When a chemical reaction occurs in an open container most of the energy gained or lost is in the form of heat .
Heat flows between the system and surroundings until the two are at the same temperature.
We are considering the enthalpic state of the system . The magnitude of D H is dependent upon the amounts of reactants consumed. Doubling the reactants, doubles the amount of enthalpy. Reversing a chemical reaction results in the same magnitude of enthalpy but of the opposite sign . The enthalpy change for a reaction depends upon the state of the reactants and products.