Final answer:
Enthalpy is a measure of the difference between the enthalpy of the products and reactants. It is determined by the energies needed to break chemical bonds and the energies needed to form chemical bonds. The change in enthalpy indicates whether a reaction is endothermic or exothermic.
Step-by-step explanation:
The change in enthalpy of a reaction is a measure of the differences in enthalpy of the reactants and products. The enthalpy of a system is determined by the energies needed to break chemical bonds and the energies needed to form chemical bonds.
Energy needs to be put into the system in order to break chemical bonds, as they do not come apart spontaneously in most cases. Bond formation to produce products will involve release of energy. The change in enthalpy shows the trade-offs made in these two processes.
Does it take more energy to break bonds than that needed to form bonds? If so, the reaction is endothermic and the enthalpy change is positive. If more energy is produced in bond formation than that needed for bond breaking, the reaction is exothermic and the enthalpy is negative.