What Is Vapour Pressure Definition Formula and Factors Affecting It
Vapour pressure or equilibrium vapour pressure is defined as the pressure exerted by a vapour in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The vapour pressure equilibrium expresses the rate of evaporation of a liquid. It is the tendency of the particles to escape the solid or the liquid state into the gaseous state. Thus the more the vapour pressure of a liquid or a solid at normal temperature, the more volatile the substance is. Thus, the vapour pressure can be alternatively expressed as the pressure exerted by the vapour over the surface of the liquid. It is because as the temperature increases, the kinetic energy in the molecules also increases that will, in turn, raise the molecules transitioning between the liquid or the solid surface to the vapour phase and therefore the vapour pressure also increases accordingly.
According to the Clausius–Clapeyron relation, the vapour pressure of any substance increases non-linearly with the increase in temperature. The vapour pressure boiling point which is also referred to as the normal boiling point is the point where the ambient atmospheric pressure becomes equal with the vapour pressure. When the temperature increases, the vapour pressure eventually surpasses the atmospheric pressure and creates vapour bubbles inside the bulk of the substance. To form the bubble deeper into the bulk of the substance requires a higher temperature due to higher fluid pressure as the vapour pressure can overcome the ambient atmospheric pressure when there is an increase in the depth. Moreover, in the shallow depths, a very high temperature is required to start the bubble formation in the bulk of the substance. The surface tension of the bubble wall creates overpressure in the initial bubbles that are very small in size.
Now the partial pressure can be defined as the vapour pressure that a single component in the mixture is contributing to the total vapour pressure of the mixture as a whole. The example of vapour pressure and partial pressure is the vapour pressure by the air at the sea level and saturated with water vapour at 20 °C has partial pressures of about 2.3 kPa of water, 78 kPa of nitrogen, 21 kPa of oxygen and 0.9 kPa of argon, totalling 102.2 kPa creating the base for the standard atm pressure. The standard unit of pressure is the measurement of vapour pressure. The SI unit of pressure is Pascal (Pa) where One pascal is one newton per square meter (N·m-2 or kg·m-1·s-2).
The relation between the vapour pressure and the temperature of the pure liquid or solid substance according to the Antoine equation is as follows:
Log P = A - [B / (C + T)]
It can be converted into the temperature explicit form as:
T = [B / (A - log P)] - C
Where,
A, B, C are the substance-specific coefficients (i.e., constants or parameters)
P is the absolute vapour pressure of a substance
T is the temperature of the substance
The log is typically either log10 or loge
Reid Vapour Pressure
Reid vapour pressure which is commonly abbreviated as RVP is a common measurement of the volatility of gasoline and other petroleum products. It is defined as the absolute vapour pressure exerted by the vapour of the liquid and any dissolved gases/moisture at 37.8 °C (100 °F). This method is known as the test method ASTM-D-32 which was developed for the first time in the year 1930 and has been revised many times since then. The vapour pressure of gasoline, volatile crude oil, jet fuels, naphtha, and other volatile petroleum products are measured by this test method but liquefied petroleum gases are not included in this test method. It is because this test method requires the liquid to be chilled at 0 to 1-degree Celsius and then poured into the apparatus but any liquid that solidifies at this temperature is not applicable for the test method. The SI unit of Reid vapour pressure is kilopascal (KPa) or it is expressed in pounds per square inch (psi). It measures the volatilization of the liquid at the vapour pressure as the test method determines the gauge pressure of the sample in a non-evacuated chamber.
The Reid vapour pressure differs from the true vapour pressure in the following subsequent ways:
Vapour Pressure of water
Vapour pressure of water is the pressure exerted by the molecules of the water vapour in the gaseous state which can either be in the pure form or in the form of a mixture of other gases such as air. The saturation vapour pressure is the pressure of the water vapour that is in the thermodynamics equilibrium with its condensed state. When the atmospheric pressure is higher than the vapour pressure, the water condenses while when the atmospheric pressure is lower than the vapour pressure, the water starts subliming or boiling. The saturation vapour pressure can be determined by the Clausius–Clapeyron relation and increases with the increase in the temperature. When the saturation vapour pressure becomes equal to the ambient vapour pressure it becomes the boiling point of the water. Commonly the meteorology uses the calculation of the saturation water vapour. The boiling point of water and the pressure is inversely related to the temperature-vapour pressure. This explains both pressure cooking and cooking at higher altitudes.
Some of the approximation formulae for the calculation of saturated vapour pressure over water and over ice is given below:
P = exp [20.386 - (5132 / T)]
Where P is vapour pressure at mmHg
T is the temperature in kelvin.
P = 0.61094 exp (17.625 T / T + 243.04)
Where, P is vapour pressure at kilopascal (KPa)
T is temperature in 0C.
.P = 0.61078 exp (17.27 T / T + 273.3)
Where, P is vapour pressure at kilopascal (KPa)
T is temperature in 0C.
P = 0.61121 exp [(18.67 - (T / 234.5) ) (T / 275.14 + T)]
Where, P is vapour pressure at kilopascal (KPa)
T is temperature in 0C.
Similarly, the vapour pressure of methanol at 250 C is 94 torr or 12.5323 KPa and the vapour pressure of ethanol is 44 torr or 5.86 Kpa at 250 C
Vapour Pressure and Boiling Point Equation
The Clausius-Clapeyron equation describes the equation between the vapour pressure. The different function of the vapour pressure with respect to the variation in the boiling point uses the Clausius-Clapeyron equation. The boiling point of a substance is inversely proportional to the increase in the boiling point and vice-versa.
Thus the Clausius-Clapeyron equation is derived from the following equation that incorporates two different temperatures.
VP2 = VP1e-△ (Hv / RT)
Thus the classic form is:-
ln(P1/P2)=Hvap/R(1/T2 – 1/T1)
Where P1 and P2 are the vapour pressures
T1 and T2 are the temperatures
Hvap is that the Enthalpy (heat) of Vaporization and
R is the universal gas constant (8.3145 J mol-1 K-1).
FAQs on Vapour Pressure in Chemistry Complete Guide
1. What is vapour pressure in chemistry?
Vapour pressure is the pressure exerted by a vapor in equilibrium with its liquid (or solid) phase at a given temperature in a closed system. It arises when molecules escape from the liquid surface into the gas phase and some return by condensation, creating a dynamic equilibrium.
- Occurs in a closed container
- Depends only on temperature and the nature of the substance
- Higher intermolecular forces → lower vapour pressure
It is usually measured in Pa, kPa, or mmHg (torr).
2. What factors affect vapour pressure?
Vapour pressure depends mainly on temperature and intermolecular forces.
- Temperature: Increasing temperature increases vapour pressure because more molecules have enough kinetic energy to escape into the vapor phase.
- Intermolecular forces: Stronger forces (e.g., hydrogen bonding in H2O) lower vapour pressure; weaker forces (e.g., in nonpolar liquids) raise vapour pressure.
- Nature of liquid: Volatile liquids like acetone have higher vapour pressure than less volatile liquids like water.
Surface area and volume do not affect vapour pressure at equilibrium.
3. Why does vapour pressure increase with temperature?
Vapour pressure increases with temperature because more molecules gain sufficient kinetic energy to overcome intermolecular forces and enter the vapor phase.
- Higher temperature → higher average kinetic energy
- More molecules escape from liquid to gas
- Equilibrium shifts to produce more vapor molecules
This relationship is quantitatively described by the Clausius–Clapeyron equation, which shows vapour pressure increases exponentially with temperature.
4. What is the formula for the Clausius–Clapeyron equation?
The Clausius–Clapeyron equation is ln(P2/P1) = −(ΔHvap/R)(1/T2 − 1/T1).
- P1, P2: Vapour pressures at temperatures T1, T2
- ΔHvap: Enthalpy of vaporization (J mol−1)
- R: Gas constant (8.314 J mol−1 K−1)
- T: Temperature in Kelvin
It is used to calculate vapour pressure at different temperatures or determine enthalpy of vaporization.
5. What is the relationship between vapour pressure and boiling point?
A liquid boils when its vapour pressure equals the external atmospheric pressure.
- Normal boiling point: when vapour pressure = 1 atm (101.325 kPa)
- Higher vapour pressure at a given temperature → lower boiling point
- Lower external pressure (e.g., high altitude) → lower boiling point
For example, water boils at 100°C at 1 atm because its vapour pressure equals atmospheric pressure at that temperature.
6. What is the difference between vapour pressure and atmospheric pressure?
Vapour pressure is the pressure exerted by a substance’s vapor at equilibrium, whereas atmospheric pressure is the pressure exerted by the air in the atmosphere.
- Vapour pressure: Depends on the liquid and temperature
- Atmospheric pressure: Caused by the weight of air molecules above a surface
- Boiling occurs when vapour pressure = atmospheric pressure
They are equal only at the boiling point of a liquid.
7. What is Raoult’s law for vapour pressure?
Raoult’s law states that the vapour pressure of a solvent in an ideal solution equals the mole fraction of the solvent multiplied by its pure vapour pressure.
Psolution = Xsolvent × P°solvent
- Xsolvent: Mole fraction of solvent
- P°solvent: Vapour pressure of pure solvent
Adding a non-volatile solute lowers vapour pressure, a phenomenon known as vapour pressure lowering, a colligative property.
8. How do you calculate vapour pressure lowering?
Vapour pressure lowering is calculated using ΔP = Xsolute × P°solvent.
- ΔP: Lowering of vapour pressure
- Xsolute: Mole fraction of solute
- P°solvent: Vapour pressure of pure solvent
Example: If Xsolute = 0.10 and P°solvent = 100 kPa, then ΔP = 10 kPa.
This applies to ideal solutions with non-volatile solutes.
9. What is meant by dynamic equilibrium in vapour pressure?
Dynamic equilibrium in vapour pressure occurs when the rate of evaporation equals the rate of condensation in a closed system.
- Liquid molecules evaporate into vapor
- Vapor molecules condense back into liquid
- Both processes occur at equal rates
At this point, the vapour pressure remains constant even though molecular motion continues.
10. Can you give an example of a substance with high and low vapour pressure?
Diethyl ether has high vapour pressure, while water has relatively low vapour pressure at the same temperature.
- Diethyl ether: Weak intermolecular forces → evaporates quickly → high volatility
- H2O: Strong hydrogen bonding → lower vapour pressure → higher boiling point
This difference explains why ether evaporates rapidly at room temperature while water evaporates more slowly.
