Variation of the standard enthalpy of formation

The standard enthalpy of formation or "standard heat of formation" of a compound is the change in enthalpy that accompanies the formation of 1 mol of a substance in its standard state from its constituent elements in its standard state (the most stable form of an element at 1 atmosphere of pressure and at 25 °C of temperature). It is denoted by Δ_fH^o.

The standard enthalpy change of formation is measured in units of energy per amount of substance. They are usually given in kilojoules per mole (kJ mol^-1), but can be given in any unit with the same dimensions. All elements in their standard states (oxygen gas, solid carbon in the form of graphite, etc.) have a standard enthalpy of formation of zero, since their formation does not involve any process.

The standard enthalpy change of formation is used in thermochemistry to find the standard enthalpy change of reaction. This is done by subtracting the sum of the standard enthalpies of formation of the reactants from the sum of the standard enthalpies of formation of the products, as shown in the following equation.

ΔH^O_r = σΔH^O_f (Products) - σΔH^O_f (Reactives)

where:

^O means "standard"

_r "reaction."

_f "of formation."

The standard enthalpy of formation is equivalent to the sum of several separate processes included in the Born-Haber cycle of synthesis reactions. For example, to calculate the enthalpy of formation of sodium chloride, we use the following reaction:

Na_(s) + (1/2)Cl_2(g) → NaCl_(s)

This process is made up of many independent sub-processes, each with its own enthalpy. Therefore we will take into account:

1. The standard sublimation of solid sodium
2. The first ionization energy of the gaseous sodium
3. The dissociation of the gaseous chlorine
4. The electronic affinity of chlorine atoms
5. The reticular energy of sodium chloride

The sum of all these values will give us the value of the standard enthalpy of formation of sodium chloride.

In addition, the application of Hess's Law shows that the sum of the individual reactions corresponding to the change in enthalpy of formation for each substance in the reaction is equal to the change in enthalpy for the total reaction, regardless of the path followed or the number of reactions involved in the calculation. In the previous example, the standard enthalpy variation of sodium chloride formation is equal to the sum of the standard enthalpy variation of each of the steps followed for the process. This is especially useful for long reactions with many steps and intermediate compounds.

This leads us to the conclusion that a standard enthalpy of formation is sometimes defined for a hypothetical reaction. For example, we cannot combine carbon and hydrogen in the laboratory to form methane, even though we define the standard enthalpy of formation for methane to be -74.8 kJ mol^-1. Finally, the negative sign of this value means that methane is formed from an exothermic reaction, that is, it is formed with the release of energy.