## Van't Hoff Eq.

$\ln K = -\frac{\Delta H^{\circ}}{RT} + \frac{\Delta S^{\circ}}{R}$

Christina_F_3F
Posts: 21
Joined: Fri Jul 22, 2016 3:00 am

### Van't Hoff Eq.

For the Van't Hoff equation, we are given two different versions of the same equation:
-ln(K2/K1) = -(deltaH*/RT2) + (deltaH*/R) where one assumes delta S naught is constant
-ln (K2/K1) = -deltaH*/R(1/T2-1/T1) where one assumes delta H naught is constant

My question is under what conditions are delta S naught and delta H naught not constant? If they are under standard conditions my understanding would be that they would be constant no matter what, because they are under a set of constant conditions.

Thanks!

Christian Hardoy 3F
Posts: 41
Joined: Fri Jul 15, 2016 3:00 am

### Re: Van't Hoff Eq.

From what I understand, we aren't being given 2 Van't Hoff equations. When the course reader says, "assume delta S naught is constant" and, "assume delta H naught is constant" it is not because there are two equations where one is constant and the other is not. The course reader says this because each line is a step in writing the Van't Hoff equation in the final form that is on the last line of page 42 in the course reader. In other words, I think the assumptions are part of rewriting and explaining the Van't Hoff equation, not two distinct equations with different assumptions.

Alexia Joseph 2B
Posts: 56
Joined: Thu Jul 27, 2017 3:01 am

### Re: Van't Hoff Eq.

From what we learned in class today, I'm assuming we only need to know how to use the equation when delta S and delta H are held constant.

Nicole 1F
Posts: 31
Joined: Fri Sep 29, 2017 7:05 am

### Re: Van't Hoff Eq.

Can someone please explain what it means for ∆S°to be constant? I think Dr. Lavelle was explaining this at the end of lecture (something to do with the difference not changing between the two ∆S° values), but I didn't quite catch it all. Thanks!

Manvir2K
Posts: 32
Joined: Fri Sep 29, 2017 7:05 am

### Re: Van't Hoff Eq.

∆S°is constant because the difference between the two ∆S°values is the same. This is because no matter what two values are taken, the difference for both values will be the same.

Michael Lee 2I
Posts: 51
Joined: Fri Sep 29, 2017 7:05 am

### Re: Van't Hoff Eq.

Both delta S and delta H remain constant from what Lavelle told us in class today.