You are Here: Home

# is this GIBBS free energy calculation WRONG? Tweet

Chemistry exam discussion - share revision tips in preparation for GCSE, A Level and other chemistry exams and discuss how they went afterwards.

Announcements Posted on
IMPORTANT: You must wait until midnight (morning exams)/4.30AM (afternoon exams) to discuss Edexcel exams and until 1pm/6pm the following day for STEP and IB exams. Please read before posting, including for rules for practical and oral exams. 28-04-2013
1. is this GIBBS free energy calculation WRONG?
7. The reaction to produce a drug has a ΔH value of –10.07 kJmol-1 and a ΔS value of 42.12 Jmol-1 K-1
at 20°C. State whether this reaction will proceed spontaneously at 25°C and explain why. (5 marks)

ΔG = ΔH – TΔS (where ΔH = -10.07 kJ mol-1, ΔS = 42.12 J mol-1 K-1 and T = 293K)
ΔG = 10.96- 2.95 = +8.01 kJ mol-1 (2 s.f.).
Reaction does not proceed as ΔG is positive
2. Re: is this GIBBS free energy calculation WRONG?
anyonjeeeeeeeeeeeeeeeeeeeeeeeeee eeeee?
3. Re: is this GIBBS free energy calculation WRONG?
check your value for ΔS - it needs to be in the same units as ΔH (once multiplied with T)
4. Re: is this GIBBS free energy calculation WRONG?
I'm not quite sure what you did there, I got a negative value for ΔG. Are you sure you've divided ΔS by 1000 to make sure it's in the same units as ΔH?
5. Re: is this GIBBS free energy calculation WRONG?
(Original post by NightBear)
I'm not quite sure what you did there, I got a negative value for ΔG. Are you sure you've divided ΔS by 1000 to make sure it's in the same units as ΔH?
I was tryna get him to realise that by himself :P
6. Re: is this GIBBS free energy calculation WRONG?
(Original post by ms607)
I was tryna get him to realise that by himself :P
Didn't see your post before I posted mine
7. Re: is this GIBBS free energy calculation WRONG?
You've got to convert the enthalpy value to the correct form by multiplying it by a 1000. So kJ mol-1 is converted to J mol-1.
From there, you should be able to work out the correct answer.
8. Re: is this GIBBS free energy calculation WRONG?
(Original post by letr)
[COLOR="#FF0000"]
ΔG = 10.96- 2.95 = +8.01 kJ mol-1 (2 s.f.).
I believe that should be "3 s.f." not "2 s.f."
9. Re: is this GIBBS free energy calculation WRONG?
I'm not quite sure what you did there, I got a negative value for ΔG. Are you sure you've divided ΔS by 1000 to make sure it's in the same units as ΔH?
Or you could do it this way or my way. Either way, you should get the right answer after your calculations.
Last edited by Cable; 27-05-2012 at 18:32.
10. Re: is this GIBBS free energy calculation WRONG?
(Original post by letr)
7. The reaction to produce a drug has a ΔH value of –10.07 kJmol-1 and a ΔS value of 42.12 Jmol-1 K-1
at 20°C. State whether this reaction will proceed spontaneously at 25°C and explain why. (5 marks)

ΔG = ΔH – TΔS (where ΔH = -10.07 kJ mol-1, ΔS = 42.12 J mol-1 K-1 and T = 293K)
ΔG = 10.96- 2.95 = +8.01 kJ mol-1 (2 s.f.).
Reaction does not proceed as ΔG is positive
25 degrees C = 298K not 293K
Last edited by wibletg; 27-05-2012 at 20:13.
11. Re: is this GIBBS free energy calculation WRONG?
(Original post by wibletg)
25 degrees C = 298K not 293K
but it states 20 degrees in the question!!p
Useful resources

## Articles:

Study Help rules and posting guidelinesStudy Help Member of the Month nominations

## Groups associated with this forum:

View associated groups