IB Chemistry SL

Revision Notes

5.1.9 Bond Enthalpy

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

Chemical Energetics Bond Breaking and Forming, downloadable AS & A Level Chemistry revision notes

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 profile, downloadable IB Chemistry revision notes

Bond enthalpy profiles

Bond Enthalpy

Average bond energy

  • Bond energies are affected by other atoms in the molecule (the environment)
  • Therefore, an average of a number of the same type of bond but in different environments is calculated
  • This bond energy is known as the average bond energy and is defined as

‘The energy needed to break one mole of bonds in a gaseous molecule averaged over similar compounds’

Average Bond Enthalpy, downloadable IB Chemistry revision notes

Average bond enthalpy of C-H in methane

  • The average bond enthalpy of C-H is found by taking the bond dissociation enthalpy for the whole molecule and dividing it by the number of C-H bonds
  • The first C-H bond is easier to break than the second as the remaining hydrogens are pulled more closely to the carbon
  • However, since it is impossible to measure the energy of each C-H bond an average is taken
  • This value is also compared with a range of similar compounds to obtain an accepted value for the average bond enthalpy

Exam Tip

A lot of students mix up endothermic/exothermic and bond breaking/ bond making.

An easy way to remember is that ENDOTHERMIC leads to the poetic phrase the ‘end o’ the bond’

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