Introduction to Bond Energies
How do we know how much energy is held by an object? How do we measure that? These are some of the questions that the students will encounter during this lesson. From this lesson, students will be able to distinguish between "energy" and "change in energy".
A chemical reaction can be viewed as a set of bonds breaking and reforming. Energy is absorbed by the bonds are broken and released when bonds are formed. Therefore, knowing how much energy is held by each bond within a compound will tell us how much energy is required to form that exact compound.
A chemical reaction can be viewed as a set of bonds breaking and reforming. Energy is absorbed by the bonds are broken and released when bonds are formed. Therefore, knowing how much energy is held by each bond within a compound will tell us how much energy is required to form that exact compound.
From the equation above, we learn that the change in enthalpy of a reaction can be determined by subtracting the sum of energy of bonds formed from the sum of energy of bonds broken. In the given example above, two gases (Hydrogen gas and Chlorine gas) is reacting together to form hydrochloric acid. Using the average bond enthalpy chart below, we can determine how much energy is in each bond. This allows us to confidently say that the change in enthalpy is exothermic in a synthesis reaction between hydrogen gas and chlorine gas.