Define the standard heat of formation of mgco3

The standard heat of formation is the energy change that occurs when 1 mole of a substance is made from its elements in their standard states (most stable form of the element at 1 bar of pressure and the specified temperature, usually 298 K or 25 degrees Celcius)

E. g for MgCO3

Mg (s) + C (s) + 3/2 O2 (g) = MgCO3