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How does the Enthalpy of vapourization change when impurities are added? I know that the boiling point of a substance increases on addition of impurities, but how does the Enthalpy of vapourization change? Is less heat needed to convert the liquid from a solution to a gas?

Related: This question deals with a similar problem, but for Enthalpy of fusion instead of Enthalpy of vapourization.

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    $\begingroup$ As long as we talk about colligative properties, this should be a simple piece of math. And yes, delta H must be lower i guess. $\endgroup$ – Karl Dec 30 '19 at 10:25
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    $\begingroup$ Are you talking about non-volatile impurities? $\endgroup$ – Chet Miller Dec 30 '19 at 14:02
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    $\begingroup$ Yes @ChetMiller $\endgroup$ – Aniruddha Deb Dec 30 '19 at 14:02
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Adding impurities(which mostly have low heat capacity, like salt) decreases the enthalpy of vaporization. This is because the impurities lower the heat capacity of the solution as a whole, making the enthalpy of vaporization lesser than the original solvent.

This is why even though the boiling point of a solution increases when impurities are added it is heated up faster(boils faster) than the one without impurities. For example, saltwater (salt is a non-volatile solute) boils faster than pure water because adding salt lowered the heat capacity of water as a result of which the enthalpy of vaporization is lesser.

Sources:

  1. https://www.studocu.com/en/document/western-governors-university/natural-science-lab/mandatory-assignments/task-1-boiling-salt-water/5667142/view

  2. https://www.thoughtco.com/adding-salt-to-boiling-water-607427

The addition of impurities lowers the heat capacity since the solvent molecules bind to the solute particles in a specific manner and hence require less heat to vaporize than they'd require when they're randomly moving.

Source:

  1. https://socratic.org/questions/how-does-salt-change-the-specific-heat-capacity-of-water#143950
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  • $\begingroup$ I think this a bit of a circular reasoning. Why does adding impurities lower the heat capacity? $\endgroup$ – Karl Jan 2 at 22:21
  • $\begingroup$ @Karl socratic.org/questions/… $\endgroup$ – Tapi Jan 3 at 6:47
  • $\begingroup$ Sorry, thats not the answer. What if the "impurity" is not so well soluble as table salt, say, ether? Also your answer should be understandable without linking to an external Q&A site. $\endgroup$ – Karl Jan 3 at 11:07
  • $\begingroup$ @Karl The OP mentions non-volatile impurities and ether is not one. High volatility and less solubility (1.5g ether/100g water) would cause ether to vaporize much before water (93.95% of ether floats on water). Since we're talking about enthalpy of vaporization, much less heat would be required to vaporize all of the ether (because of the aforementioned reasons) than the same amount of water. Hence ether-water mix would have a lesser enthalpy of vaporization than water. $\endgroup$ – Tapi Jan 3 at 14:35
  • $\begingroup$ OK, replace "ether" by "propylene glycol". $\endgroup$ – Karl Jan 3 at 14:38

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