Bonds & Energy
- When bonds are broken or made enthalpy changes take place
- A chemical bond is a force of attraction between two atoms
- Breaking the bond requires the input of energy it is therefore an endothermic process
- The energy change required to break the bond depends on the atoms that form the bond
- The energy required to break a particular bond is called the bond dissociation enthalpy
- This is usually just shortened to bond enthalpy or bond energy
- Bond formation is the opposite of bond breaking and so energy is released when bonds are formed
- It is therefore an exothermic process
To break bonds energy is required from the surroundings and to make new bonds energy is released from the reaction to the surroundings
- The amount of energy released when a particular bond is formed has the same magnitude as the energy taken in when the bond is broken but has the opposite sign
Overall enthalpy changes
- If more energy is released when new bonds are formed than energy is required to break bonds, the reaction is exothermic
- The products are more stable than the reactants
- If more energy is required to break bonds than energy is released when new bonds are formed, the reaction is endothermic
- The products are less stable than the reactants
- The relationship between bond breaking and bond making can be shown graphically like this:
Bond enthalpy profiles