Bond Enthalpies (Oxford AQA International A Level Chemistry)

Mean Bond Enthalpy Calculations

• During a reaction, enthalpy changes take place because bonds are being broken and formed

• Energy (in the form of heat) is needed to overcome attractive forces between atoms

  • Bond breaking is therefore endothermic

• Energy is released from the reaction to the surroundings (in the form of heat) when new bonds are formed

  • Bond forming is therefore exothermic

Making and breaking bonds 

• If more energy is required to break bonds than energy is released when new bonds are formed, the reaction is endothermic

• If more energy is released when new bonds are formed than energy is required to break bonds, the reaction is exothermic

• In reality, only some bonds in the reactants are broken and then new ones are formed

Bond Dissociation Enthalpy

• The amount of energy required to break one mole of a specific covalent bond in the gas phase is called the bond dissociation enthalpy

• Bond dissociation energy (E) is also known as exact bond energy or bond enthalpy

Mean bond enthalpy

• Bond enthalpies 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 enthalpy is known as the mean bond enthalpy

• Since bond enthalpies cannot be determined directly, enthalpy cycles are used to calculate the average bond enthalpy

How bond enthalpies are affected by other atoms in the molecule

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

  • E.g. the Haber Process

• The equation to calculate the standard enthalpy change of reaction using bond enthalpies is:

 ΔH^θ = enthalpy change for bonds broken - enthalpy change for bonds formed

• Enthalpy changes calculated using mean bond enthalpies differ from those determined using Hess's law

• This is due to bond enthalpies being mean values, whereas a Hess's law cycle uses enthalpy data that has been taken from individual compounds where a bond has a specific value