Enthalpy

If a chemical change is carried out at constant pressure and the only work done is caused by expansion or contraction, the change is called the enthalpy change with the symbol ΔH, or ΔH°298 for reactions occurring under standard state conditions.

The value of ΔH for a reaction in one direction is equal in magnitude, but opposite in sign, to ΔH for the reaction in the opposite direction, and ΔH is directly proportional to the quantity of reactants and products. Examples of enthalpy changes include enthalpy of combustion, enthalpy of fusion, enthalpy of vaporization, and standard enthalpy of formation.

Standard enthalpy of formation

The standard enthalpy of formation, ΔH°f, is the enthalpy change accompanying the formation of 1 mole of a substance from the elements in their most stable states at 1 bar (standard state).

Many of the processes are carried out at 298.15 K. If the enthalpies of formation are available for the reactants and products of a reaction, the enthalpy change can be calculated using Hess’s law: If a process can be written as the sum of several stepwise processes, the enthalpy change of the total process equals the sum of the enthalpy changes of the various steps.

Enthalpy of fusion

The enthalpy of fusion of a substance (or latent heat of fusion), is the change in its enthalpy resulting from providing energy, typically heat, to a specific quantity of the substance to change its state from a solid to a liquid, at constant pressure.

Enthalpy of condensation

The enthalpy of condensation (or latent heat of condensation) is by definition equal to the enthalpy of vaporization with the opposite sign: enthalpy changes of vaporization are always positive (the substance absorbs heat), whereas enthalpy changes of condensation are always negative (the substance releases heat).

Enthalpy of vaporization

The enthalpy of vaporization (or latent heat of vaporization) is the amount of energy needed to change a liquid into vapor once it has reached its boiling point. Together with heat capacity, it is an essential property in determining how effectively a solvent can regulate the internal temperature of an organism.

The fact that water has both a high heat of vaporization and a high heat capacity makes it ideal in this respect and is one of the reasons it is so essential to life as we know it.

Enthalpy of sublimation

The enthalpy of sublimation (or latent heat of sublimation) is the heat required to change one mole of a substance from solid state to gaseous state at a given combination of temperature and pressure, usually standard temperature and pressure (STP).

The heat of sublimation is usually expressed in kJ/mol, although the less customary kJ/kg is also encountered.

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