Edexcel A2 Chemistry Exams -6CH04 (14th June) and 6CH05 (22nd June) Discussion Thread

Original post by iStudent96

You see I am lost when you say 'so increasing the temperature would favour the forward direction hence increasing the concentration of hydrogen ions and thus lowering the pH', but the forward direction produces both H+ and OH-. Why would it only increase conc of H+ and not OH-?

It does increase both the conc of H+ and OH-. However, remember that pH = -log([H+]) so it doesn't take into consideration the hydroxide ion concentration. This is why pure water is still neutral at higher temperatures despite a decrease in pH because the
[H+] = [OH-] still :smile:

Reply 803

iStudent96

1

Original post by Whizbox

It wouldn't, it increases both by the same amount hence why it stays neutral, however pH is purely a measure of [H+] so pH decreases as [H+] increases

Thank you so much!! I get it now. Stupid me lool I should have put a little more thought into it!

Reply 804

How's everyone feeling about tomorrow?
I haven't been posting much this year but hello again to people who've seen me spamming the chem AS thread last year haha :lol:

Reply 805

Can someone provide me with clear definitions for enthalpy change of hydration and enthalpy change of solution? There are so many different ones and I don't know who to believe and who not to believe smh I'm developing trust issues.

@Ayman! @C0balt @samb1234 @AnyoneelsepleasehelpmeIamsolost.

(edited 7 years ago)

Reply 806

iStudent96

1

Original post by Don Pedro K.

It does increase both the conc of H+ and OH-. However, remember that pH = -log([H+]) so it doesn't take into consideration the hydroxide ion concentration. This is why pure water is still neutral at higher temperatures despite a decrease in pH because the
[H+] = [OH-] still :smile:

Thank you very much! I don't have any idea why my brain filtered out this simple logic lool

Reply 807

Original post by C0balt

How's everyone feeling about tomorrow?
I haven't been posting much this year but hello again to people who've seen me spamming the chem AS thread last year haha :lol:

Co!!!

Going for that 120/120 again this year xD?

Reply 808

Original post by Don Pedro K.

Co!!!

Going for that 120/120 again this year xD?

Heya

Yeee going for dat 600/600 in A level chem :rofl:

Reply 809

sabahshahed294

12

Original post by iStudent96

You see I am lost when you say 'so increasing the temperature would favour the forward direction hence increasing the concentration of hydrogen ions and thus lowering the pH', but the forward direction produces both H+ and OH-. Why would it only increase conc of H+ and not OH-?

BOTH increase. I think a mathematical explanation will help better. What happens is that when we increase the temperature, the Kw value increases because the equilibrium shifts to the right and result in an increase in both the OH and H ions. Now, assuming both have a concentration equal to x so Kw=x^2. Let's say, the Kw value is now greater than 1x10^-14. Calculate the hydrogen ion concentration using the expression(which is x, x^2 represents product of conc of H and OH). Then find the pH. This way, you'll see that the pH is lower. The effect is mainly due to Kw actually.

(edited 7 years ago)

Reply 810

Whizbox

7

Original post by C0balt

How's everyone feeling about tomorrow?
I haven't been posting much this year but hello again to people who've seen me spamming the chem AS thread last year haha :lol:

I reckon unit 4 should be alright, unit 5 can be a completely different beast though, what do you think?

Reply 811

Original post by Whizbox

I reckon unit 4 should be alright, unit 5 can be a completely different beast though, what do you think?

Yeah, unit 4 is usually the same thing over and over but unit 5 has such a wide range of topics and easier for them to make the paper more difficult! I really don't like electrochemistry :lol:

Reply 812

ihaspotato

2

∆S total is '+'ve

I understand that Temperature is inversely proportional to ∆S surr. so if it (temp) increases ∆S surr decrease which therefore gives a lower ∆S total
But...
∆S sys is unchanged /remains constant... why? explain. Increasing temp doesn't have effect on the entropy of the product/reactant?

Reply 813

robinfairy978

1

Original post by Don Pedro K.

Can someone provide me with clear definitions for enthalpy change of hydration and enthalpy change of solution? There are so many different ones and I don't know who to believe and who not to believe smh I'm developing trust issues.

@Ayman! @C0balt @samb1234 @AnyoneelsepleasehelpmeIamsolost.

Enthalpy of Hydration: Enthalpy Change when one mole of gaseous ions dissolve in sufficient solvent to produce an infinitely dilute solution.

Enthalpy of Solution: Enthalpy Change when one mole of an ionic solid dissolve in sufficient solvent to produce an infinitely dilute solution.

Reply 814

Original post by robinfairy978

Enthalpy of Hydration: Enthalpy Change when one mole of gaseous ions dissolve in sufficient solvent to produce an infinitely dilute solution.

Enthalpy of Solution: Enthalpy Change when one mole of an ionic solid dissolve in sufficient solvent to produce an infinitely dilute solution.

So the difference is that one is for gaseous ions and one is ionic solid! Okay, danke :biggrin:

Reply 815

Funky_Giraffe

10

Original post by ihaspotato

∆S total is '+'ve

I understand that Temperature is inversely proportional to ∆S surr. so if it (temp) increases ∆S surr decrease which therefore gives a lower ∆S total
But...
∆S sys is unchanged /remains constant... why? explain. Increasing temp doesn't have effect on the entropy of the product/reactant?

You normally assume (or are told to assume) that ∆S sys remains constant. It doesn't in reality, but we don't have to know that :smile:

Reply 816

Funky_Giraffe

10

Original post by C0balt

Yeah, unit 4 is usually the same thing over and over but unit 5 has such a wide range of topics and easier for them to make the paper more difficult! I really don't like electrochemistry :lol:

Totally agree with this. Although being the last main-sit exam in this series, the potential is there for them to pull out all the stops and make this unit 4 a hard one...

Reply 817

ihaspotato

2

Original post by Funky_Giraffe

You normally assume (or are told to assume) that ∆S sys remains constant. It doesn't in reality, but we don't have to know that :smile:

Phew. I don't want to know about it anyways :colonhash:

Reply 818