A heat of formation is A) The heat produced in a reaction when 1 mole of a compound is produced. B) The heat absorbed in a reaction when 1 mole of reactants react. C) The heat required to make 1 mole of a compound from its elements in their standard states. D) The heat absorbed or produced in a combination reaction.

Heat of formation is defined as change in enthalpy during the formation of 1 mole of compound from its constituent elements in its standard states.it is denoted by symbol [tex]\Delta H^o_{f}[/tex]. Its units are Kilo joules per mole.

If [tex]\Delta H^o_{f}[/tex] <0, the change in enthalpy is negative then it means that energy is released during the formation of 1 mole of substance.
If [tex]\Delta H^o_{f}[/tex] >0 , the change in enthalpy is positive then it means that energy is absorbed during the formation of 1 mole of substance.

For example:

[tex]2H_2(g)+O_2(g)\rightarrow 2H_2O(l),\Delta H_f^{o}=-286 kJ/mol[/tex]

286 kilo Joules of energy given out when 1 mole of liquid water is formed.