Vapour Pressure

Definition

Vapour pressure is the pressure measured when a system reaches equilibrium

Vapour pressure is dependent on temperature and the type of liquid

Table

Vapour pressure of water
0 0.6113 4.5851 0.0060
5 0.8726 6.5450 0.0086
10 1.2281 9.2115 0.0121
15 1.7056 12.7931 0.0168
20 2.3388 17.5424 0.0231
25 3.1690 23.7695 0.0313
30 4.2455 31.8439 0.0419
35 5.6267 42.2037 0.0555
40 7.3814 55.3651 0.0728
45 9.5898 71.9294 0.0946
50 12.3440 92.5876 0.1218
55 15.7520 118.1497 0.1555
60 19.9320 149.5023 0.1967
65 25.0220 187.6804 0.2469
70 31.1760 233.8392 0.3077
75 38.5630 289.2463 0.3806
80 47.3730 355.3267 0.4675
85 57.8150 433.6482 0.5706
90 70.1170 525.9208 0.6920
95 84.5290 634.0196 0.8342
100 101.3200 759.9625 1.0000
Example

As a result of a chemical reaction, is produced and maintained at a temperature of in a closed flask. Will the water be present as a liquid only, vapour only, or vapour-liquid in equilibrium?

The total volume of the product is . Estimate the mass of liquid and the mass of vapour using the Ideal Gas Law.

solution
Assume liquid-vapour equilibrium.

From the table at , we have . Converting to atm gives .

Now to find the mass of vapour and liquid:

Note that this assumes that the volume occupied by the vapour phase is equivalent to the volume of the container. Since the liquid phase occupies at most , this is a reasonable assumption.

Note that when starting with this assumption, and calculate a mass larger than the total mass, you conclude that the system contains only vapour at a pressure lower than the vapour pressure.

Is the sample only in the liquid phase?
The density of water is , so of water will only occupy . Since the sample occupies it is not only a liquid.

Is the sample only in the vapour phase?
Determine the pressure of the vapour assuming that all is in the vapour phase.

Since this pressure is greater than the vapour pressure of water at , this means some of the water vapour will condense until a vapour pressure of is achieved. The system must contain vapour and liquid in equilibrium.

Note that if the pressure were less than the vapour pressure, the system would contain only vapour and we would be finished

Volatility

Info

Weak intermolecular forces -> more likely to evaporate -> high vapour pressure (volatile)
Strong intermolecular forces -> less likely to evaporate -> low vapour pressure (non-volatile)

Evaporation vs Boiling

Evaporation:

Boiling:

Boiling Point

Definition

The temperature at which the vapour pressure is equal to the external pressure is the boiling point temperature

Normal Boiling Point

Definition

The temperature at which the vapour pressure is equal to is the normal boiling point temperature ()

Normal Boiling Points of Some Hydrides

Normal Boiling Points.png

Why is higher at the beginning? Because N, O, and F form H-bonds.

Example

Rank the following from highest to lowest normal boiling point:
Normal Boiling Point Q.png

solution

  1. Ethylene glycol, 2 hydrogen bonds ()
  2. Ethanol, 1 hydrogen bond ()
  3. Carbon disulphide, higher molar mass = stronger LDF ()
  4. Butane, lower molar mass = weaker LDF ()

Clausius-Clapeyron Equation

Vapour Pressure Curves of Some Liquids

Vapour Pressure Curves.png

  1. Diethyl ether,
  2. Benzene,
  3. Water,
  4. Toluene,
  5. Aniline,

These kinda look like exponential functions, or .

Clausius-Clapeyron Equation.png

From we can get . with , where is the universal gas constant.

We can get rid of B by taking the difference between two points:

Equation

Note that we assume does not vary with time. We can also use this equation for and other phase transitions.

Example

Today's weather report states: "The temperature is , barometric pressure is , and the relative humidity is ." From this information, estimate the mole fraction of water in the air.

Useful information: at , at

solution
Approximate the vapour pressure of water at from the Clausius-Clapeyron equation knowing that the normal boiling point of water is .

then using Dalton's law: