Calculating bond energy
Calculating bond energy
Worked example 3
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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 3:
Methane (CH4) reacts with chlorine (Cl2)
to form chloromethane (CH3Cl) and hydrogen chloride (HCl).
Calculate the overall energy change for the reaction above. (6)
The word equation is:
Methane + Chlorine --------> Chloromethane + Hydrogen chloride
The symbol equation showing all the bonds is:
Step 1. Calculate energy taken in when breaking bonds (1)
(4 x C - H) + (1 x Cl - Cl)
(4 x 412) + (1 x 242) = 1648 + 242 = 1890
Step 2. Calculate energy released from making bonds (1)
(3 x C-H) + (1 x C - Cl) + (1 x H - Cl)
(3 x 412) + (1 x 328) + (1 x 431) = 1236 + 328 + 431 = 1995
Step 3: Calculate total energy change (1)
Overall energy change = breaking bonds total energy – making bonds total energy
Overall energy change = 1890 - 1995
Overall energy change = – 105 kJ/mol
Step 4: Explain if the reaction is an endothermic or an exothermic process. (3)
Overall energy change = – 122 kJ/mol
the energy released from making bonds is higher than energy taken in when breaking bonds
so, overall more energy has been lost to the surrounding
the reaction is exothermic.
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