Specific Question on Kaplan PS Sectional 2

Suffusion

10+ Year Member
Hopefully this is in the right forum/I'm allowed to ask it.

I took the second Kaplan PS sectional and had a problem with question 25. It asks if entropy increases or decreases for photosynthesis. We're told that delG for the reaction is 686 kcal/mol and that delH is 2800 kJ/mol. Kaplan says entropy decreases by analyzing the equation (you wind up with less moles, solids instead of gas, etc), I say it has to increase to make G=H-TS work.

Am I missing something here? Thermodynamics is one of my weak spots so probably G1SG2

10+ Year Member
5+ Year Member
Hopefully this is in the right forum/I'm allowed to ask it.

I took the second Kaplan PS sectional and had a problem with question 25. It asks if entropy increases or decreases for photosynthesis. We're told that delG for the reaction is 686 kcal/mol and that delH is 2800 kJ/mol. Kaplan says entropy decreases by analyzing the equation (you wind up with less moles, solids instead of gas, etc), I say it has to increase to make G=H-TS work.

Am I missing something here? Thermodynamics is one of my weak spots so probably In the information you provided, delta G and delta H are both positive. In the equation G=H-TS, you would end up with a positive delta G if the term TS was negative, because then you would end up with G=H-(-TS)=G=H+TS, giving you a positive value for delta G.

OP
S

Suffusion

10+ Year Member
In the information you provided, delta G and delta H are both positive. In the equation G=H-TS, you would end up with a positive delta G if the term TS was negative, because then you would end up with G=H-(-TS)=G=H+TS, giving you a positive value for delta G.
But doesn't the fact that G is smaller than H when they are both positive indicate that S must be positive but not large enough to make G negative?

minutemen11

7+ Year Member
hmm yeah that is true.. but i guess you just have to look at the big picture instead of the overall equation.. its definitely true that entropy decreases in photosynthesis tho..

G1SG2

10+ Year Member
5+ Year Member
But doesn't the fact that G is smaller than H when they are both positive indicate that S must be positive but not large enough to make G negative?
Yeah, I thought about that at first as well. I suppose you have to look at the equation of photosynthesis to see that entropy is decreasing, as the poster above suggests.