Liquid State- Vapor Pressure

Vapor Pressure

It is the pressure exerted by the vapor on a liquid or solid in an equilibrium state. It can be defined as the pressure exerted by vapor formed over the surface of the liquid in thermodynamic equilibrium.

How fast a liquid will vaporise or transform into the gaseous state is determined by the equilibrium vapor pressure of the fluid. In short, it is equivalent to the vaporisation rate of a liquid. vapor pressure is directly proportional to temperature. It rises with an increase in temperature.

The boiling point of a liquid is when the pressure exerted by the surrounding is equal to the pressure exerted by the vapor formed by the liquid over its surface. 

vapor pressure also depends on the nature of the liquid.

Characteristics Of vapor Pressure

1.     The vapor pressure of a pure liquid is higher than its solution.
2.     vapor pressure is inversely proportional to the force of attraction of the molecules of the liquid.
3.     vapor pressure increases with temperature because the molecules gain kinetic energy, which results in quick transformation into vapors.

Factors Affecting vapor Pressure

Nature Of The Liquid

Liquids have weak intermolecular forces than solids. When the liquid is heated, the molecules’ forces break and convert them into vapor.

The intermolecular forces depend on the nature of the liquid. Some have stronger intermolecular forces than others. For example, the vapor pressure of water is lower than that of acetone at a given temperature.

Weaker intermolecular forces mean it will break easily when heated. Thus, a liquid with weak intermolecular forces of the molecules will have a higher vapor pressure.

Strong intermolecular forces will take longer to break off when heated than weak ones. Liquids with strong intermolecular forces have a low vapor pressure as only a few molecules will break and convert to vapor.

Temperature

vapor pressure of a liquid depends on its temperature. It rises with an increase in temperature as the molecules in the liquid gain more kinetic energy at higher temperatures and vaporises quickly. 

Raoult’s Law

According to Raoult’s law, the partial pressure of a solvent in a solution is equal to its mole fraction of in solution.multiplied by vapour pressure of the pure solvent

That is,

pA ∝ xA

pA = pA0 xA

Where,

pA 0 = vapor pressure of pure liquid component A.

In the same way, the partial pressure of B will be

pB ∝ xB

pB = pB 0 xB

Where,

pB 0 = vapor pressure of pure liquid component B.

Now, when we apply Dalton’s law of partial pressures:

Dalton’s law of partial pressure states that the total pressure of the solution in a container equals the sum of the partial pressure of the solvent and the solute.

ptotal = pA + pB

ptotal = pA 0 xA + pB 0 xB

Also since, xA + xB = 1 , we can write the relation as:

ptotal  = pA 0 + (pB 0 – pA 0) XB

Conclusion

vapor pressure is an essential topic in chemistry that you cannot miss. The vapor pressure lets you determine how quickly or slowly the liquid will convert to a vapor state or gaseous state.

vapor pressure of liquid-liquid solutions and liquid-solid solutions can be easily calculated using Raoult’s law. You can use the equation stated above to evaluate the total vapor pressure of any solution.