Chemistry build up and aftermath

ironclaw

MissRealist wrote: »

Oh oh oh! I think I actually know this one!:eek:. Cl is an oxidising agent and messes with the rxn.

Correct.

Des23

not bakunin wrote: »

ok.........so, how many questions can i nail without going near maths in the paper? any ideas...........im diabolical with numbers, ive a close and figure devoid mind.

i acknowledge that i will have to admit defeat and get out the old calculator and become puzzled-face boy with-lots-of-scribbles-on-his-answerbook-and-a-stupid-answer-with-made-up-maths, but how far can i get before reaching that point?

that, my edelweisses, is the question.

I'd honestly say that all the hardest questions on the paper are the theory based ones, in terms of picking up marks. I mean Q1 calculations are primary school standard, just learn the formula.

You'd have to avoid the fuels question because thermo comes up there usually.

Equilibrium usually has calculations. Stoichimetry usually comes up in Q10 or 11.

That said calculations can be introduced in any question, bar Q5 probably in any given year so you really are limiting youself.

eoins2345

I thought the reason was that kMno4 would oxidise CL- TO cl2 interfering with the reaction

MissRealist

ironclaw wrote: »

Correct.

I know it's ridiculous but that actually makes me giddy!:p:rolleyes:

ironclaw

eoins2345 wrote: »

I thought the reason was that kMno4 would oxidise CL- TO cl2 interfering with the reaction

Ok, have to be careful here:

CL- is the ion (Pronounced "CL minus one" / Chlorine ion / Chlorine radical)

CL2 can mean two things.

It can mean the loss of two electrons So "CL two plus"

Or it can mean the two chlorine atoms combined (aka Chlorine Gas / CL2 molecule)) e.g.:



Depends where the two is, I presume you meant the latter?

KMnO_4 oxidises the chlorine ion (Cl^-) to the chlorine molecule (Cl_2).

yummy91

you know in the titration rxn of kmno4 and ammonium iron(II) sulfate, Is the sulphuric acid added first to stop the air oxidising Fe^2 to Fe^3 and then added again secondly to supply the H+ ions??

herisson

hey guys

my friend predicted what might be coming up i trust him coz he got the bio predictions right but anyway:

1 of the bleach exp, EDTA exp or BOD

for organic exp either cloves or ethyne or ethene

the rest: volatile liquid or bezoic acid

oh we may get a full question on rate of reaction

half question equilibrium and pH

The anion tests ALWAYS come up

stoicheometry generally comes up in q2

My teacher gave us tips too:

if your given Kc you use x (equilibrium or rate of reaction or pH i forget which 1)

know difference between water/sewrage treatment

learn the rations in the volumetric analyst

oh and know the colour changes

MOST IMPORTANT

KNOW THE DEFINITIONS!!! they pop up EVERYWHERE

thats all i know at the mo hoped its useful for people :D:D

Fringe

yummy91 wrote: »

you know in the titration rxn of kmno4 and ammonium iron(II) sulfate, Is the sulphuric acid added first to stop the air oxidising Fe^2 to Fe^3 and then added again secondly to supply the H+ ions??

Yeah that's right. You only need it in excess though.

Alexl

yummy91 wrote: »

you know in the titration rxn of kmno4 and ammonium iron(II) sulfate, Is the sulphuric acid added first to stop the air oxidising Fe^2 to Fe^3 and then added again secondly to supply the H+ ions??

Fringe wrote: »

Yeah that's right. You only need it in excess though.

The first part is right.
Sorry to contradict but you wont get full marks for saying "to supply H+ ions".
The proper answer is to ensure that the Mn (VII) is fully reduced to Mn (II) and does not get caught in the Mn(IV) intermediary phase, which is a brown colour. Hope this helps

Killaqueen!!!

Has anyone kinda given up on the LC now? The first 2 weeks I was cramming like mad but lately I've just been chillaxing...haven't looked at chemistry this weekend but I did study it a few weeks before exams started.

I think after biology I felt my exams were over so I wan't stressed at all even though I had German and Chemistry left - two of my worst subjects!

It's not like me to be this calm...I think I may have been drugged! :pac:

celtic723

Killaqueen!!! wrote: »

Has anyone kinda given up on the LC now? The first 2 weeks I was cramming like mad but lately I've just been chillaxing...haven't looked at chemistry this weekend but I did study it a few weeks before exams started.

I think after biology I felt my exams were over so I wan't stressed at all even though I had German and Chemistry left - two of my worst subjects!

It's not like me to be this calm...I think I may have been drugged! :pac:

i'd agree with the feeling i felt when Bio was over. i thought i was free but it hit me yesterday that i'm not quite free.(YET)

mikeglee

celtic723 wrote: »

i'd agree with the feeling i felt when Bio was over. i thought i was free but it hit me yesterday that i'm not quite free.(YET)

Me too and I still have chemistry and music.

Alexl

mikeglee wrote: »

Me too and I still have chemistry and music.

I still have physics chamestry and apllied maths left, thats half of my points:(:(.

Even worse,
my friend still has music latin physics and applied maths!!!! I doubt anyone here has as bad of a last week!!!!?

lorna100

I still have chem, spanish, classics and religion left! last week I only had irish, maths and french, so this week will be much worse!

JSK 252

yummy91 wrote: »

you know in the titration rxn of kmno4 and ammonium iron(II) sulfate, Is the sulphuric acid added first to stop the air oxidising Fe^2 to Fe^3 and then added again secondly to supply the H+ ions??

1. To prevent hydrolysis. ( reaction of substance or substances with water leading to the formation of hydrogen and hydroxide ions, which may proceed to parcipitate in additional reactions). This reason wasnt mentioned in an earlier post.

2. To be more specific, to prevent the fe 2+ ions being oxidised to fe 3+ ions from the oxygen in the air but ALSO the oxygen dissolved in the water.

aine-maire

Will someone PLEASE explain the following to me?

It's in relation to the unreactivity of benzene :

Chemistry Live wrote:

We can imagine the six valence electrons, one from each carbon atom, belonging to the whole molecule, instead of being localised in three double bonds. This gives the benzene molecule extra stability...

Anyone want to translate that? Thanks in advance

Dmullar

Basically, none of the valence electrons are bound to a specific carbon atom within the benzene molecule, they form a "cloud" around the molecule. This makes it extra stable.

Thats why the benzene molecule is drawn as a hexagon with a circle in it; the circle represents this "cloud" of electrons.

aine-maire

Yeah, but which electrons are they exactly?

I'm probably just way over complicating this, but aren't there four valence molecules in each carbon atom? So which ones are the cloud?

Thanks btw

Dmullar

Carbon does have 4 valence electrons, but in Benzene, 1 is bonded to a hydrogen and 2 are bonded to the carbons either side of it(by a sigma bond i think) which leaves 1 electron that can be donated to the cloud.

Hope that somehow makes it easier.:)

aine-maire

Oh ok, yeah it totally does! I'm thinking I need some sleep when I can't count...:pac:

Bring on Tuesday lol........

Piste

Think of the Benzene molecule as being flat first of all, as you're looking down on it imagine each carbon shares an electron with 1 hydrogen and 2 other carbon atoms. These are all sigma bonds. Now its 4th electron is in a pi bond, which is perpendicular to the plane of the benzene molecule (so imagine the molecule on its side now if you can). The pi bonds overlap, giving a ring above and a ring below the benzene atoms around which the electrons can move. As the pi bonds overlap, the electrons are free to move around all the pi bonds, not just the one belonging to its carbon, and so can move around the whole benzene molecule.

Chemistry live has a diagram on p355 that explains this.

Sorry if my explanation was a little confusing!

aine-maire

Piste wrote: »

Chemistry live has a diagram on p355 that explains this.

Sorry if my explanation was a little confusing!

No,it wasn't at all,totally get it now

See I was revising the chapter 'fuels and heats of reactions' and the explanation there is a little inadequate(well I found it so)...
Haven't got as far as p355 in my revision lol

But yeah,phew,Tuesday should be OK.:o

Thanks btw!

MissRealist

Electronegativity question I need an answer to pronto!

My Chem teacher has always taught us that 0.0-0.4 is a non polar bond but my less stress book says that the difference has to be 0.0

Is anybody 100% certain which is right???:eek::(

Piste

well o.4 isn't going to be completely not polar, it'll be the tiniest bit polar. The only true non-polar value is 0, but o.4 is so tiny it's negligible polarity.

RayCarley

I don't think they'd ask anything like that, which might have a dodgy answer. They usually just ask the obvious values, so you can be sure it's Polar, non polar or ionic.

MissRealist

RayCarley wrote: »

I don't think they'd ask anything like that, which might have a dodgy answer. They usually just ask the obvious values, so you can be sure it's Polar, non polar or ionic.

I've defo done a question where they did ask. It's more the polar values i'm worried about. If my non polar is marked wrong then that means my polar values are incorrect too...

Fringe

Think they asked a question like this where the number was less than 0.4 and it pretty much ruined the entire question since none of us were exactly sure. My teacher completely went over that and the marking scheme. If it's 0, it's pure covalent. 0 - 0.4, it's slightly polar but it's pretty much negligible so you wouldn't consider it a polar molecule. Over 0.4, it's polar covalent. Over 1.7, it's ionic.

Idjit

Anybody out there doing ordinary level chemistry?
What are you guys studying tonite?

mikeglee

Jade182 wrote: »

Anybody out there doing ordinary level chemistry?
What are you guys studying tonite?

I think that I will. I am just going to do papers all night