Bond Enthalpies (Edexcel International AS Chemistry): Revision Note

Bond Enthalpy - Definitions

• 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

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 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

• Bond enthalpies can be used to predict which bonds will break first in a reaction

  • Bonds with high bond enthalpy values require more energy to break them

    • Reactions involving these types of bond often require heat and / or the use of a catalyst

  • Bonds with low bond enthalpy values require less energy to break them

    • Reactions involving these types of bonds are more likely to happen at room temperature

  • Therefore, bonds with lower bond enthalpy values are easier to break and are likely to be the first bonds to break in a reaction

Bond Enthalpy - Calculations

• Bond energies are used to find the ΔH_r^ꝋ 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:

Δ_rH^ꝋ = enthalpy change for bonds broken + enthalpy change for bonds formed

• These two worked examples show how to lay out your calculation