Bond Enthalpy Calculations (DP IB Chemistry: HL)

• 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:

Formula for calculating the standard enthalpy change of reaction using bond energies

 

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

Answer:

Step 1: The chemical equation for the Haber process is:

    N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)

    N≡N       3 H-H         6 N-H

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

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

Step 3: 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 be therefore simplified such that only one mole of ethyne reacts in excess oxygen:

H-C≡C-H + 2 ½ 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