Equilibria, Energetics and Elements (F325) - June 2011 Exam.

Reply 420

tallysingh

Hi can someone help me with entropy. As temperature increases, how does it affect gibbs free energy?

Reply 421

wilsea05

1

Original post by tallysingh

Hi can someone help me with entropy. As temperature increases, how does it affect gibbs free energy?

it depends on the reaction.
G=H-TS (sorry for lack of delta symbols)
so as T increases, if entropy is positive (increase in disorder), G will become more negative as you're increasing the number that you're taking away from H
if entropy is negative and you're increasing the temperature, G will become more positive as Tx-S = negative overall and H--TS = positive.

hope this answers your question and im not being too vague

Reply 422

Medifield

11

Original post by tallysingh

Hi can someone help me with entropy. As temperature increases, how does it affect gibbs free energy?

So if the enthaply change is more exothermic then, the entropy change must be less exothermic for the reaction to be feasible.

If the enthalpy change is endothermic then the entropy change must be very endothermic (or postive) for the reaction to be feasible.

In short, the entropy change must be less negative than enthalpy change.

(Entropy change > enthaply change)

What if they were both negative values?

Then temperature would affect whether its feasible or not- temp for entro must be more postive compared to enthalpy. Temperate must be very low

To create a stable chemical system more enthaply must be given out than taken in.

(edited 12 years ago)

Reply 423

arvin_infinity

3

Original post by Teva

Do we need to know the colour of chromium compounds?

It was on past papers! Thats why am saying we need to know them very well ..thats not something u can work it out in the exam

But then bear in mind its only 1-2 marks if ever comes up

U def know cr2o7 2- (orange)
cr04 2- (the same ox. no. but different colour!)

(edited 12 years ago)

Reply 424

wilsea05

1

Original post by arvin_infinity

It was on past papers! Thats why am saying we need to know them very well ..thats not something u can work it out in the exam

But then bear in mind its only 1-2 marks if ever comes up

well Cr6+ is gonna be orange and Cr3+ is green (should know it from F324, identifying aldehydes and ketones)
they're like the only 2 i know, apart from Fe(II)+NaOH -> Fe(OH)2 (green) and Fe(III)+NaOH -> Fe(OH)3 (brown) if you look at the summary page of 2.3 theres the colours of Fe and Co when adding with NaOH, dont know any others tho :/

Reply 425

Norfolk_Enchance

Got a low A last year. Resitting in this gap year as i want A*AA instead of A*AB. Forgot about it all till yesterday. 10 Days of revision enough? The transition metal chapter is easy, the rates chapter is ****ed, the other is ok.

will let you guys know what happens :biggrin:

and no im not crazy.

I did the same thing with AQA A BIOLOGY UNIT 5 last year, came out with a 132/140 in the end lol.

Reply 426

tallysingh

Original post by Medifield

So if the enthaply change is more exothermic then, the entropy change must be less exothermic for the reaction to be feasible.

If the enthalpy change is endothermic then the entropy change must be very endothermic (or postive) for the reaction to be feasible.

In short, the entropy change must be less negative than enthalpy change.

(Entropy change > enthaply change)

What if they were both negative values?

Then temperature would affect whether its feasible or not- temp for entro must be more postive compared to enthalpy. Temperate must be very low

To create a stable chemical system more enthaply must be given out than taken in.

I totally do not understand what you are saying. for a recation to be feasible, G must be negative. If they are both negative, entropy has to be accompained by a temperature high enough to overcome H? Only then is it feasible? Do higher temperatures take enthalpy out of the chemical system?

Reply 427

Medifield

11

Original post by tallysingh

I totally do not understand what you are saying. for a recation to be feasible, G must be negative. If they are both negative, entropy has to be accompained by a temperature high enough to overcome H? Only then is it feasible? Do higher temperatures take enthalpy out of the chemical system?

When both enthalpy and entropy are negative the temperature must be negative also.

Reply 428

kej817

5

Hi, please can someone help me with this exam question?

As part of an investigation, a student needed to prepare a buffer solution with a pH value of 8.71. From the Ka value of phenol, the stuent thought that a mixture ofphenol and sodium phenoxide could be used to prepare this buffer solution. The student decided to use a 0.2moldm^-3 solution of phenol, mixed with an equal volume of sodium phenoxide.
Use your knowledge of buffer solutions to determine the concentration of sodium phenoxide solution that the student would need to mix with the 0.2moldm^-3 phenol solution.

I found the concentration of H+ to be 1.95 x 10^-9, and have the rearranged equation of C6H5O- = Ka[phenol]/[H+]
But I only have the concentration of H+ and phenol? Would I use 0.2 for phenol ? Are we expected to know the Ka value :s-smilie:

I'm wondering whether, because this is from an exam pack made from the school, part of the question is missed out? :s-smilie:

I'm so confused

Reply 429

lbavfc

Sorry if this question has been answered before, but can someone answer me this about haemoglobin? In the revision guide it states that the Fe2+ ion forms four coordinate bonds with nitrogen from haem, one coordinate bond with nitrogen from globin, and one coordinate bond with water. Where as in the textbook it doesn't mention water and says that two coordinate bonds are formed with nitrogen from globin?

Also, after the haemoglobin has delivered oxygen, does it pick up water or carbon dioxide to take back to the lungs?

Could anyone clear this up please?

Reply 430

blush.ox

15

Original post by lbavfc

Sorry if this question has been answered before, but can someone answer me this about haemoglobin? In the revision guide it states that the Fe2+ ion forms four coordinate bonds with nitrogen from haem, one coordinate bond with nitrogen from globin, and one coordinate bond with water. Where as in the textbook it doesn't mention water and says that two coordinate bonds are formed with nitrogen from globin?

Also, after the haemoglobin has delivered oxygen, does it pick up water or carbon dioxide to take back to the lungs?

Could anyone clear this up please?

It doesnt mention it in the book and i've actually come across this Question somewhere, maybe a past exam Q? :colonhash:

. I'll be waiting for the right answer :tongue:

.

Reply 431

tripodd

Original post by lbavfc

Sorry if this question has been answered before, but can someone answer me this about haemoglobin? In the revision guide it states that the Fe2+ ion forms four coordinate bonds with nitrogen from haem, one coordinate bond with nitrogen from globin, and one coordinate bond with water. Where as in the textbook it doesn't mention water and says that two coordinate bonds are formed with nitrogen from globin?

Also, after the haemoglobin has delivered oxygen, does it pick up water or carbon dioxide to take back to the lungs?

Could anyone clear this up please?

What book says 2 nitrogen bonds form with globin? Don't see that in the OCR textbook

Reply 432

haydyb123

9

Original post by arvin_infinity

Thats right

Am slightly worried about colour of transition metals:

WOuld be epic if someone list them all here

1. Cro4 2- Yellow
2.cucl2- Pale blue
..

Cu2+(aq) Cu(OH)2
Co2+ (aq) Co(OH)2 **
Fe2+ (aq) Fe(OH)2 *
Fe3+ (aq) Fe(OH)3

*Oxidised to Fe3+ on standing, and changes colour accordingly.
**Oxidised to Co3+ on standing, changes colour to Beige

I can do complexes as well if you would like?

Reply 433

haydyb123

9

I've got this feeling that they're going to ask a question about the body's buffer systems, it was Ligand sub in Hamoglobin last year, so I suspect it will be the buffer system this year.

Reply 434

sportycricketer

Original post by haydyb123

I've got this feeling that they're going to ask a question about the body's buffer systems, it was Ligand sub in Hamoglobin last year, so I suspect it will be the buffer system this year.

Thats quite good judgement actually! Cos it includes biochemistry and buffers!

Reply 435

haydyb123

9

Original post by sportycricketer

Thats quite good judgement actually! Cos it includes biochemistry and buffers!

Just a stab in the dark really, we'll see... I can't believe it's 10 days away now, I just want to get it over with, the same can be said for the rest of these exams, I'm tired, and bored to tears with revision.

Reply 436

sportycricketer

Original post by haydyb123

Just a stab in the dark really, we'll see... I can't believe it's 10 days away now, I just want to get it over with, the same can be said for the rest of these exams, I'm tired, and bored to tears with revision.

Tbh who isn't?! :/ What other subjects do you do?? And what UMS do you need in this exam for an A/A*?

Reply 437

blush.ox

15

Original post by haydyb123

I can do complexes as well if you would like?

I'd like them plz if you dont mind :flutter:

xD

Reply 438

haydyb123

9

Original post by sportycricketer

Tbh who isn't?! :/ What other subjects do you do?? And what UMS do you need in this exam for an A/A*?

That's very true my friend, this is the exam I really need to destroy actually, I just haven't done as well I should have in Chemistry exams. I need to get roughly 126-130 UMS to get an A overall, wasn't helped by my coursework. But still I've only got myself to blame, no excuses. I'm going to be embarrassed if I end-up going to study a masters in Chemistry, with a B in Chemistry.

How about you?

Reply 439

haydyb123

9

Original post by blush.ox

I'd like them plz if you dont mind :flutter:

xD

Sure, I'll be right on it :wink:

you disappeared for a while!?