You are Here: Home

entropy a2 ocr feasibility and free energy watch

1. hi
can someone confirm my understanding is correct?
when free energy delta G is less than 0 the reaction is feasible
and spontanenous (whats the difference) for exothermic reactions
for endothermic reactions delta S must be positive and the temperature is high enough so temp* delta S is more negative/greater than delta H??

also with electrode potentials -its feasible when the answer is a positive voltage- i think thisis where i got abit confused

thank you so much! pos rep
2. (Original post by maik13)
hi
can someone confirm my understanding is correct?
when free energy delta G is less than 0 the reaction is feasible
and spontanenous (whats the difference) for exothermic reactions
for endothermic reactions delta S must be positive and the temperature is high enough so temp* delta S is more negative/greater than delta H??

also with electrode potentials -its feasible when the answer is a positive voltage- i think thisis where i got abit confused

thank you so much! pos rep

Reaction is spontaneous (means it goes without requiring an input of workwork such as electrical work. This has no relation to how fast the reaction is, e.g. diamond -> graphite is spontaneous at standard conditions) if DeltaGr is negative.
DeltaG=DeltaH-TDeltaS, so at high T entropy is more important. If the reaction is endothermic (+ve for DeltaH) this can be outweighed by the TDeltaS term to make it spontaneous.

DeltaG=-nFEcell, so +Ecell is spontaneous.

G is the actual energy 'currency' that counts, H ignores entropy so doesn't describe the position of equilibrium unless at absolute zero kelvin.
3. (Original post by maik13)
hi
can someone confirm my understanding is correct?
when free energy delta G is less than 0 the reaction is feasible
and spontanenous (whats the difference) for exothermic reactions
for endothermic reactions delta S must be positive and the temperature is high enough so temp* delta S is more negative/greater than delta H??

also with electrode potentials -its feasible when the answer is a positive voltage- i think thisis where i got abit confused

thank you so much! pos rep
Yes, you're pretty much there.

There is no real difference between thermodynamically spontaneous and feasible. They both mean that the reaction is possible under the stated conditions.

The relationship between ΔG and electrode potentials is:

ΔGº = - nFEº

where n is the number of electrons transferred, and F is faraday's constant. Hence if ΔG is negative then Eº is positive.

The rule of thumb is that if Eº is positive and greater than 0.3 the reaction is spontaneous and would go to completion. Between 0 and 0.3V it would arrive at equilibrium.
4. The difference between feasible and spontaneous is how easily it is done. A feasible reaction is a reaction which is thermodynamically possible, but might not happen at a given temperature, while a spontaneous reaction is a reaction which will just happen of its own accord at that temperature.
And yes, when G<0 a reaction is feasible.

Electrode potentials which are greater than +0.6V will happen. Below 0 they are impossible. Inbetween i'm a bit fuzzy on, i think they can happen under certain circumstances but are unlikely to.

Enthalpy deals with whether a reaction is spontaneous. If deltaS total is positive it is spontaneous. [DeltaS total = DeltaS surroundings + DeltaS system]

For the second to last line: deltaG = DeltaH - T(DeltaS system), so you want T*DeltaS of system to exceed Delta H. In other words, you want T*DeltaS to be a bigger positive number than DeltaH

Hope this helps!

Edit: FSM damn it, double sniped!
5. (Original post by Mathisfun!)
The difference between feasible and spontaneous is how easily it is done. A feasible reaction is a reaction which is thermodynamically possible, but might not happen at a given temperature, while a spontaneous reaction is a reaction which will just happen of its own accord at that temperature.
And yes, when G<0 a reaction is feasible.

Electrode potentials which are greater than +0.6V will happen. Below 0 they are impossible. Inbetween i'm a bit fuzzy on, i think they can happen under certain circumstances but are unlikely to.

Enthalpy deals with whether a reaction is spontaneous. If deltaS total is positive it is spontaneous. [DeltaS total = DeltaS surroundings + DeltaS system]

For the second to last line: deltaG = DeltaH - T(DeltaS system), so you want T*DeltaS of system to exceed Delta H. In other words, you want T*DeltaS to be a bigger positive number than DeltaH

Hope this helps!

Edit: FSM damn it, double sniped!
Sorry, that isn't strictly true.... A reaction is spontaneous if it satisfies DeltaGr (gibbs free energy) < 0, not enthalpy.
6. (Original post by JMaydom)
Sorry, that isn't strictly true.... A reaction is spontaneous if it satisfies DeltaGr (gibbs free energy) < 0, not enthalpy.
Edexcel A2 Chemistry, page 42:

'It is the total entropy change which determines whether this reaction is spontaneous or not'

It follows on page 45:

' Since DeltaS must be positive for a change to occur spontaneously, it follows that such changes have DeltaG less than zero'

Seems like there you can take from both, none taking priority
7. (Original post by Mathisfun!)
Edexcel A2 Chemistry, page 42:

'It is the total entropy change which determines whether this reaction is spontaneous or not'

It follows on page 45:

' Since DeltaS must be positive for a change to occur spontaneously, it follows that such changes have DeltaG less than zero'

Seems like there you can take from both, none taking priority
Mr Josiah Willard Gibbs defined ΔG as being equal to the term -TΔS(universe)

Hence when the universal entropy increases (change is positive) Gibbs free energy decreases (change is negative).
8. (Original post by Mathisfun!)
Edexcel A2 Chemistry, page 42:

'It is the total entropy change which determines whether this reaction is spontaneous or not'

It follows on page 45:

' Since DeltaS must be positive for a change to occur spontaneously, it follows that such changes have DeltaG less than zero'

Seems like there you can take from both, none taking priority
That must be out of context as that is just wrong.... Sorry but university level chemistry is more reliable than exam board specifications.
9. (Original post by JMaydom)
That must be out of context as that is just wrong.... Sorry but university level chemistry is more reliable than exam board specifications.
Considering the amount of lies i discovered were told to me in gsce, i'll buy that :L But i tried to keep it as in context as possible
10. (Original post by Mathisfun!)
Considering the amount of lies i discovered were told to me in gsce, i'll buy that :L But i tried to keep it as in context as possible
Entropy does not make a reaction spontaneous. It is Gibbs free energy for systems at constant pressure, or Helmholtz free energy at constant volume.
Entropy is just a constituent term for both of these free energies.
The physical requirement of entropy is that for any process the entropy of the universe must increase. This applies to both spontaneous and non-spontaneous reactions.

I can run you through a bit of thermodynamics quickly if you would like? It really is not covered well at A-level so you don't get a real idea of what the functions actually are.
11. (Original post by JMaydom)
Entropy does not make a reaction spontaneous. It is Gibbs free energy for systems at constant pressure, or Helmholtz free energy at constant volume.
Entropy is just a constituent term for both of these free energies.
The physical requirement of entropy is that for any process the entropy of the universe must increase. This applies to both spontaneous and non-spontaneous reactions.

I can run you through a bit of thermodynamics quickly if you would like? It really is not covered well at A-level so you don't get a real idea of what the functions actually are.
Yeah sure, i like learning stuff :L But don't worry about to much detail, no need wasting your time
12. I'm doing entropy for my Unit four exam / I've got some good notes - if you want them?
By the way how can you send images via I phone ? (I'm new on here!)
13. (Original post by MissMariya)
I'm doing entropy for my Unit four exam / I've got some good notes - if you want them?
By the way how can you send images via I phone ? (I'm new on here!)
I can PM you my email?
To upload it on here, the box you type in with the reply has a little row above it with font, colour, bold and one of the icons is an image and you can upload via that.
14. (Original post by maik13)
I can PM you my email?
To upload it on here, the box you type in with the reply has a little row above it with font, colour, bold and one of the icons is an image and you can upload via that.

No problem - I'm not actually at home at the minute - but ill try and get them for you asap! It's just over two spreads - I found it quite difficult - but once you get your head round it -its not that bad !
15. It won't let me - upload the images - when I click the icon and upload image it remains blank ?
16. (Original post by MissMariya)
It won't let me - upload the images ������- when I click the icon and upload image it remains blank ?
oh I dont know I'll PM my email
17. (Original post by charco)
The rule of thumb is that if Eº is positive and greater than 0.3 the reaction is spontaneous and would go to completion. Between 0 and 0.3V it would arrive at equilibrium.
I know that 0.3V is the actual value at which an EC cell will go to completion, but at a-level, or in OCR A at least, I've been taught, and it says, that it's 0.4V
18. (Original post by Funtry)
I know that 0.3V is the actual value at which an EC cell will go to completion, but at a-level, or in OCR A at least, I've been taught, and it says, that it's 0.4V
What do you mean by go to completion? This never came up when I did A-level and doesn't make sense from the point of view of thermodynamics.
19. (Original post by JMaydom)
What do you mean by go to completion? This never came up when I did A-level and doesn't make sense from the point of view of thermodynamics.
I don't know about to completion, it just says the reaction will 'go' when it's 0.4V or greater, as in Ecell = Epos-Eneg.
20. In Edexcel chemistry it was quoted as +0.6v to go to completion

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: April 12, 2013
Today on TSR

Tuition fees under review

Would you pay less for a humanities degree?

Can I get A*s if I start studying now?

Discussions on TSR

• Latest
Poll
Discussions on TSR

• Latest

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE