Difference between Clausius-Clapeyron and Van't Hoff equation I was wondering what is the difference between the Clausius-Clapeyron equation and the Van't Hoff equation.  They appear to have the exact same physical meaning and are often used interchangeably.
 A: They're two forms of the same equation, but Clausius-Clapeyron uses vapor pressure ($p^*$) where Van 't Hoff uses the reaction equilibrium constant ($K$).
Why does this work out? Well, think of vaporization as a chemical reaction:
$$ \text{X} (l) \longrightarrow \text{X} (\text{g}) $$
The equilibrium constant is defined in terms of activity (a):
$$ K=\left.\frac{a_\text{X(g)}}{a_{\text{X}(l)}}\right|_\text{equil} $$
For an ideal liquid solution at modest pressure, a is just the mole fraction x. And for an ideal gas, a is just its partial pressure in bar:
$$ K=\left.\frac px\,\right|_\text{equil}$$
One equilibrium condition is $x=1\ $ and $p=p^*,\ $ so...
$$ K=p^* $$
Or $p^*$ is the equilibrium coefficient for vaporization. Now since K is characteristic of a reaction at a given temperature, this would imply that if we change x, then the new partial pressure at equilibrium would change according to
$$ K=p^*=\frac{p}{x} \qquad\Rightarrow\qquad p=xp^*$$
And that's exactly what happens. Neat, huh?
