Doubts from Jan06 Edexcen unit 5 paper

Just had few doubts in the jan 06 paper, I have the marking scheme but dont really understand clearly
Il write out my answers the way i understand, correct me where i go wrong

Q1 c (iii)
- Zn has a more negative E than Sn
- If Zn used it will be oxidized prefrentially to Fe (this means Zn gets oxidized and not Fe?)
-But if Sn used, Fe would get oxidized

Q2 (d) - U first mark the tie lines and show it ends up at 80
- As you heat the mixture at a specific conc X it will boil at T1 and give a vapour conc of Y.
- If you condense and reboil again, it will boil at T2 and give a mixture conc of Z
- Similarily simultaneous condensation and reboiling will distill off benzene at the top
- Because vapour collected gets richer with benzene
-Ethylbenzene remains in the flask

Q3 (d)
I dont understand this at all. How is change in oxidation no. 20? i understand that As gets oxidized from +3 to +5 but how are 4 electrons lost?

Reply 1

icanneverremember

usman_s

Just had few doubts in the jan 06 paper, I have the marking scheme but dont really understand clearly
Il write out my answers the way i understand, correct me where i go wrong

Q1 c (iii)
- Zn has a more negative E than Sn
- If Zn used it will be oxidized prefrentially to Fe (this means Zn gets oxidized and not Fe?)
-But if Sn used, Fe would get oxidized

Q2 (d) - U first mark the tie lines and show it ends up at 80
- As you heat the mixture at a specific conc X it will boil at T1 and give a vapour conc of Y.
- If you condense and reboil again, it will boil at T2 and give a mixture conc of Z
- Similarily simultaneous condensation and reboiling will distill off benzene at the top
- Because vapour collected gets richer with benzene
-Ethylbenzene remains in the flask

Q3 (d)
I dont understand this at all. How is change in oxidation no. 20? i understand that As gets oxidized from +3 to +5 but how are 4 electrons lost?

1)c)iii) I think tin just coats the iron so that no iron is exposed. If the can was damaged, the iron would then be oxidised.

3)d) For As: overall ox no changes from (5 x +3) to (10 x +5). This gives a change in + 20. Therefore the overall change in ox no of Mn is equal and opposite, therefore -20.

Hope that helps.

Btw, would you be able to attach the Jan 06 Unit 5 and 6 papers online?

Thanks.

Reply 2

mizfissy815

12

For every mole of As₂O₃, two As atoms are oxidized from +3 to +5. So a total of 4 is lost by every mole of As₂O₃.
So for every 5 moles of As₂O₃, 20e^- are lost.
These 20e^- are taken up by 4 moles of MnO^-. So each mole of MnO^- gains 5 electrons(20/4). So the oxidation state of Mn+7 decreases to +2

Reply 3

=Keith=

OP

3

Icanneverremember i have the unit 5 paper but its mainly photos given to me by Jean. I can forward u the email if u want
Misfissy thats the answer in the markign scheme which i dotn understand

(5 x +3) = 15
(10 x +5) = 50
change in +35.. how is it +20?

and what do u mean by saying 4 electrons are lost per mole?

Reply 4

mizfissy815

12

There are 4 molecules As₂O₃ present right? In each molecule there are 2 atoms of As and each of these loose 2 electrons. So each molecule of As₂O₃ loose 4 electrons (2 electrons per As atom...)

Reply 5

mizfissy815

12