Bond Enthalpies (OCR AS Chemistry)

• The amount of energy required to break one mole of a specific covalent bond in the gas phase is called the bond dissociation energy
• Bond dissociation energy, E, is usually just simplified to bond energy or bond enthalpy
• In symbols, the type of bond broken is written in brackets after E
  • For example, E (H-H) is the bond energy of a mole of single bonds between two hydrogen atoms

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

Bond energies are affected by other atoms in the molecule, so average bond enthalpies are listed in data tables

Calculating enthalpy change from bond energies

• Bond energies are used to find the ΔH_r^ꝋ of a reaction when this cannot be done experimentally
• The formula is:

The formula for calculating the standard enthalpy change of reaction using bond energies

• As energy is required to break bonds, bond breaking is endothermic
  • ΔH is positive
• As energy is released making new bonds, bond forming is exothermic
  • ΔH is negative
• The difference between the energy required for bond breaking and the energy released by bond making determines whether an overall reaction is exothermic or endothermic

Answer

Step 1: Use the equation to work out the bonds broken and formed and set out the calculation as a balance sheet as shown below:

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)

Note! Values for bonds broken are positive (endothermic) and values for bonds formed are negative (exothermic)

Step 2: Calculate the standard enthalpy of reaction

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

   = (+2253 kJ mol^-1) + (-2346 kJ mol^-1)

   = -93 kJ mol^-1

Answer

Step 1: The enthalpy of combustion is the enthalpy change when one mole of a substance reacts in excess oxygen to produce water and carbon dioxide

The chemical reaction should therefore be simplified such that only one mole of ethyne reacts in excess oxygen:

H-C≡C-H + 2½ O=O → H-O-H + 2O=C=O

Step 2: Set out the calculation as a balance sheet as shown below:

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

   = (+2912 kJ mol^-1) + (- 4142 kJ mol^-1)

   = -1230 kJ mol^-1