How Does Vapour Pressure Affect Daily Life and Physics Problems?
All liquids exhibit a tendency for evaporation. The process of evaporation occurs at the surface of the liquid. If the kinetic energy of liquid molecules overcomes the intermolecular force of attraction in the liquid state, then the molecules from the surface of the liquid escape into space above the surface. The process is called 'evaporation.' If evaporation is carried out in a closed container system, then the vapours of liquid remain in contact with the liquid's surface.
Like gas molecules, the condensation of molecules also executes the continuous random motion. During these motions, molecules collide with each other and, even with the walls of the container, they lose their energy and return to the liquid state. This process is called 'condensation.'
Evaporation And Condensation
Evaporation and condensation are continuous processes. Hence, after some time, an equilibrium is established at a constant temperature between evaporation and condensation. At the equilibrium number of molecules in the vapour, the state remains stable at a constant temperature.
The liquid's vapour pressure depends on the nature of the fluid and temperature, with an increase of intermolecular force of attraction Vapour pressure of liquid decreases, and with a rising temperature vapour pressure of liquid increases. A mercury manometer may be used to determine the vapour pressure of a liquid.
What is vapour pressure?
Vapour pressure is the tendency of a material to change into a gaseous state. And generally, vapour pressure is the pressure exerted by the vapour of the liquid in Thermodynamic equilibrium with the condensed phases in a closed system. If the temperature of any liquid increases then its vapour pressure also increases. The vapour pressure of a liquid can be measured in various ways. The simplest method to measure vapour pressure is using a manometer in a closed container or flask.
Factors Affecting Vapour Pressure
There are various factors on which vapour pressure depends. They are:
1. Nature of Liquid
The nature of the liquid is explained based on its intermolecular forces. That is to say, as the magnitude of the intermolecular forces rise up, the vapour pressure will dwindle down
2. Effect of Temperature
As the temperature of the liquid increases, the kinetic energy associated with the liquid also increases. And due to this increase in kinetic energy, the escaping tendency of the molecule increases; hence vapour pressure increases. So we can draw the inference that vapour pressure is directly proportional to temperature.
3. Concentration of Solute
The existence of a solute in the liquid will significantly reduce the vapour pressure. And this fall in vapour pressure also differs with respect to the concentration of solute.
4. Vapour Pressure Is Independent of Humidity.
Temperature is the only property that affects the vapour pressure for a certain amount of water vapour in the air. Humidity will act only if all the other variables are constant. So don't get any sort of confusion between the effect of temperature and humidity.
5. Volume
Vapour pressure does not tend to get affected by the volume of the container. As we know, that liquid in the box will be in equilibrium with the vapour Now, suppose the volume of surface S is divided into infinite elementary volumes such that volume is changed, say decreased, then some of the container's vapour turns into a liquid state. And if the volume rises up, some of the liquid is bound to change into its vapour state.
6. Surface Area
Usually, vapour pressure is independent of surface area.
Factors Affecting Vapour Pressure of Liquid
However, the following factors affect the vapour pressure of a liquid at equilibrium.
1. Intermolecular Forces of Attraction
The forces that mediate an interaction between atoms, including powers of attraction or repulsion are called the Intermolecular forces (IMF). For example, the covalent bond, involving sharing electron pairs between atoms, is much stronger than the parties intermolecular force present between neighbouring molecules.
2. The Volume of the Liquid Present Does Not Affect the Vapour Pressure of a Liquid at Equilibrium.
We can change the volume of a liquid (keeping temperature constant), but the vapour pressure of a fluid at equilibrium will remain the same.
3. The Temperature of the Liquid.
Weaker are the intermolecular forces of attraction, or higher is the temperature of the liquid, higher is the vapour pressure of a fluid at equilibrium.
Unit of Vapour Pressure
The most common unit for vapour pressure is the torr. One torr = 1 mm Hg (one millimetre of mercury).
Most materials have external vapour pressures. For example, water has a vapour pressure of approximately 20 torrs at room temperature (22 °C = 72 °F). Note that the vapour pressure increases with the temperature; water will have a vapour pressure of 760 torr = 1 atm at its boiling point of 100 oC (212 oF).
Boiling Point
The boiling point refers to the temperature of any substance at which the vapour pressure of a liquid becomes equivalent to the pressure encompassing the liquid, and the fluid converts into a vapour.
Relation Between the Standard Boiling Point And The Vapour Pressure of Liquids
The higher the vapour pressure of a liquid at a given temperature, the lower the standard boiling point (i.e., the boiling point at atmospheric pressure) of the liquid.
The vapour pressure chart to the right has graphs of the vapour pressures versus temperatures for a variety of liquids.
As can be seen on the map, the liquids with the highest vapour pressures have the lowest standard boiling points.
For example, at any given temperature, methyl chloride has the highest vapour pressure of any of the liquids in the chart. It also has the lowest standard boiling point (−24.2 °C), which is where the vapour pressure curve of methyl chloride (the blue line) intersects the horizontal pressure line of one atmosphere (atm) of absolute vapour pressure.
FAQs on Vapour Pressure: Definition, Examples & Applications
1. What is vapour pressure and what is its SI unit?
Vapour pressure is the pressure exerted by a vapour when it is in thermodynamic equilibrium with its condensed phase (liquid or solid) inside a closed system at a specific temperature. It is a measure of a substance's tendency to evaporate. The SI unit for vapour pressure is the Pascal (Pa), although other units like atmospheres (atm) or millimetres of mercury (mm Hg) are also commonly used.
2. What are the main factors that affect the vapour pressure of a liquid?
The vapour pressure of a liquid is primarily influenced by two key factors:
Nature of the Liquid (Intermolecular Forces): Liquids with weak intermolecular forces (like ethanol) are more volatile and have a higher vapour pressure. Liquids with strong intermolecular forces (like water, due to hydrogen bonding) are less volatile and have a lower vapour pressure.
Temperature: As the temperature of a liquid increases, its molecules gain more kinetic energy. This allows more molecules to overcome the intermolecular forces and escape into the vapour phase, thus increasing the vapour pressure.
3. What are some real-world examples and applications of vapour pressure?
Vapour pressure is a concept we observe in daily life and is crucial in many industrial applications. Some examples include:
Pressure Cookers: They work by increasing the pressure inside, which raises the boiling point of water, allowing food to cook faster. This demonstrates the relationship between external pressure and boiling point, which is governed by vapour pressure.
Drying Clothes: Clothes dry faster on a warm, windy day because the increased temperature raises the vapour pressure of water, and the wind removes the water vapour, encouraging more evaporation.
Industrial Applications: It is critical in processes like distillation for separating liquids, in designing fuel systems, and in meteorology for predicting humidity and dew point.
4. How is the concept of vapour pressure important for understanding a liquid's boiling point?
The boiling point of a liquid is the temperature at which its vapour pressure becomes equal to the surrounding atmospheric pressure. A liquid with a high vapour pressure does not need to be heated as much to reach this point. Therefore, there is an inverse relationship: the higher a liquid's vapour pressure at a given temperature, the lower its normal boiling point.
5. How does Raoult's Law for ideal solutions relate to vapour pressure?
According to the NCERT syllabus for Class 12, Raoult's Law states that for an ideal solution of volatile liquids, the partial vapour pressure of each component in the solution (Pᵢ) is equal to the vapour pressure of the pure component (P°ᵢ) multiplied by its mole fraction (xᵢ) in the solution. The formula is Pᵢ = P°ᵢ * xᵢ. The total vapour pressure of the solution is the sum of the partial pressures of all components.
6. Why doesn't the vapour pressure of a liquid depend on the surface area or the volume of the container?
This is a common point of confusion. Vapour pressure is an intensive property measured at equilibrium. At equilibrium, the rate of evaporation (molecules leaving the liquid) equals the rate of condensation (molecules returning to the liquid). While a larger surface area increases the rate of evaporation, it also provides a larger area for condensation to occur. These two effects cancel each other out, so the final equilibrium pressure, which depends only on molecular energy (temperature) and intermolecular forces, remains independent of the surface area.
7. How can one distinguish between saturated and unsaturated vapour pressure?
The distinction lies in the state of equilibrium:
Saturated Vapour Pressure: This is the maximum vapour pressure a substance can exert at a given temperature. It occurs in a closed container when the rate of evaporation equals the rate of condensation. The space is 'saturated' with vapour.
Unsaturated Vapour: If the actual vapour pressure is less than the saturated vapour pressure, the vapour is unsaturated. In this state, the rate of evaporation is greater than the rate of condensation, meaning more liquid can still evaporate if available.
8. If a non-volatile solute like salt is added to water, how does it affect the vapour pressure and why?
Adding a non-volatile solute to a pure solvent lowers the vapour pressure of the solvent. This phenomenon is a key colligative property. The reason is that the solute particles occupy some of the liquid's surface area. This reduces the number of solvent molecules at the surface that are able to escape into the vapour phase. With fewer solvent molecules escaping, the equilibrium vapour pressure decreases. This is known as the relative lowering of vapour pressure.
9. Compare the vapour pressure of water and diethyl ether at the same temperature. Which is higher and why?
At the same temperature, diethyl ether will have a much higher vapour pressure than water. The reason lies in their intermolecular forces:
Water: Molecules are held together by strong hydrogen bonds, which require a significant amount of energy to break, making water less volatile.
Diethyl Ether: Molecules are held together by much weaker dipole-dipole interactions and London dispersion forces. Less energy is needed for ether molecules to escape into the vapour phase, making it highly volatile and resulting in a higher vapour pressure.
