In a combustion reaction, element or molecule reaction with oxygen to form corresponding oxide.
In present case, 2 mol of solid magnesium mg combines with 1 mole of oxygen gas 2 mol of solid magnesium oxide is formed and 1204 kj of heat is released. The balanced thermochemical equation for the combustion reaction is
2Mg + O2 → 2 MgO + 1204 kJ
here, 1204 kJ is the energy liberated by the system during the reaction. Liberation of energy suggest the it is an exothermic reaction and the resultant product formed is thermodynamically more stable than reactant.
