Why is vaporisation rate increased by a decrease in pressure

[Roroy]

In my textbook, it says that the rate of vaporisation can be increased by (a) increasing the temperature or (b) reducing the pressure.

I understand why an increase in temp can lead to more molecules breaking away from the liquid and entering the vapor state. More temp = more energy = more likely for the bonds to break and for molecules to become vapor.

However, I don't understand why a reduction in pressure would have this effect. An increase in pressure is similar to an increase in temperature, isn't it?

My logic is that an increase in pressure leads to more molecular collisions, which results in an increase in energy = more likely for bonds to break. Therefore, why does a reduction in pressure lead to an increase in vaporisation rate?

Appreciate any help.

 

[fresh_42]

The air acts like a cap on the fluid, its weight is the pressure. Less pressure means less force to overcome by the molecules to vaporize (and displace air molecules). This effect can also be observed by different boiling points of water in different heights.

 

[DrStupid]

The air acts like a cap on the fluid, its weight is the pressure. Less pressure means less force to overcome by the molecules to vaporize (and displace air molecules). This effect can also be observed by different boiling points of water in different heights.

What pressure are we talking about? The vapour pressure or the total pressure? The vaporization rate depends on the vapour pressure because it affects the ratio between the mass transfer from the liquid into the gas phase and vice versa. But I do not think that changing the total pressure at constant vapour pressure has a significant effect on vaporization as long as it is above the equilibrium vapour pressure.

 

[fresh_42]

More air pressure means the vaporization pressure must be higher to reach the equilibrium and vice versa.

 

[Borek]

More air pressure means the vaporization pressure must be higher to reach the equilibrium and vice versa.

Not at all. Equilibrium pressure of the saturated vapor depends only on the temperature.

 

[Roroy]

So how does the system work?
Cause I've been googling and all I can find is that above explanation that a reduction in pressure means that the equilibrium is reached faster.

 

[Borek]

Part of the problem is, the statement

rate of vaporisation can be increased by (...) reducing the pressure

is a bit ambiguous, as it doesn't clearly state - reducing pressure of what. And there are two pressures involved here - the total pressure (including all the inert gases) and the partial pressure of vapor.

I take it it was intended to mean "removing the gases present above the liquid" - in which case both the total and the partial pressure are getting lower. That means moving system farther from the equilibrium, so the evaporation gets faster. The same effect would be achieved by reducing pressure of the vapor only (without changing the pressure of the inert gases).

That's all assuming temperature is low enough that the liquid doesn't boil and that the temperature remains constant (which means enough heat is delivered to the liquid to not let the liquid cool down, evaporation requires energy).

 

[Roroy]

So, if Saturated Vapor depends on temp, then wouldn't it be right in saying:

In a system where the atmospheric pressure (all inert gases, as you mentioned) is high, then a higher temp would be required to increase vapour pressure enough to equal the atmospheric pressure (at which point the liquid reaches its boiling point).

Similarly, in a system with low atmospheric pressure (again, only counting the pressure exerted by air molecules), then a lower temp is fine to increase vapour pressure enough to equal the atmospheric pressure (liquid reaches its boiling point).

Applying this on two real life systems: a pressure cooker vs Mt. Everest.

In a pressure cooker, the high pressure causes the liquid to reach boiling point at a much higher temp (let's say that water has a boiling point of 200C in a pressure cooker). This means that the food cooks in 200C water, which means it cooks faster.

On Mt. Everest, the low pressure means that the liquid needs a lower temp to reach boiling point (let's say water has a boiling point of 90C on Mt. Everest). This means that food cooks in 90C water, hence cooks slower.

Is that all correct? Cause it seems to match all of the answers given, and all of the research I've done.

 

[Borek]

Actually much less than 90°C at Mt Everest (something around 71) and it doesn't go up to 200°C in a pressure cooker (around 120, depending on the valve). But yes, you are about right.

 

[Roroy]

Thanks for the help

 

[Chestermiller]

More air pressure means the vaporization pressure must be higher to reach the equilibrium and vice versa.

I agree with Borek. It's only the partial pressure of the water vapor in the air above the liquid (I.e., the relative humidity) that matters in terms of pressure (for straight evaporation below the boiling point). Once the boiling point is reached, the surface area for evaporation increases.

 

[DrDu]

Not at all. Equilibrium pressure of the saturated vapor depends only on the temperature.

I remember vaguely that vapour pressure over a liquid increases with increasing pressure.

 

[Chestermiller]

I remember vaguely that vapour pressure over a liquid increases with increasing pressure.

Within the context of the OP's question, how important do you feel it was to mention this typically minor non-ideality (related to the so-called "Poynting effect" of total pressure on the fugacity of the liquid) at the risk of confusing the OP? Please explore this for us with some sample calculations to illustrate the magnitude of the correction. For example, at 1 atm total pressure, how much does this correction affect the equilibrium vapor pressure of water?

 

[DrDu]

So this is called Poynting effect. Just looked it up on wikipedia. Does not seem to be due to nonideality and they cite even an interesting application of it. But you are right, it is apparently too small to be relevant for the vaporisation of water at ambient pressure.
A more relevant aspect of pressure dependence of vaporisation is the increased free pathlength for particles leaving the surface or higher diffusivity at lower pressure. I.e. the air on a water surface acts like a lid and the better so the higher its pressure.