ecessary to know the bonds present in both the reactants and products

How to complete bond energy calculations

• Write a balanced equation if none is present already

• Optional - draw the displayed formula in order to identify the type and number of bonds more easily

• Add together all the bond energies for all the bonds in the reactants – this is the ‘energy in’

• Add together the bond energies for all the bonds in the products – this is the ‘energy out’

• Calculate the enthalpy change:

Enthalpy change (ΔH)  = Energy taken in - Energy given out

Worked Example

Hydrogen and chlorine react to form hydrogen chloride gas:

H2  + Cl2 ⟶ 2HCl

The bond energies are given in the table below.

Calculate the overall energy change for this reaction and use this value to explain whether the reaction is exothermic or endothermic.

Answer:

• Calculate the energy in

  • 436 + 242 = 678 (kJ)

• Calculate the energy out

  • 2 x 431 = 862 (kJ)

• Calculate the energy change

  • 678 - 862 = –184 (kJ)

• Since the energy change is a negative number, energy is being released (to the surroundings)

  • Therefore, the reaction is exothermic

Examiner Tips and Tricks

When calculating enthalpy change using bond energies, it is helpful to write down a displayed formula equation for the reaction before identifying the type and number of bonds, to avoid making mistakes.

So, the reaction for the above worked example is:

H-H + Cl-Cl → H-Cl + H-Cl

Worked Example

Hydrogen reacts with iodine to form hydrogen iodide.

H2  + I2  ⟶ 2HI

The relevant bond energies are shown in the table below.

 Calculate the overall energy change for this reaction and use this value to explain why the reaction is exothermic.

Answer:

• Calculate the energy in

  • 436 + 151 = 587 (kJ)

• Calculate the energy out

  • 2 x 295 = 590 (kJ)

• Calculate the energy change

  • 587 - 590 = -3 (kJ)

• The reaction is exothermic because:

  • More energy is released than taken in 

Worked Example

Hydrogen bromide decomposes to form hydrogen and bromine:

2HBr  ⟶ H2  + Br2

The overall energy change for this reaction is +103 kJ.

The relevant bond energies are shown in the table below.

 Calculate the bond energy of the Br–Br bond.

Answer:

• Calculate the energy in

  • 2 x 366 = 732 (kJ)

• State the energy out

  • 436 + Br–Br 

• Overall energy change = energy in - energy out

  • +103 = 732 - (436 + Br–Br)

  • +103 = 732 - 436 - Br–Br

• Calculate the bond energy of the Br–Br bond

  • Br–Br = 732 - 436 - 103

  • Br–Br = +193 (kJ)