Equilibria, Energetics and Elements (F325) - June 2011 Exam.

Reply 720

Jtking3000

13

Original post by 182blink

CAn you send it to me via email pleasee ?? :smile:

if i pm you my email

http://www.qfpost.com/download.do?get=55fdf8ddc8c4ba8fcf13b8f2f3ff8d26

:smile:

Reply 721

wilsea05

1

Original post by Princess_perfect786

why do you have to do this part?

the question asks for concentration, not moles

Reply 722

science rules! :)

Original post by cws121

How come when a system increases in entropy, it becomes energetically more stable?
I thought if entropy increased, T(DeltaS) would increase, and that means that (Delta)G<0 which means the reaction is spontanoeous?? Or am i just overcomplicating things

just remember that if entropy increases, it becomes more +ve, and is more energetically stable (I think of a bedroom, it becomes a mess, this makes it more energetically stable according to book and teachers- so just have to accept :tongue:

)

gibbs free energy is affected by the entropy of the system, the enthalpy with surrondings and the temperature.
For a reaction to take place spontaneously it must be below zero.

Remember when working out gibbs free energy convert all the units to either joules or kilojoules, and use temp in kelvin (convert from degrees if needed) :smile:

yeh, that section is complicated if you use the gibbs free energy equation and sub the positive or negative signs in you can see if gibbs free energy is positive, or negative and feasible.

(edited 12 years ago)

Reply 723

hey-hey-hey

10

Original post by KateSW

I think that you not only have to balance the numbers, but also the charges.
So looking at their answer the charge on either side of the equation = +12
This wouldn't be the case if you balanced only using oxidation numbers.
I hope this makes sense, let me know if i'm not being clear!

hey thanks for the help, but i still dont understand..how? so first balance the equation then the charges..hmmm i tried and failed...I asked my teacher this today and he couldnt even do this.. :s-smilie:

I was like what?!!!

Reply 724

182blink

Original post by Jtking3000

http://www.qfpost.com/download.do?get=55fdf8ddc8c4ba8fcf13b8f2f3ff8d26

:smile:

thanks alottt !!! :biggrin:

Reply 725

Student21

8

Can anyone give me a link to some born haber cycle and lattice entalhpy questions. Im having trouble with these and need loads of practice. Thankyou

Reply 726

student777

8

Original post by Student21

Can anyone give me a link to some born haber cycle and lattice entalhpy questions. Im having trouble with these and need loads of practice. Thankyou

Look at the legacy papers- the Trends and Patterns ones. Should be on the ocr website :smile:

Reply 727

Charlee14

I am so unprepared for this exam :frown:

any ideas

Reply 728

KateSW

Original post by hey-hey-hey

hey thanks for the help, but i still dont understand..how? so first balance the equation then the charges..hmmm i tried and failed...I asked my teacher this today and he couldnt even do this.. :s-smilie:

I was like what?!!!

Okay, I didn't actually try to do it before, and you're right it's really hard!
Obviously I can see that the answer is balanced, but i have no idea how you'd go about doing that! If your teacher can't do it, i've got no chance i'm afraid! Sorry I wasn't more help.

Reply 729

M_I

Do we need to know this stuff for our exam?

Alloys

Colour Change in Titrations

Silver Nitrate

Absorption Spectrum (I don't think we need this, but just in case)

Questions Like:
Barium carbonate and magnesium carbonate both decompose when heated.
Describe and explain the difference between the temperature at which barium carbonate and magnesium carbonate decompose. (3 marks)

Anything to do with NaSCN (NaSCN added to complex ions)

Decribing and explaining sturucture and bonding and electrical conductivity and melting point and reactinf with water, e.g. of MgO and SiCl4

If we do need to know these things can you tell me what we need to know?

These have come up in the legacy so I'm a bit unsure.

Also, MgCO3 a gaint ionic lattice?

Thanks so much. :smile:

Reply 730

wilsea05

1

Original post by hey-hey-hey

Can someone help me with this :
FeO4 2- + H+ -------> O2 + H20 + Fe 3+
Q asks to balance this? I think they use oxidation numbers for this: the answer is :
4FeO4 2- + 20H+ ------->3 O2 + 10H20 + 4Fe 3+

How did they do this??? I could just balance it when looking at the nunber of oxygens, hydrogens etc but i get different answer, Can someone please please help me out... :smile:

FeO4 2- + H+ -------> O2 + H20 + Fe 3+
Fe6+ Fe3+ - reduced by 3, so you want an even number really as O2- -> O and you can't have 1.5O2- lol
so if you try
2Fe6+ -> 2Fe3+ overall reduction of 6
you have 8 O2- ions now, but you only want 3 of them to be oxidized to O2
so that means you'll have 1.5 O2, and 5H2O
you don't really want halves though, so double it again to...
4Fe6+ ->4Fe3+ overall reduction of 12
16O2-, so you want 6 O2- to be reduced, giving you 3O2 molecules
this leaves you with 10 O2-, which stay as O2-
10 O2- means you must have 10H2O, so to balance that you need 20H+

giving overall equation 4FeO42- + 20H+ -> 4Fe3+ + 10 H2O + 3O2

probably would've taken me a LOT longer if you didn't post the answer :smile:

haha

Reply 731

arvin_infinity

3

Could someone link me up with june 09-10 old legacy ??

Ive got a mark scheme for it

Reply 732

M_I

Original post by M_I

Do we need to know this stuff for our exam?

Alloys

Colour Change in Titrations

Silver Nitrate

Absorption Spectrum (I don't think we need this, but just in case)

Questions Like:
Barium carbonate and magnesium carbonate both decompose when heated.
Describe and explain the difference between the temperature at which barium carbonate and magnesium carbonate decompose. (3 marks)

Anything to do with NaSCN (NaSCN added to complex ions)

Decribing and explaining sturucture and bonding and electrical conductivity and melting point and reactinf with water, e.g. of MgO and SiCl4

If we do need to know these things can you tell me what we need to know?

These have come up in the legacy so I'm a bit unsure.

Also, MgCO3 a gaint ionic lattice?

Thanks so much. :smile:

Anyone?

Reply 733

haydyb123

9

Right I'm back from the realms of History, if anyone has any questions I would be glad to answer!

Reply 734

fellowgrip

Can someone help me with this please?

Ka value of phenol is 1.3 x10 -10
pH value of a buffer is 8.71
Student used 0.200 mol dm-3 solution of phenol, mixed with the same volume of sodium phenoxide.
Determine the concentration of phenoxide?

I get the answer 0.013 mol dm-3 when the mark scheme says it is 0.13 moldm-3... I can't see where i've gone wrong, can someone please explain this :smile:

Reply 735

haydyb123

9

Original post by M_I

Anyone?

No I think it's all legacy! :smile:

Reply 736

Limesasquatch

11

TBH, this unit is a piece of p*ss. Yet i have been unable to score well in any past paper!!

Reply 737

Limesasquatch

11

Original post by fellowgrip

Can someone help me with this please?

Ka value of phenol is 1.3 x10 -10
pH value of a buffer is 8.71
Student used 0.200 mol dm-3 solution of phenol, mixed with the same volume of sodium phenoxide.
Determine the concentration of phenoxide?

I get the answer 0.013 mol dm-3 when the mark scheme says it is 0.13 moldm-3... I can't see where i've gone wrong, can someone please explain this :smile:

Is the pH value of the buffer referring to the equilibrium between
C6H5OH <--> C6H50- + H+
?

Reply 738

haydyb123

9

Original post by fellowgrip

Can someone help me with this please?

Ka value of phenol is 1.3 x10 -10
pH value of a buffer is 8.71
Student used 0.200 mol dm-3 solution of phenol, mixed with the same volume of sodium phenoxide.
Determine the concentration of phenoxide?

I get the answer 0.013 mol dm-3 when the mark scheme says it is 0.13 moldm-3... I can't see where i've gone wrong, can someone please explain this :smile: