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Van't Hoff Equation

Posted: Sun Feb 17, 2019 8:53 am
by Alyssa Wilson 2A
I'm a little confused on why the Van't Hoff equation shows the temperature dependence of K. I understand that it relates the difference in standard Gibbs free energy to K, because it's easy to measure, but why does it show that the temperature depends on K?

Re: Van't Hoff Equation

Posted: Sun Feb 17, 2019 9:58 am
by David S
The Van't Hoff equation is meant to show the Temperature dependance of K, and while it is derived from an expression that relates G to K, in its ultimate for the equation excludes G.

Derivation:

1) Given and :



2) Divide both sides by -RT to isolate ln(K):



3) Use the above equation, plug in K1 at T1 to make the equation for initial K and plug in K2 at T2 to make the equation for Final K. Subtract the final K equation by initial K equation:



4) Simplify the equation by using ln(a)-ln(b) = ln(a/b) and cancelling out the entropy terms:




5) Now you factor out -delta H/R to get the final form that we use:



Note how the process of deriving this equation eventually allows us to compute a final K @ Tfinal from an initial K @ Tini knowing only the enthalpy of reaction.