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Aqa chem 4/ chem 5 june 2016 thread

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Original post by Sniperdon227
Can someone clarify how many marks I would get for using; SO2+ 0.5O2 --> SO3 , in each question involving;
Enthalpy: My answer was half the correct (0) marks/2
Entropy: Since using the SO2+ 0.5O2 --> SO3 equation my entropy was also half (0 or 2 ? as follow through of error in eq) marks/2
Gibbs: My answer was calculated using the half the correct values (follow through based on previous answers)/3


Am I awarded 3/7 or 5/7 ?


-2.

Original post by Aethrell
Yeah, so why did increasing pressure affect the oxygen half equation if they weren't all gaseous?


I thought the water produced was gaseous?

Original post by Cadherin
Guys, would I get 'ethanol' as a suitable reagent for reducing Cr3+ to Cr2+? (I put Cr2+ is blue, so that's one mark at least!).


The main principle is a reducing agent with its conditions, hence Zn acidified with HCl.

Personally, I don't believe you would.
Original post by Aethrell
Also, wouldn't increasing the surface area of the platinum electrodes speed up both reactions lead to a faster transfer of electrons and give a higher emf?


Original post by zombaldia
As you said... it speads up both reactions. So has no effect overall. Could be wrong though


Question stated that the SA of both electrodes increased, meaning emf remains the same.
Original post by zombaldia
As you said... it speads up both reactions. So has no effect overall. Could be wrong though


Nah, I'm probably wrong lol

just thought that if the left cell had say -0.3V and the right was 0.3V, the E would be 0.6V. Doubling both would give E as 1.2V

Guessing there's some majorly wrong chemistry there though
Original post by ChronicBoredom
For the emf graph, I drew a 'straight' line free-hand because I didn't have a ruler... will I lose the marks lol? It was a pretty shakey line..


ngl you should have just asked for a ruler or gotten another pencil/pen to use as a ruler

i reckon you'll still pick this mark up if it is clearly meant to be horizontal, and couldn't be mistaken for a line that is intentionally fluctuating up and down. good luck :smile:
Does anyone know what the highest grade boundaries for Chem 5 has been?
Original post by Aethrell
Nah, I'm probably wrong lol

just thought that if the left cell had say -0.3V and the right was 0.3V, the E would be 0.6V. Doubling both would give E as 1.2V

Guessing there's some majorly wrong chemistry there though


You don't double them. That's the point. E values aren't about the speed of the reaction, I believe

Only similar thing I can think of is effect of catalyst on KC... speeds up reaction equally both ways= no effect. Doesn't double it
Original post by hello2you
Does anyone know what the highest grade boundaries for Chem 5 has been?


I just looked for june12-june 15 and highest was 84 last year. I reckon it'll be higher than that though this year.
Original post by zombaldia
You don't double them. That's the point. E values aren't about the speed of the reaction, I believe

Only similar thing I can think of is effect of catalyst on KC... speeds up reaction equally both ways= no effect. Doesn't double it


Fair enough, was counting it as a dead mark anyway.

Do you know why a higher pressure of oxygen shifted the fuel cell equilibrium is they weren't all in gaseous state?
Original post by Charco63
image.jpeg
I put phase boundaries at both ends of the representation of a cell like this example in the picture 😭😭 will I lose the marks for that question?


That's what i did? One was soIid and the other was aqueous so sure that's what it wouId be.
Original post by palacefloor
That's what i did? One was soIid and the other was aqueous so sure that's what it wouId be.


theyre not needed at the end
Original post by Azzer11
I got - 67.5 because I halved my values beforehand as 2 moles of product were being formed. Standard enthalpy of formation by definition is the formation of 1 mole of a compound.

Posted from TSR Mobile


Units for dH are kJmol-1 so you wouldnt halve it as the value is a per molar value, also, you wouldn't halve dS because entropy change would actually be the same regardless of moles because quantity (in terms of molar quantity) is not a unit when determining entropy change for a reaction.
Original post by Sniperdon227
theyre not needed at the end


I don't think i incIuded the end Iines but not entireIy sure. Is it considered fuIIy wrong or couId the examiner just ignore it?
Original post by rosslegg
Units for dH are kJmol-1 so you wouldnt halve it as the value is a per molar value, also, you wouldn't halve dS because entropy change would actually be the same regardless of moles because quantity (in terms of molar quantity) is not a unit when determining entropy change for a reaction.


I think the case has been closed on this one.
Original post by rosslegg
Units for dH are kJmol-1 so you wouldnt halve it as the value is a per molar value, also, you wouldn't halve dS because entropy change would actually be the same regardless of moles because quantity (in terms of molar quantity) is not a unit when determining entropy change for a reaction.



Whats your take on this then........?

Can someone clarify how many marks I would get for using; SO2+ 0.5O2 --> SO3 , in each question involving;
Enthalpy: My answer was half the correct (0) marks/2
Entropy: Since using the SO2+ 0.5O2 --> SO3 equation my entropy was also half (0 or 2 ? as follow through of error in eq) marks/2
Gibbs: My answer was calculated using the half the correct values (follow through based on previous answers) marks/3


Am I awarded 3/7 or 5/7 ?
Original post by Azzer11
I think the case has been closed on this one.


What was the consensus?
Is there an unofficial mark scheme or shall I try and start one? I wrote down how many marks the overall questions were worth each and I remember about 50% of it but with help it could be full.
Original post by Aethrell
What was the consensus?


The division of the molar ratio in the equation by 2 was the wrong method. It will most likely result in the loss of 2/3 marks.
Original post by mgill17
Is there an unofficial mark scheme or shall I try and start one? I wrote down how many marks the overall questions were worth each and I remember about 50% of it but with help it could be full.


Someone has already made one.

http://www.thestudentroom.co.uk/showthread.php?t=4180942
Original post by hello2you

Great! Thanks!
i got -134953 Jmol-1 for delta G, is this okay?

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