My textbook states the following:

Textbook statement

Qualitatively, I understand why the boiling point of a substance increases when the pressure is increased. However, I learned that if the pressure is increased when the reaction system is at equilibrium, the volume of the system decreases and hence, the reaction will proceed in that direction in which the volume of the system increases. In the above scenario, however, it has been given that water in the gaseous form occupies more volume than its liquid form and so, shouldn't the given reaction proceed in the forward direction, hence reducing the boiling point?

Please note that I am convinced and I believe that it makes sense for the boiling point to increase when the pressure increases but can somebody please explain the flaw in my reasoning with regard to the given equilibrium?

Much thanks in advance :) Regards.

  • 2
    $\begingroup$ what's the reaction in boiling of water? $\endgroup$ – JM97 Aug 30 '16 at 9:36
  • $\begingroup$ I'm afraid I don't understand. What do you mean by "reaction of boiling of water"? $\endgroup$ – user33789 Aug 30 '16 at 11:41
  • $\begingroup$ There is no reaction in boiling water, why do you apply le chatliers principle? $\endgroup$ – JM97 Aug 30 '16 at 12:16
  • $\begingroup$ There is no chemical reaction but there is a physical reaction, no? An equilibrium does exist between the gas phase and the liquid phase of water(or any liquid, for that matter). $\endgroup$ – user33789 Sep 1 '16 at 3:30
  • $\begingroup$ at equilibrium ratio of concentration of products to concentration of reactants remain constant. (K_c) . How would you apply this to a physical change? $\endgroup$ – JM97 Sep 1 '16 at 3:37

Your flaw in the logic lies in that you think that the volume of the system includes the liquid. Le Chatelier's principle on pressure equilibrium only applies to when the compound involved is in gaseous phase, in an ideal sense. Therefore if you increase the pressure (decreasing the volume), then the reaction will progress towards the direction where gas volume DECREASES, which is towards liquid phase. Overall, the entire apparatus increases the vapor pressure on the gas (gas pressure on the external pressure), which increases the boiling point.

  • $\begingroup$ OK, but on decreasing volume, why does the reaction progress in that direction in which the volume further decreases? $\endgroup$ – user33789 Sep 1 '16 at 3:29
  • $\begingroup$ Le Chatelier's principle states that a reaction that undergoes a change in pressure, temperature, or volume, the equilibrium shifts towards the direction of the reaction that counteracts and subsists with that change. In this case, the change in volume causes an equilibrium shift that subsists by decreasing the volume $\endgroup$ – phi2k Sep 1 '16 at 3:43
  • $\begingroup$ The shift in equilibrium should be such that it counteracts the stress(change) applied, yes? So why, on decreasing the volume, will the equilibrium shift in a direction in which the volume further decreases?! $\endgroup$ – user33789 Sep 1 '16 at 3:46
  • $\begingroup$ Decreasing the volume increases the pressure. The volume decreases because the pressure increase causes the gas to contract. Look at this link for more info: en.wikipedia.org/wiki/… $\endgroup$ – phi2k Sep 1 '16 at 3:49
  • $\begingroup$ So if I decrease the pressure, the volume increases and the reaction will shift in that direction in which the volume further increases? Seems very absurd sir/ma'am. I think it should shift in that direction in which the volume decreases, causing the pressure to increase. $\endgroup$ – user33789 Sep 1 '16 at 3:57

It's because when we increase pressure, we are actually increasing external pressure so according to Le Chatelier's principle, to counteract the effect it should increase the internal pressure. This is the reason that the reaction shifts where internal volume decreases, so that internal pressure increases.

  • $\begingroup$ Could you please edit your post so that you are usually complete words? It is difficult to understand the content of your answer with the incomplete words. $\endgroup$ – Ben Norris Apr 14 '17 at 17:23

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