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Thermodynamics

I really need your opinion in this one:

thermoq.PNG

I think it is meant to be delta G at standard conditions (not sure). So I calculated delta G and it's negative so option A? I think it's either A or B

Many thanks. Help will be appreciated
Original post by Daniel Atieh
I really need your opinion in this one:

thermoq.PNG

I think it is meant to be delta G at standard conditions (not sure). So I calculated delta G and it's negative so option A? I think it's either A or B

Many thanks. Help will be appreciated


Without an equation it certainly cannot be determined ...

Option D
Original post by charco
Without an equation it certainly cannot be determined ...

Option D


The equation is
deltaG= delta + RTln ([products]/[reactants])
Original post by Daniel Atieh
The equation is
deltaG= delta + RTln ([products]/[reactants])


I meant the equation for the reaction ...
Original post by charco
I meant the equation for the reaction ...


hmm I really don't know. But the reaction is feasible, and we have the concentrations. Lets suppose i have the equation, how i should go about solving it?
Original post by Daniel Atieh
hmm I really don't know. But the reaction is feasible, and we have the concentrations. Lets suppose i have the equation, how i should go about solving it?


Which reaction? There is none stated!
Original post by charco
Which reaction? There is none stated!

I would assume this (didn't check the image, sorry):

A + B <--> C + D
Original post by Daniel Atieh
I would assume this (didn't check the image, sorry):

A + B <--> C + D


If it is that equation, and its an equilibrium, then at equilibrium ∆G = 0 so ∆G° = -RT lnK and you can work out the equilibrium ratios. Then just decide whether [A] and will increase or decrease to get to that ratio. (That is assuming the ∆G you've been given is in fact ∆G°. If its ∆G instead, you don't have enough information. I don't really trust the notation of whoever gave you the question though!)
(edited 8 years ago)
Original post by KombatWombat
If it is that equation, and its an equilibrium, then at equilibrium ∆G = 0 so ∆G° = -RT lnK and you can work out the equilibrium ratios. Then just decide whether [A] and will increase or decrease to get to that ratio. (That is assuming the ∆G you've been given is in fact ∆G°. If its ∆G instead, you don't have enough information. I don't really trust the notation of whoever gave you the question though!)

Hey

Yes, I confirmed it is delta G at standard conditions. I am given the concentration of each species in the question (1M A, 1M B, 0.1 M C, 0.1 M D).
So shall I calculate delta G? If so, I got a large negative value, and accordingly, can I say it goes in forward direction? Is this approach correct?

I still feel pretty weak at grasping what's going on in this question :/
Original post by Daniel Atieh
Hey

Yes, I confirmed it is delta G at standard conditions. I am given the concentration of each species in the question (1M A, 1M B, 0.1 M C, 0.1 M D).
So shall I calculate delta G? If so, I got a large negative value, and accordingly, can I say it goes in forward direction? Is this approach correct?

I still feel pretty weak at grasping what's going on in this question :/


My take on this is that the easiest way to tackle it is to use:

&#916;G = -RTlnK

To work out K

Then plug in the values given to find out the reaction quotient K'

If K' is larger than K the reaction goes in reverse and vice versa.
Original post by charco
My take on this is that the easiest way to tackle it is to use:

&#916;G = -RTlnK

To work out K

Then plug in the values given to find out the reaction quotient K'

If K' is larger than K the reaction goes in reverse and vice versa.


Original post by KombatWombat
If it is that equation, and its an equilibrium, then at equilibrium ∆G = 0 so ∆G° = -RT lnK and you can work out the equilibrium ratios. Then just decide whether [A] and will increase or decrease to get to that ratio. (That is assuming the ∆G you've been given is in fact ∆G°. If its ∆G instead, you don't have enough information. I don't really trust the notation of whoever gave you the question though!)

Many thanks both of you. I utterly got what you mean, and it worked well.

My last question is whether I can simple find delta G and according to that I can decide which side it will favour. In our example, it is negative so forward. What do you think?
Original post by Daniel Atieh
Many thanks both of you. I utterly got what you mean, and it worked well.

My last question is whether I can simple find delta G and according to that I can decide which side it will favour. In our example, it is negative so forward. What do you think?


But you are given &#916;G in the question!
Original post by charco
But you are given &#916;G in the question!

Delta G at standard conditions.
Original post by Daniel Atieh
Delta G at standard conditions.


The value of &#916;G changes during the reaction.

Do you mean to find &#916;G at the beginning of the reaction with only A and B present in stoichiometric amounts? (Don't forget that temperature does not feature in standard conditions)

I don't see how this is relevant for your question.
Original post by charco
The value of &#916;G changes during the reaction.

Do you mean to find &#916;G at the beginning of the reaction with only A and B present in stoichiometric amounts? (Don't forget that temperature does not feature in standard conditions)

I don't see how this is relevant for your question.

Yes, at that point where we have 1 molar of reactants and 0.1 molar of products. I can calculate delta G for that point and see whether it will occur or not. As I got a negative value, it is feasible and will go towards products. Correct?

I got your point that delta G changes in the course of the reaction, yeah.
Original post by Daniel Atieh
Yes, at that point where we have 1 molar of reactants and 0.1 molar of products. I can calculate delta G for that point and see whether it will occur or not. As I got a negative value, it is feasible and will go towards products. Correct?

I got your point that delta G changes in the course of the reaction, yeah.


According to your question the reaction will not start under standard conditions from the reactants A & B

&#916;G is positive.

But you know that at equilibrium &#916;G = 0

Use the standard &#916;G value to work out the equilibrium constant

And do your calculations using the molar amounts given to find the quotient = [C][D]/[A][ B]
Original post by charco
According to your question the reaction will not start under standard conditions from the reactants A & B

&#916;G is positive.

But you know that at equilibrium &#916;G = 0

Use the standard &#916;G value to work out the equilibrium constant

And do your calculations using the molar amounts given to find the quotient = [C][D]/[A][ B]

Thank you. Yes, I got this approach and it worked, but I am still keen to know whether the other pathway makes sense or not.

deltaG= delta + RTln ([products]/[reactants])
delta G = (0.4 Kj x 10^3) + [8.314 J/K/mol x 298 K x ln (0.1 x 0.1)/1)] = -11000 J

Sorry for disturbance, but it is important for me to know why the above method does/doesn't work

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