Define the term enthalpy of reaction

A chemical reaction occurs when bonds are broken and formed between atoms. Energy is taken in in order to break bonds; when a bond is formed the molecule becomes more stable, so energy is given out. The enthalpy of reaction is the net change in energy during a reaction, and is denoted by delta H(dH). When the energy released by bond formation is greater than the energy required to break the bonds, heat is given out, dH is negative, and the reaction is said to be exothermic. If more energy is required to break bonds than is released by bond formation, dH is positive and the reaction is said to be endothermic.The enthalpy of reaction can be calculated by adding up the bond energies of the reactants and subtracting the bond energies of the products.