intregueing chemestry question?

Question

ok i was doing a heat release question where i am given two molecules and the bond energies of each of their pieces and a reaction
i was asked to calculate the change in heat for the reaction
i found that the reactants side was 345 less than the products side
i know the formula says that i am to take the products side and subtract from the reactants side but explain to me how that makes logical sense
that would mean you get an answer reactants-products=-345
i knew this but i thought about it and i wrote +345 (i got it wrong) because if it were the other way that would mean that the reactants give up an amount of energy less than that which the products need to form and the reaction gives off energy this is impossible
(no free lunch rule)

how can u need more energy than u have and still give off energy
(energy is measured from the point of view of the system remember)

Answer 1

I am confused about your question. If delta H is negative it means that the products are lower in energy than the reactants. The reaction would be exothermic. Delta H is found by products - reactants. I cannot determine whether your reaction was actually endothermic or exothermic without the specific chemicals in the problem. If you get a chance to list this question again with the actual problem I will be happy to help you with it.

Answer 2

In a reaction enthalpy (the amount of heat is either given off or needed by the reaction) is given in terms of + or -.
Activation energy is the energy needed to start the reaction and is not considered when calculating the final change in energy. Every reaction does need a small/large amount of energy provided to begin the reaction.
Enthalpy is calulated by the 'energy of products'-'energy of reactants'. If the products energy is lower then that of the reactants, then some energy has been lost in terms of heat.