June 02, 2020, 09:55:48 AM
Forum Rules: Read This Before Posting


Topic: contradiction? equilibrium question  (Read 2888 times)

0 Members and 1 Guest are viewing this topic.

Offline 21385

  • Regular Member
  • ***
  • Posts: 55
  • Mole Snacks: +2/-1
contradiction? equilibrium question
« on: November 18, 2007, 01:06:10 AM »
I don't really get the effects of increasing the temperatures on the equilibrium. Because of le chatelier's principle, the equilibrium of any endothermic reaction will increase if the temperature is increased. However, looking at G=-RTlnK equation, we can see that G can still be negative even if it is endothermic reaction by having a positive entropy change. Thus, if G is negative, negative signs from both sides cancel, and from this equation, we can see that an increase in temperature would actually decrease K. Can anyone explain this to me? Thanks

Offline Yggdrasil

  • Retired Staff
  • Sr. Member
  • *
  • Posts: 3210
  • Mole Snacks: +482/-21
  • Gender: Male
  • Physical Biochemist
Re: contradiction? equilibrium question
« Reply #1 on: November 18, 2007, 01:55:51 AM »
Maybe things will be more clear if we express the equation this way:

K = e^(-ΔG/RT)

So, we want to know how K changes with T.  Specifically, we want to know the sign of dK/dT.  The difficulty is that ΔG also depends on T.  We can write out this explicit dependence on T by writing:

ΔG = ΔH - TΔS

so that

K = e^(-ΔH/RT) e^(ΔS/R)

Assuming that ΔH and ΔS do not depend on temperature

dK/dT = e^(ΔS/R) e^(-ΔH/RT) * ΔH/RT2

Since e^x is always positive and RT2 is always positive, the sign of dK/dT is equal to the sign of ΔH.

Offline 21385

  • Regular Member
  • ***
  • Posts: 55
  • Mole Snacks: +2/-1
Re: contradiction? equilibrium question
« Reply #2 on: November 18, 2007, 02:12:49 AM »
Thank you for your answer, I guess the original formula wasn't really clear since G varies with T. Thanks

Sponsored Links