A formation reaction is a reaction in which a compound is formed from its elements.

Under standard conditions, the enthalpy change of a formation reaction is referred to as the standard enthalpy of formation Δ_fH°. The standard molar enthalpy of formation is the enthalpy change when one mole of a compound is formed from its elements under standard conditions.

For formation reactions, "standard conditions" are internationally accepted to be Standard Ambient Temperature and Pressure (SATP). At SATP, the temperature is 25 degrees Celsius and the pressure is 100 KPa.

The table in your Chemistry 30 data booklet (pages 4 and 5) lists standard molar enthalpies of formation for many common compounds. Notice that no elements are listed on the table. This is because elements in their most stable state are regarded to have zero enthalpy at SATP.

Reference Energy State - A reference point at which the potential energy of the elements in their most stable form at SATP is defined as zero.

Example

According to the Table of Standard Molar Enthalpies of Formation, the standard molar enthalpy of formation for aluminum oxide is -1675.7 kJ/mol. In other words, when aluminum oxide is formed from its elements at SATP, 1675.7 kJ of potential energy are released.

This information can be represented by the following formation reaction. The degree symbol indicates that the reaction is occurring at standard state.

\( \mathrm { 2Al(s) + \frac{3}{2}O_2(g) \rightarrow Al_2O_3(s) } \)

\( \mathrm { \Delta_fH^{\circ} = -1675.7 \frac{kJ}{mol} } \)

This reaction is classified as a formation equation. Can you think of another classification for it? Perhaps you said combustion. If you did, you are correct. When elements react with oxygen, the reaction can be classified as both formation and combustion.

Most of the compounds listed on the Table are formed exothermically from their elements. As such, most of the ΔfH° values for the formation reactions are negative.