Answer to Question #106173 in Physical Chemistry for Nikesh

Question

Define standard enthalpy of formation and describe a method for its direct
determination with the help of an example

Answer 1

The standard enthalpy of formation (ΔH^of) or standard heat of formation of a compound is the change of enthalpy during the formation of 1 mole of the substance from its constituent elements, with all substances in their standard states.

Standard enthalpies of formation  are determined under standard conditions: a pressure of 1 atm for gases and a concentration of 1 M for species in solution, with all pure substances present in their standard states (their most stable forms at 1 atm pressure and the temperature of the measurement).

For example, the standard enthalpy of formation of carbon dioxide would be the enthalpy of the following reaction under the above conditions:

C(s, graphite) + O₂(g) → CO₂(g).