AQA CHEM5 A2 Chemistry - 19th June 2013

Reply 860

lifeisgood2012

Hey guys.... do we need to know any of the half equations in the hydrogen fuel cell?

Reply 861

Gazfink

Original post by lifeisgood2012

Hey guys.... do we need to know any of the half equations in the hydrogen fuel cell?

Yeah pretty sure we do. I remember them as...
2H2 -----> 4H+ + 4e-
O2 + 4H+ + 4e- -------> 2H2O

which will balance out to give...2H2 + O2 -----> 2H2O

Reply 862

StudentAnon

Does anyone have the advantages/disadvantages associated with the use of the different electrochemical cells?

Are we meant to do non-rechargeable vs rechargeable vs fuel cells or for each individual cell?

Reply 863

mulac1

Original post by samfreak

if u worked out the temperature T from the gibbs free energy change, and the reaction was exothermic, what would be the range of temperature that the reaction would be feasible for? is it <t or >t ?

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go from the formula as normal:

eg feasible when deltaH - TdeltaS < 0

just remember that when you divide by a negative number, you reverse the inequality sign..

Reply 864

SamEastie

2

Original post by lifeisgood2012

Hey guys.... do we need to know any of the half equations in the hydrogen fuel cell?

Original post by Gazfink

Yeah pretty sure we do. I remember them as...
2H2 -----> 4H+ + 4e-
O2 + 4H+ + 4e- -------> 2H2O

which will balance out to give...2H2 + O2 -----> 2H2O

what he said is in acidic conditions.
the other ones you need to know are;
4e- + 2H2O + O2 ---> 4OH-
4e- + 4H2O ---> 4OH- + 2H2

(both written as reductions)

sum of both equations is the same overall as in acidic conditions
2H2+O2--->2H2O. same cell emf.

Reply 865

lifeisgood2012

guys do we need to know any half equations for the titration calculation question thats gone come up?

Reply 866

alex7892

2

I got an a overall for as. Howver I am looking to get an a*. In unit 4 I got 108ums which is exactly and a* however I got 36/50 on my a2 isa not sure what this equals?? Would I still be able to get an a* in the a2 from chem5? Thanks

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Reply 867

lifeisgood2012

what reacations do we need to know that are in the catalyisis topic?

Reply 868

Affliction

Original post by Gazfink

We DO need to know them.
Sodium...yellow flame...white powder
Magnesium...white flame...white powder
Alluminum...white flame...white powder
silicon...white flame...white powder
Phosphorous...white flame...white powder
Sulphur...blue flame...choking fumes produced.

as far as I know this is correct. But please correct me if I'm wrong. Hope that helps.

We don't need to know the flame tests. There's nothing on the spec to suggest that: ImageUploadedByStudent Room1370412458.537811.jpg

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Reply 869

lifeisgood2012

Original post by SamEastie

what he said is in acidic conditions.
the other ones you need to know are;
4e- + 2H2O + O2 ---> 4OH-
4e- + 4H2O ---> 4OH- + 2H2

(both written as reductions)

sum of both equations is the same overall as in acidic conditions
2H2+O2--->2H2O. same cell emf.

So the one you did is in alkali conditions?

Reply 870

Dalts

1

Original post by Affliction

We don't need to know the flame tests. There's nothing on the spec to suggest that: ImageUploadedByStudent Room1370412458.537811.jpg

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Have you done a past paper recently?

Reply 871

Affliction

Original post by Dalts

Have you done a past paper recently?

None so far, I was just using the spec to make my point

Reply 872

Dalts

1

Original post by Affliction

None so far, I was just using the spec to make my point

Well they do ask about those points that you think aren't in the spec, so I do suggest you start doing the past papers, besides you have less than 2 weeks left :L

Reply 873

DELETED ACCOUNT

15

Original post by alex7892

I got an a overall for as. Howver I am looking to get an a*. In unit 4 I got 108ums which is exactly and a* however I got 36/50 on my a2 isa not sure what this equals?? Would I still be able to get an a* in the a2 from chem5? Thanks

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You potentially scraped an A in the ISA.You can still get an A* if you get around 113 UMS out of 120 in Chem5. To achieve this, you'd need approx 84/85 out of 100 in the actual exam.

~~This is all assuming you got a low B or higher in your AS modules.~~
You got an A at AS. :smile:

But you'd still need an average of 90 UMS. :smile:

(edited 10 years ago)

Reply 874

Gazfink

Original post by Affliction

We don't need to know the flame tests. There's nothing on the spec to suggest that: ImageUploadedByStudent Room1370412458.537811.jpg

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Point 75: Write equations for the formations of oxides from elements & describe the trends.

Its very vauge, but the specification is. But can't it be argued that there is a trend in what colour exists when reacting with oxygen. Oxides are usually formed by burning them.

Reply 875

Affliction

Original post by Dalts

Well they do ask about those points that you think aren't in the spec, so I do suggest you start doing the past papers, besides you have less than 2 weeks left :L

Okay, thanks for the advice but I started my chem4 papers 2 weeks before the exam as well, I don't like doing them too early.

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Reply 876

Affliction

Original post by Gazfink

Point 75: Write equations for the formations of oxides from elements & describe the trends.

Its very vauge, but the specification is. But can't it be argued that there is a trend in what colour exists when reacting with oxygen. Oxides are usually formed by burning them.