Definition of heat of vaporisation
Also known as enthalpy of vaporisation, the heat of vaporisation (∆Hvap) is defined by the amount of enthalpy (thermal energy) required to transform a liquid substance into a gas or vapour. It is measured in joules per mole (J/mol) or sometimes in calories (C).
Explanation of the heat of vaporisation
The heat of vaporisation always has a positive value because enthalpy is always added to a system to vaporise a liquid. As molecules gain more kinetic energy, they are more likely to separate from the liquid and become a gas.
The required increase in internal energy can be described as the energy needed to break the intermolecular interactions in the liquid. The weaker the bond between atoms, the less energy is needed to break those bonds.
The amount of energy required is a function of the pressure at which the transformation takes place and depends on the temperature. The hotter the liquid, the less energy is required. At higher pressures, more energy is required. There is a critical temperature at which the heat of vaporisation disappears (T r = 1). After this critical temperature, the substance is no longer distinguishable as either a liquid or a vapour. Instead, it is known as a supercritical fluid.
In a solution containing both liquid and gaseous states, the kinetic energy of the vapour is higher than that of the liquid because the vapour particles can flow more easily. The greater movement of gas particles compared to liquid particles creates heat and pressure.
Heat of vaporisation formula
A very basic equation for calculating the heat of vaporisation is:
ΔH VAP = H STEAM – H LIQUID
This calculates the difference in internal energy of the vapour phase compared to the liquid phase.
However, this equation does not take into consideration the additional energy required for the gas particles to recoil against atmospheric pressure to allow for the increase in volume when a liquid boils.
Therefore, a more complete equation for calculating the heat of vaporisation is:
ΔH VAP = ΔU VAP + PΔV
Where ΔU vap is the internal energy difference between the vapour phase and the liquid phase (ΔU vap = H steam – H liquid ), and pΔV is the work done against ambient pressure.
Heat of vaporisation of water
Water has a specific heat high. This measure describes the amount of energy needed to raise the temperature of water by 1 degree Celsius. As such, water also has a high heat of vaporisation. In fact, water needs more than 40,000 joules per mole to vaporise. This is extremely important for life on Earth.
Since most of the Earth is made of water, large changes in the amount of solar energy the Earth receives are counteracted by water. Water absorbs heat slowly and releases it when there is less sun. This helps to counteract drastic temperature changes, which would be devastating for life. In comparison, if the world were composed mainly of ethanol, the temperature would fluctuate rapidly because ethanol has a much lower heat of vaporisation and specific heat.
However, this high heat of vaporisation may not be up to the task of regulating temperature in the face of human actions. Climate change, and global warming specifically, is adding a lot of heat to the atmosphere. While the ocean can absorb much of this heat, it has limits. Also, as the ocean absorbs heat, the molecules expand. This expansion will lead to much of the flooding currently estimated by climate scientists.
Differences in heat of vaporisation
The main influences on the heat of vaporisation are the interactions between molecules in a solution. In a liquid, the molecules move towards each other but interact constantly. Some form bonds of hydrogenwhile other substances form other types of light bonds between molecules. These bonds contain energy and keep the liquid in a lower energy state. The heat of vaporisation describes how much energy is needed to separate these bonds.
Water has a high heat of vaporisation because hydrogen bonds are easily formed between the oxygen of a molecule and the hydrogens of other molecules. These bonds hold the molecules together. For water to vaporise, you have to increase the temperature so that the molecules move faster. At a certain point, the molecules will begin to break away from the liquid and vaporise.
Metals have an even higher heat of vaporisation. Many metals form complex interactions with other metal atoms. This holds the molecules together even tighter than water molecules. As such, the heat of vaporisation of metals is much higher than that of water.