Liquids: Vapour Pressure

A pure compound’s vapour pressure is the pressure of a vapour in equilibrium with its liquid or solid state at any given temperature. Vapour pressure is a measure of a compound’s capacity to connect with itself; compounds with good bonding have a low vapour pressure (less inclination to escape to the vapour phase), while compounds with poor bonding have a high vapour pressure. This can be seen from a thermodynamic aspect using the well-known Clausius–Clapeyron expression. In a broader sense, the Clausius Clapeyron equation captures the connection between pressure and temperature in two-phase equilibrium. The two phases in sublimation could be vapour and solid, or solid and liquid in melting. Finally, thermodynamic concepts are used to obtain the Clausius Clapeyron equation.

Characteristics of Vapour Pressure

• A pure liquid has more vapour pressure than a liquid solution.
• It’s inversely proportional to the attraction between a liquid’s molecules.
• When the temperature rises, it rises with it. Because the molecules gather kinetic energy, they rapidly vapourize.

A volatile solute and solvent may be present in a liquid solution. We can observe that the solvent is volatile in the majority of cases, but the solute is not. We’ll look at it in two ways based on this: Liquid-Liquid Solutions and Solid-Liquid Solutions.

Vapour Pressure of Liquid-Liquid Solutions

We will take the two volatile liquid solutions to perform this test. Let’s call their liquid components A and B for now. After putting the volatile liquid and its components in a closed vessel, we discover that equilibrium between the liquid and vapour phases has been established. Let P_A and P_B be the partial vapour pressures of components A and B, respectively, and Ptotal be the overall vapour pressure at equilibrium. Furthermore, the mole fractions of the two components are xA and xB. Raoult’s Law is used to determine the vapour pressure of a volatile liquid.

Raoult’s Law

The partial pressure is directly proportional to the mole fraction of the solute component, according to the law. According to the result of Raoult’s Law, the partial pressure of A’s will be

P_A ∝ x_A

P_A = PA⁰ x_A

where PA0 is the pure liquid component A’s vapour pressure.

B’s partial pressure will be similar

P_B ∝ x_B

P_B = PB⁰ x_B

where PB0 is the pure liquid component B’s vapour pressure. We’ll now apply Dalton’s partial pressures law. The total pressure (Ptotal) of a solution placed in a container is equal to the sum of the partial pressures of its constituents, according to this law. That is

Ptotal = P_A + P_B

Ptotal = PA⁰ x_A + PB⁰ x_B

Also since, x_A + x_B = 1 ,the relation can be written as:

Ptotal = PA⁰ + (PB⁰ – PA⁰) xB

Vapour Pressure of the Solutions of Solids in  the Liquids

Let’s take a look at the other form of solution, which is solids in liquid.The solute is a solid, while the solvent is a liquid, in this case. The solute is non-volatile in these situations. Vapour pressure is lower than the solution’s pure vapour pressure. Let’s look at how we can calculate the overall vapour pressure of a solution like this.

Consider a solution in which the solvent is A and the solute is B. The partial vapour pressure of a single component (solute/solvent) is directly proportional to its mole fraction, according to Raoult’s Law.

When we add a non-volatile solute, we must ensure that all of the vapour pressure comes from the solvent. This is due to the fact that they are the only component present in the vapour phase. As a result, if PA is the solvent’s vapour pressure, xA  is its mole-fraction, and PA0  is the pure solvent’s vapour pressure, Raoult’s law yields the following relationship:

P_A ∝ x_A

P_A = PA⁰xA

When a graph is drawn between the mole fraction of solvent to the vapour pressure, we observe that it is linear in character.

Vapour Pressure of pure Liquid

Vapour pressure, which rises with temperature, is used to quantify a substance’s tendency to change to a gaseous or vapour form. The temperature at which the vapour pressure at a liquid’s surface equals the pressure exerted by its surroundings is known as the boiling point. The vapour pressure of a liquid at any temperature is the pressure exerted by the vapour present above the liquid in equilibrium with the liquid at that temperature.

Heat of Vaporization

When we heat a liquid, its energy increases, which leads to a rise in the overall temperature. The extra heat is used up by the molecules at the boiling point to overcome the intermolecular force of attraction in the liquid and shift to a gaseous form. The amount of heat provided by this process when 1 mole of liquid is changed into a gaseous state is known as the Heat of Vaporization.

Conclusion

According to the Clausius–Clapeyron relationship, the vapour pressure of any substance grows non-linearly with temperature. The temperature at which the vapour pressure matches the ambient air pressure is known as the atmospheric pressure boiling point of a liquid (also known as the normal boiling point). The vapour pressure grows sufficient to overcome atmospheric pressure and raise the liquid to create vapour bubbles inside the bulk of the substance with any incremental increase in temperature. Bubble formation at a deeper depth in the liquid necessitates a higher temperature due to the increased fluid pressure, which rises above atmospheric pressure as depth rises. The increased temperature necessary to start bubble production is more critical at shallow depths. The bubble wall’s surface tension causes an overpressure in the extremely small, first bubbles.