Final answer:
Bond dissociation enthalpies with positive values are indicative of endothermic reactions, where energy is absorbed because breaking covalent bonds requires energy. Calculations of enthalpy change using bond energies can help determine if a reaction is exothermic or endothermic.
Step-by-step explanation:
Bond dissociation enthalpies have positive values, indicating that these reactions are endothermic. During an endothermic reaction, energy is absorbed from the surroundings because the bonds being broken in the reactants require an input of energy. This is the reason why bond energy is always a positive value - it takes energy to break a covalent bond, whereas energy is released during bond formation. When the bonds in the products are weaker than those in the reactants, the result is a reaction that absorbs heat, denoted by a positive enthalpy change (ΔH positive).
We can calculate the enthalpy change for a reaction by using the bond dissociation energies, which allows us to determine whether a chemical process is exothermic or endothermic. This calculation involves summing the bond energies for all the bonds broken and subtracting the energies for the bonds formed. As bond energies are typically averages for one type of bond in many different molecules, this method provides an approximation rather than an exact value for the reaction's enthalpy change.