Question

2mg+O2 2MgO +energy

Answer 1

Well you did not provide full question.
But this is the exothermic reaction which cause the burning of magnesium in the presence of air as the magnesium is a highly reactive metal .It just need the source in order to burn in the presence of the oxygen or air .This reaction results in the magnesium oxide production as well as the energy or heat release being the exothermic reaction.

Exothermic reactions are those reactions which release energy .

Answer 2

Since the question is not clear, I am giving a general explanation of the given equation.

Magnesium (Mg) and Oxygen (O₂) are the reactants of the reaction, whereas the product is Magnesium oxide (MgO).

This is a combination reaction where two or more simple substances react together to form a compound.

When Magnesium is burned in the presence of oxygen, Magnesium oxide is formed and energy is released.

2Mg + O₂ → 2MgO + Energy