Explain standard enthalpy of formation `(Delta_(f)H^(@))`.

Explanation

It is defined as the change in enthalpy accompanying the formation of one mole of a compound in its standard state from the elements in their standard states.
Example : `H_(2(g)) + (1)/(2)O_(2(g)) rarr H_(2)O_((l)) , Delta_(f)H^(@) = -285.8 kJ mol^(-)1`.