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

the energy released from making bonds is lower than energy taken in when breaking bonds
so, overall more energy has been taken in from the surrounding
the reaction is endothermic.

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