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Confusing Chemistry AS exam question

Hi! Am revising for my exam tomorrow and have come across this question which I am struggling with. Rep to anyone who can help!

The question is about position of equilibrium.

H2 + I2 -------->(reversible) 2HI

The question states that when pressure is increased, the purple colour becomes deeper (prv Q about increasing temp). However, I thought the equilibrium position would only change if there was a different amount of gases on both sides of the reaction. :confused:

Am I being stupid? lol, thanks!

Reply 1

TwentyNine

l0uis

Hi! Am revising for my exam tomorrow and have come across this question which I am struggling with. Rep to anyone who can help!

The question is about position of equilibrium.

H2 + I2 -------->(reversible) 2HI

The question states that when pressure is increased, the purple colour becomes deeper (prv Q about increasing temp). However, I thought the equilibrium position would only change if there was a different amount of gases on both sides of the reaction. :confused:

Am I being stupid? lol, thanks!

Is that a question on a past paper? If so which one?

Reply 2

LadySmythe

19

So the question actually states that it will go more purple if pressure is increased? I'd agree with you coz there is 2 moles of molecules either side of the equilibrium. What direction would it move to make this purple colour?

Reply 3

Interchange

OP

14

Well to make more purpleness it should move to the left, but it wouldnt due to 2 moles on either side. This question is from OCR Summer 2008 How Far, How Fast? 2813/01

Edit: Thanks for speedy replies so far :smile:

Reply 4

EierVonSatan

21

I₂ is not a gas so:

1 mole of gas ----> 2 moles of gas :smile:

Reply 5

exursist

EierVonSatan

I₂ is not a gas so:

1 mole of gas ----> 2 moles of gas :smile:

I think he's right

Reply 6

Interchange

OP

14

EierVonSatan

I₂ is not a gas so:

1 mole of gas ----> 2 moles of gas :smile:

I thought of this too but state symbols are given in the question and it says I2 is a gas.

Reply 7

Yellowmonkeyman

10

l0uis

I thought of this too but state symbols are given in the question and it says I2 is a gas.

In that case, I think the answer is wrong and you're right!

Reply 8

LadySmythe

19

I'll tell you this now what you put in your 1st post is correct. You'd get 2 marks for it.

3rd point relates to collision theory; Increase in pressure means less space for molecules to move around, so the I2 concentration/density increases, making the colour deeper.

1st and 2nd: equilibrium doesn't alter; same no of moles each side

Reply 9

LadySmythe

19

EierVonSatan

I₂ is not a gas so:

1 mole of gas ----> 2 moles of gas :smile:

http://www.ocr.org.uk/download/pp_08_jun/ocr_30961_pp_08_jun_l_gce_2813_01.pdf question 2a)

But you'd be correct, this is AS chemistry so it lies.

Reply 10

Interchange

OP

14

Loz17

I'll tell you this now what you put in your 1st post is correct. You'd get 2 marks for it.

3rd point relates to collision theory; Increase in pressure means less space for molecules to move around, so the I2 concentration/density increases, making the colour deeper.

1st and 2nd: equilibrium doesn't alter; same no of moles each side

Yeah that makes sense. Thanks, and to everyone else who contributed! Also Loz, did you get this off the mark scheme? If so, was it on the OCR website? If not, could you link me please? Thanks

Reply 11

LadySmythe

19

l0uis

Yeah that makes sense. Thanks, and to everyone else who contributed! Also Loz, did you get this off the mark scheme? If so, was it on the OCR website? If not, could you link me please? Thanks

Yea I got it from MS. I'll have to find it again lol. Will be 2 mins.

Reply 12

LadySmythe

19

l0uis

x

http://www.ocr.org.uk/download/ms_08/ocr_16547_ms_08_l_gce_jun.pdf

Page 24 onwards (although there are links)

Reply 13

cookiemonster1

oh i remember doing this question in the exam, it has a graph right? totally didnt know what i was doing in that exam lol.

Reply 14

EierVonSatan

21

l0uis

I thought of this too but state symbols are given in the question and it says I2 is a gas.

Ah, well that changes it then :p:

You're right that the equilibrium doesn't shift due to pressure changes :yes:

It's a bit of a daft question to say that it becomes more purple because the volume decreases. You could easily raise the pressure of the system by putting an inert gas in there making it less purple looking. Stoopid A-level /rant