Calculating bond energy

Worked example 2

Bond breaking is an endothermic process. 

Bond making is an exothermic process. 


Overall energy change =  breaking bonds total energy –  making bonds total energy 

Worked example 2:

Hydrogen bromide decomposes to form hydrogen and bromine:

2HBr ⟶ H2  + Br2

2 H – Br → H – H + Br – Br

Calculate the overall energy change for the reaction above. (6)

The table below shows the bond energies. 

Step 1. Calculate energy taken in when breaking bonds (1) 

(2 x H - Br) 

(2 x 366) = 732

Step 2. Calculate energy released from making bonds (1) 

(1 x H - H) + (1 x Br - Br)

(1 x 436) + (1 x 193) = 629

Step 3: Calculate total energy change (1) 

Overall energy change =  breaking bonds total energy –  making bonds total energy 

Overall energy change = 732 - 629

Overall energy change =  103 kJ/mol 

Step 4: Explain if the reaction is an endothermic or an exothermic process. (3)

Overall energy change = + 130 kJ/mol