Flexi Says: The enthalpy of reaction allows us to determine if a given reaction is exothermic or endothermic. The way in which a reaction is written influences the value of the enthalpy change for the reaction. Many reactions are reversible, meaning that the products of the reaction are capable of combining and reforming the reactants. If a reaction is written in the reverse direction, the sign of the ΔH changes. An exothermic reaction, in which heat is released by the reaction to the surroundings, has a negative ΔH value. When we reverse the same reaction the energy is consumed in the reaction from the surroundings and that changes the enthalpy to a positive ΔH value.
