Thermochemical Properties and Molecular Stability

Abstract

The enthalpy of formation of one mole of compound A _m B _n C _o from its elements under standard conditions is called the heat of formation (∆H _f):

$$ mA + nB + oC \to {{A}_{m}}{{B}_{n}}{{C}_{o}} \ldots \Delta {{H}_{f}} $$

(15-1)

The enthalpy of elements in their standard state is taken to be zero by convention. The standard state is the stable state of elements at a pressure of 101 kPa at the reaction temperature. For the formation of carbon dioxide from carbon and oxygen [Eq. (15–2)], molecular oxygen is considered to be the standard state of oxygen and graphite is the standard state of carbon. If the temperature is not stated, the data are considered to refer to 25 °C.

$$ C\left( s \right) + {{O}_{2}}\left( g \right) \to C{{O}_{2}}\left( g \right),\Delta {{H}_{f}} = - 389.79kJ/mol, $$

(15-2)

where (s) denotes the solid and (g) the gaseous state.