Answer: third choice, not created or destroyed.
Explanation:
1) During a chemical reaction energy is not either created or destroyed. It is only transferred and the total amount of energy remains constant.
2) That is the first law of thermodynamics, which means that it is a universal true and is verified in any process either chemical or physical.
3) Of course we constantly experience that some form of energy that we observe "dissapears". Well it does not. It is just transformed into other form of energy.
4) In the case of the chemical reactions, whe usually observe that in the form of heat: for example when H₂ reacts with O₂ to form H₂O a considerable amount of heat evolves. That is because potential chemical energy in the bonds of the reactants (O₂ and H₂) is released, this is, some potential chemica energy is transformed into heat and part of the chemical potential energy is in the product bonds.
That is why you can write the expression:
ΔH rxn = ∑ΔH products - ∑ΔH reactants, or
∑ΔH products = ΔH rxn + ∑ ΔH reactants.
Which you must interpret as that the energy of the reactants ends up as energy on the products or heat released by the reaction.
