Latent Heat of Vaporization LHV

The Latent Heat of Vaporization is the amount of energy a substance requires to transform from the liquid to the vapor state. If the liquid is being vaporized, this heat is absorbed; if it is condensing, the heat is released. The reverse, condensing the same amount of liquid, releases the same amount of energy. This is sometimes called the Latent Heat of Condensation (LHC), but as they are the same amounts, we'll just use LHV to mean both.

The following table lists the LHV for several substances you may encounter: water and the first four alcohols. The figures quoted are for standard atmospheric pressure.

 Substance LHV cal/gm LHV kJ/mol LHV Cal/mol Water 539 40.639 9.7 Methyl alcohol 263 35.3 8.4 Ethyl alcohol 204 39.330 9.4 Propyl alcohol 160 40.3 9.6 Butyl alcohol 142 43.9 10.5

The figures expressing the LHV in calories per gram differ widely, so it's sometimes assumed that water needs far more heat to vaporize than any of the alcohols. This is true, on a weight basis, but if you look at the calories per mol, you can see that ethanol and water molecules require just about the same amount of energy to vaporize. To a first approximation, each requires 40 kJ to change from a liquid to a vapor, and then occupies the same space as any other mol of vapor - 30 liters at 90°C.

If some of those units are confusing, the following list explains what each one is (the mol was defined earlier).

A calorie ( cal ) is a unit of energy, and is the amount of heat required to raise the temperature of 1 gram water through 1C (it varies slightly with temperature)

A Calorie ( Cal ) is 1000 calories, and is the "Calorie" referred to when talking about food energy. A Joule ( J ) is also a unit of energy, and is the amount of energy expended in one second by a current of 1 ampere flowing through a resistance of 1 ohm. A kiloJoule (kJ) is 1000 Joules

From experiment, it's been determined that 1 calorie = 4.185 Joule, so 1 Calorie = 4.185 kJ

A Watt (W) is a unit of power, one Watt being defined as the rate at which energy is expended by a current of 1 ampere flowing through a resistance of 1 ohm. So 1 Watt = 1 Joule/sec.