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Bond Enthalpy Calculations (DP IB Chemistry: SL)

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Bond Enthalpy Calculations

  • Bond energies are used to find the ΔHr 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:

Chemical Energetics Equation Enthalpy Change of Reaction using Bond, downloadable AS & A Level Chemistry revision notes

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

 

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

Worked example

Calculate the enthalpy of reaction for the Haber process reaction. The relevant bond energies are given in the table below:Worked example - Calculating the enthalpy change in Haber process, downloadable IB Chemistry revision notes

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:

Chemical Energetics Table 2_5, downloadable AS & A Level Chemistry revision notes

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

Step 3: Calculate the standard enthalpy of reaction

ΔHr = enthalpy change for bonds broken + enthalpy change for bonds formed

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

= -93 kJ mol-1

Worked example

The complete combustion of ethyne, C2H2 , is shown in the equation below:

2C2H2 (g)  + 5 O2 (g)      2H2O (g) + 4CO2 (g)

Using the average bond enthalpies given in the table, what is the enthalpy of combustion of ethyne?

Bond Enthalpy - Worked Example 2 Data table, downloadable IB Chemistry revision notes

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:

Bond Enthalpy - Worked Example 2 Answer, downloadable IB Chemistry revision notes

ΔHr = enthalpy change for bonds broken + enthalpy change for bonds formed

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

= -1230 kJ mol-1

Examiner Tip

The key to success in bond enthalpy calculations is to be very careful when accounting for every bond present. Always draw out the full displayed structures of the molecules so you don't miss any of the bonds.

Watch out for coefficients in the balanced equations as students often miss those, forget to multiply them by the bond enthalpies and get the answer wrong!

It is super important to show your steps because bond enthalpy calculations often carry 3 marks, 2 of which could be for workings if you get the final answer wrong

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Stewart

Author: Stewart

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.