Aqa chem 4/ chem 5 june 2016 thread

Original post by SubwayLover1

The mass of an electron is tiny!!

Ah fair enough - thanks!

Reply 464

lahigueraxxx

Original post by Limerence.

The mass of an electron is pretty much close to 0. mass number is equal to protons + neutrons

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Thank you! Makes sense now :smile:

Reply 465

emsieMC

5

Hey guys I'm really stuck on how they worked out the moles of so3 to be 5.2( 8-2.8?i guess, I don't know why) but I got the 2.8 for so3.
Questions is this: 2SO3 <--> 2SO2 + O2
At start so3 is 8 moles
At equilibrium O2 is 1.4
Determine amount of so2 and so3 at equilibrium
Usually I don't have any troubles with these, maybe I'm having a mental block, chem4 I'm pretty much sound at everything else. Thanks!

Reply 466

Matt Johnson

Can they ask questions about the AS mechanisms in the unit 4 exam? or will it just be the A2 mechanisms like Nucleophillic addition-elimination?

Reply 467

Original post by emsieMC

Hey guys I'm really stuck on how they worked out the moles of so3 to be 5.2( 8-2.8?i guess, I don't know why) but I got the 2.8 for so3.
Questions is this: 2SO3 <--> 2SO2 + O2
At start so3 is 8 moles
At equilibrium O2 is 1.4
Determine amount of so2 and so3 at equilibrium
Usually I don't have any troubles with these, maybe I'm having a mental block, chem4 I'm pretty much sound at everything else. Thanks!

You've made 1.4mol of O₂ and SO₃:O₂ is a 2:1 ratio, so 2.8mol of SO₃ has been used up. You started off with 8 so 8-2.8.

Reply 468

lahigueraxxx

Good luck everyone! Let's smash this

Reply 469

badaman

Original post by Matt Johnson

Can they ask questions about the AS mechanisms in the unit 4 exam? or will it just be the A2 mechanisms like Nucleophillic addition-elimination?

Yes they very much can ask for AS mechanisms like in previous years.

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Reply 470

Lilly1234567890

9

why is it that the larger the ka value the stronger the weak acid?

Reply 471

emsieMC

5

Original post by Parallex

You've made 1.4mol of O₂ and SO₃:O₂ is a 2:1 ratio, so 2.8mol of SO₃ has been used up. You started off with 8 so 8-2.8.

Thank you but shouldn't it also minus the 2.8 from so2? because 2.8 miles of so2 is also formed

Reply 472

sean.17

1

Original post by Lilly1234567890

why is it that the larger the ka value the stronger the weak acid?

From what I know, I'd suppose it was something to do with the fact that a larger Ka value means higher [H+] and [X-] as they go on the top of the Ka equation and therefore as there is higher concentration of both, the acid will dissociate more thus stronger acid as strong acids fully dissociate :smile:

Reply 473

Super199

20

http://filestore.aqa.org.uk/subjects/AQA-CHEM4-QP-JUN12.PDF

3bi can someone explain?

Reply 474

Original post by emsieMC

Thank you but shouldn't it also minus the 2.8 from so2? because 2.8 miles of so2 is also formed

For every 2mol of SO₃that breaks down, 2mol of SO₂ and 1 mol of O₂ are formed. We're not saying that some SO₃ breaks down to form SO₂ and some breaks down to O₂, it's that each mole of SO₃that breaks down forms one mole of SO₂ AND half a mole of O₂.

Reply 475

cowie

7

Original post by Lilly1234567890

why is it that the larger the ka value the stronger the weak acid?

Ka = (h+)((x-)/(hx) so Ka is proportional to h+ which higher in a stronger weak acid

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Reply 476

Original post by Super199

http://filestore.aqa.org.uk/subjects/AQA-CHEM4-QP-JUN12.PDF

3bi can someone explain?

H₂O is protonated going from the LHS to the RHS, so it's acting as a base. It accepts a H⁺ to form H₃O⁺.

Now look at the backwards reaction by itself, CH₃COO^-accepts a H⁺ to form CH₃COOH. That is also acting as a base as it is being protonated.

Reply 477

Super199

20

Original post by Parallex

H₂O is protonated going from the LHS to the RHS, so it's acting as a base. It accepts a H⁺ to form H₃O⁺.

Now look at the backwards reaction by itself, CH₃COO^-accepts a H⁺ to form CH₃COOH. That is also acting as a base as it is being protonated.

So with the second one CH3Nh2 gains a H+ so is an acid? What about the water?

Reply 478