Define and explain the standard enthalpy of formation `(Delta_(r)H^(theta))`.

Explanation

the enthalpy of formation is the heat change accompanying the formation of one mole of a compound from its constituent elements. It is generally denoted by `Delta_(l)H`, for examples the enthalpy of formation of carbon dioxide can be represented as
`C_(("graphite"))+O_(2(g))toCO_(2(g)), DeltaH=-393.5kJ`
When all the species of the chemical reaction are in their standard states, the enthalpy of formation is called standard heat of formation. It is denoted by `DeltaH^(theta)`.
The standard heat of formation is defined as the heat change accompanying the formation of one mole of accompanying the formation elements all the substances being in their standard states (1 bar pressure and 298k.)