Structural Biochemistry/Unique Properties/High Heat of Vaporization

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General Information[edit]

One unique property of water is its high heat of vaporization.

Heat of vaporization refers to the energy required to convert one gram of liquid into a gas at boiling point. This required energy will break down the intramolecular attractive forces in water. At 100oC, which is the boiling point of water, water has a heat of vaporization equal to 2260 J/g. This means that 2260 J of heat is needed in order to convert 1 gram of water into 1 gram of steam at boiling point. When water is converted from liquid to steam, 2260 J of heat will be absorbed into the water. But when 1 g of steam is converted back into 1 g of liquid, 2260 J will be released into the water's surroundings.[1]

The dominant intramolecular force that leads to this high heat of vaporization in water is hydrogen bonding. Because liquid water consists of numerous H2O molecules, a great amount of hydrogen bonds are formed between the water molecules. Although hydrogen bonds are not the strongest forces (especially when considering ionic or covalent bonds, which are much stronger), the great multitude of hydrogen bonds that need breaking leads to the high heat of vaporization because so much energy can be released. When water changes its physical state during vaporization, there is a large change in volume for the water molecule. For example, one mole of water contains 18 grams of liquid; but in the gas phase, the same mole of water would amount to 22.4 liters of gas. [2]