How do you calculate the energy change in a reaction using bond enthalpies?

The energy change in a reaction is calculated by subtracting the total energy required to break bonds from the total energy released when new bonds are formed.

In a chemical reaction, bonds in the reactants are broken and new bonds are formed in the products. This process involves energy changes. The energy required to break a bond is known as bond dissociation energy or bond enthalpy, and it is always a positive value because energy is absorbed to break a bond. On the other hand, energy is released when a new bond is formed, and this is a negative value.

To calculate the energy change in a reaction, you need to know the bond enthalpies of all the bonds in the reactants and the products. The total energy required to break all the bonds in the reactants is calculated by multiplying the bond enthalpy of each bond by the number of such bonds, and then adding up these values. Similarly, the total energy released when all the bonds in the products are formed is calculated.

The energy change in the reaction, also known as the enthalpy change of the reaction, is then calculated by subtracting the total energy required to break the bonds in the reactants from the total energy released when the bonds in the products are formed. This is based on Hess's law, which states that the total enthalpy change in a reaction is the sum of the enthalpy changes of the individual steps, regardless of the path taken.

Remember, if the energy released when new bonds are formed is greater than the energy required to break the initial bonds, the reaction is exothermic (energy is released, so the enthalpy change is negative). If the energy required to break the initial bonds is greater than the energy released when new bonds are formed, the reaction is endothermic (energy is absorbed, so the enthalpy change is positive).

In summary, calculating the energy change in a reaction using bond enthalpies involves determining the total energy required to break the bonds in the reactants and the total energy released when the bonds in the products are formed, and then subtracting the former from the latter.