Bond Enthalpy Calculations (DP IB Chemistry): Revision Note

Bond enthalpy calculations

• 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

Diagram to show bond breaking and bond making

• 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:

Diagram to show the energy profiles of both exothermic and endothermic reactions

Bond enthalpy profiles

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

• 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

Bond enthalpy calculations

• Bond energies are used to find the ΔH^ꝋ_r of a reaction when this cannot be done experimentally

• The process is a step-by-step summation of the bond enthalpies of the all the molecules present finishing with this formula:

ΔH^Ꝋ_r = enthalpy change for bonds broken + enthalpy change for bonds formed