonolysis. Note, however, with ozonolysis you require a follow-up (reductive work-up) step to reduce the cyclic intermediate (molozonide rearranged to ozonide) to the carbonyl compounds, using dimethyl sulphide (to produce aldehydes and ketones) or sodium borohydride (to produce alcohols) or hydrogen peroxide (to produce carboxylic acids).

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

Thus, the volume of ozone decreases as it is used up to oxidize alkenes to carbonyl compounds (ie. aldehydes and/or ketones), alcohols or carboxylic acids. Notice, in the (flawed) question, you're not given data on what the alkene is, it's physical state or any solvent used, or how much alkene is used, or which is the excess/limiting reactant, etc. A 'contraction' in volume seems to imply a physical change rather than a chemical change, which is unreasonable. What actually happens, is that the ozone is used up to oxidize the alkene.

3) What happens when you heat ozone? Let's consider the conversion of ozone (O3) to dioxygen (O2). Note that bond breaking of 2O3 to 6O is endothermic, and bond forming of 6O to 3O2 is exothermic. Overall, conversion of 2O3 to 3O2 is exothermic. However, note that the forward reaction results in an increase in entropy. Since G = H - TS, thus even though the forward reaction is exothermic (ie. disfavoured by high temperatures where enthalpy is concerned), a high temperature multiplied by a positive change in entropy results in a more strongly negative or favourable Gibbs free energy, thus high temperatures actually encourage the exothermic decomposition of ozone (O3) to dioxygen (O2).

Hence, in the context of the question, heating results in conversion of ozone to dioxygen, which explains why an 'expansion' occurs (since more molecules of gas are formed). Again, the iteration or choice of terminology of this question, is dubious.

-----------------------------------------------------------

My recommendation to students is, when stuck with time-consuming dubious questions, skip the question and utilize your precious time to complete the rest of the paper as quickly as you can. All too often, students score lower than they should in exams, because they spent too much time on a couple of problematic questions, and fail to complete the paper in the limited time duration allocated.

Out of curiosity, arigatoast, are you taking any Chemistry examinations this year or the next? If so, which syllabus and level are you sitting for?

a level chemistry qns help replied by arigatoast @ Tue, 24 Jun 2008 17:20:52 +0800

arigatoast — Tue, 24 Jun 2008 17:20:52 +0800

a 250 cm3 sample of ozonized oxygen is treated with an alkene ,a contraction in volume occurs.

When a 250cm3 sample of ozonized oxygen is heated to 200 degree celcius and then cooled, an expansion occurs, which is half the contraction observed in the previous treatment . Formula of oxygen is O2, what is the formula of ozone. (ans is not O3).

i have no idea where to start calculating, any help is appreciated. thanks.

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a level chemistry qns help replied by arigatoast @ Thu, 26 Jun 2008 00:25:05 +0800

a level chemistry qns help replied by UltimaOnline @ Wed, 25 Jun 2008 20:58:21 +0800

a level chemistry qns help replied by arigatoast @ Wed, 25 Jun 2008 15:12:49 +0800

a level chemistry qns help replied by UltimaOnline @ Wed, 25 Jun 2008 00:43:04 +0800

a level chemistry qns help replied by arigatoast @ Tue, 24 Jun 2008 17:20:52 +0800