Chemistry

Vapor pressure (Pv)


At any temperature, the molecules of any liquid are always in motion, some at higher speeds than others. Therefore, they can escape the liquid, passing into the atmosphere.

This is the case, for example, with clothes drying on the clothesline or a disappearing pool of water. The evaporation rate of the liquid is equal to the condensation velocity of its vapors.

We then say that there is a dynamic balance between the liquid and its vapors. The liquid vapors have reached the state of saturated vapors and was reached vapor pressure of the liquid.

Vapor pressure (Pv) is the pressure exerted by their vapors when they are in dynamic equilibrium with the liquid. It can also be said that it is the pressure exerted by the liquid solvent molecules against their surface to pass into the vapor state.

The higher the Pv, the more volatile the liquid. That is, the more pressure the liquid puts against its surface, the more it will vaporize and evaporate faster. Some factors influence vapor pressure, such as:

- temperature
- nature of liquid

When heating a liquid, the amount of steam tends to increase over time, which will cause the vapor pressure to increase as well.

The increase in temperature causes the molecules to stir. The liquid evaporates more intensely and causes higher vapor pressure. Take the case of pure water:

T (° C)

Pv (mmHg)

0

4,6

10

9,2

20

17,5

30

31,8

40

55,3

50

92,5

60

149,4

70

233,7

80

355,1

90

525,8

100

760,0

110

1.074,6

120

1.489,1

The higher Pv, the more volatile (more evaporates)

Consider a bottle containing water and one containing alcohol, both in liquid form and with the same volume and temperature. In both bottles there is a pressure gauge to measure the vapor pressure of the substances.

After a while, the pressure indicated by the alcohol was higher than the pressure indicated for the water, because in the alcohol bottle there was a formation of a larger amount of vapors, since alcohol is a more volatile substance.

It follows that the vapor pressure of a substance depends only on its chemical nature and not on its quantity.

More volatile liquids than water, such as common ether, ethyl alcohol and acetone, evaporate more intensely and have higher vapor pressure.

The lower the temperature, the more it evaporates, the higher Pv.


Source: esds.edu.pt/antiga/afa/quimica/Imagens/I_17.png