Trouton's rule says that the ratio of the molar heat of vaporization of a liquid to its normal boiling point (in kelvin) is approximately the same for all liquids: ∆H_vap/T_bp ≈ 88 J/(K*mol) (b) Explain why liquids tend to have the same value of ∆Hvap/Tbp.

Video transcript

Hello. Everyone in this video, We're giving this table here. This includes some liquids as well as their boiling points and their delta age of vaporization. So we're being told that according to trans law that all liquids have a roughly constant ratio between their mole heat of vaporization and a normal boiling point. So that's that the delta age of vaporization divided by the temperature of the boiling point is equal to 88 joules per kelvin times small. So what vaporization is is the phase change from liquid to gas. Let's actually write this out. So vaporization is the phase change from liquid going to gas. And this process occurs at the boiling point. So at this point liquids have a equal delta delta S of vaporization values. So let's recall that the formula for delta S of vaporization is equal to our delta H. Of vaporization divided by the boiling point temperature. So I'll just put T sub bp. So liquids have the same delta age of vaporization over the boiling point temperature value because they have similar delta S. Of vaporization values. So the similarity in the delta S vaporization values of liquids is related to the inter molecular forces. So how inter molecular forces come into play is that if we have stronger. So I guess one would be if we have strong into molecular forces such as hydrogen bonding that it's harder for liquids to vapor eyes. And if this is the case then they will have a higher delta age of vaporization value. Which would then lead to higher delta age of vaporization over the boiling point temperature. And the reason being is that if we know we have a fraction here and the numerator on top is bigger. Our overall value of course is going to be bigger as well. In our second cases if we have a weaker inter molecular force, what happens then is that it's easier for liquids to vaporize. This is a case that we will have a lower delta H of vaporization and if we have that then of course we also have a lower delta age of vaporization over boiling point temperature. Alright, let me just go ahead and scroll down to write our final answer. So for our final answer then you can say that liquids have the same delta H of vaporization over boiling point temperature value because they have similar delta S. Vaporization values during our process of vaporization. Again, globalization is the face change from liquid to gas. So this end highlight is going to be my final answer for this problem.