Bond Energy Calculations (Edexcel GCSE Chemistry): Revision Note

Example 1

Hydrogen and chlorine react to form hydrogen chloride gas:

H₂  + Cl₂ ⟶ 2HCl

The table below shows the bond energies. Calculate the energy change for the reaction and deduce whether it is exothermic or endothermic.

Answer:

Step 1: Calculate the energy in:

Energy in = 436 + 243 = 679 kJ mol^-1

Step 2: Calculate the energy out:

Energy out = 2 x 432 = 864 kJ mol^-1

Step 3: Calculate the energy change:

Energy change = 679 - 864 = -185 kJ mol^-1

Step 4: Deduce the type of reaction:

• The energy change is a negative number, which means energy has been lost to the surroundings

• Therefore the reaction is exothermic

Example 2

Hydrogen bromide decomposes to form hydrogen and bromine:

2HBr  ⟶ H₂  + Br₂

The table below shows the bond energies. Calculate the energy change for the reaction and deduce whether it is exothermic or endothermic.

Answer:

Step 1: Calculate the energy in:

Energy in = 2 x 366 = 732 kJ mol^-1

Step 2: Calculate the energy out:

Energy out = 436 + 193 = 629 kJ mol^-1

Step 3: Calculate the energy change:

Energy change = 732 - 629 = +103 kJ mol^-1

Step 4: Deduce the type of reaction:

• The energy change is a positive number, which means energy has been taken in from the surroundings

• Therefore the reaction is endothermic

For bond energy questions, 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.