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

Somebody tell me why it's not B?

@Funky_Giraffe @samb1234

would it be right to say thermodynamically stable and thermodynamically feasible are are opposites?

This question really got me too haha. If you work out the moles of the acid and alkali, you'll find that the acid is in excess. When you subtract the moles of alkali from the moles of acid to see how many moles of acid you have left over, you'll find that it is the same as the moles of ethanoate ions formed. Thus, [HA] = [A-], which means that Ka = [H+] (think about the expression for Ka and you'll see why this is the case). So, to find [H+], you just use your value for Ka!

I got C, 3 moles of ester at equilibrium so there are also 3 moles of water. Minus 3 from initial moles of both reactants gives you 1mol of methanoic acid and 3 moles of ethanol, I put them into kc expression and got 3 (C)

1 to 1 ratio: therefore equilibrium moles of methanol acid would be 4-3= 1 mol and ethanol 6-3 = 3 mol therefore moles of ester and water must be 3. overall experession is 9/3 which is 3

Thanks for that, but now I am a little confused as to why ethanoate ions are also 0.05.... thanks again

Well, check this out:

OHHHHHHHHHHH!!!!!!!! I get it! Thank you. you superstar!

Do you leave water out of a Kc expression??

Could someone please explain why there are different answers to similar questions?

They look similar but they're not the same. The first one asks what happens if you add a catalyst, which doesn't affect the position of equilibrium but makes it be reached quicker. The second one asks what happens if you increase pressure, think about La Chateliers principle and how equilibrium shifts to the side with less moles when pressure increases, at A2 you need to know why it does that and it's because Kp stays constant so equilibrium moves to alter the partial pressures of the reactants and products to keep Kp constant.

Please could someone's explain the relationship between entropy change and the equilibrium constant?

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It'll probably be clear from the question and answers that you work out for equilibrium concs. etc. if you leave it out or not

Can you clarify , because i assumed you left it out due to the concentration of water being constant, but when i worked out the Kc, of the multiple choice question, i got the right answer when i left it in

Total entropy change = RlnK (where R is the gas constant 8.31kJmol-1)

So basically, total entropy change is proportional to lnK.

Can anyone help with this question please? I'm kind of confused.


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