Calculating bond energy
During a chemical reaction,
bonds are broken in the reactants and bonds are formed in the products.
Energy must be supplied to break bonds in reactants.
Bond breaking is an endothermic process.
Energy is transferred to the surroundings when bonds in products are formed.
Bond making is an exothermic process.
Overall energy change = breaking bonds total energy – making bonds total energy
Worked example:
Methane (CH4) reacts with chlorine (Cl2)
to form chloromethane (CH3Cl) and hydrogen chloride (HCl).
Calculate the overall energy change for the reaction above.
The word equation is:
Methane + Chlorine --------> Chloromethane + Hydrogen chloride
The symbol equation showing all the bonds is:
Step 1:
work out the type and number of broken bonds, and the energy needed in kJ/mol
Expand to reveal answer
Bond broken Number of bonds Energy needed in kJ/mol
C - H 4 4 x 413 = 1652
Cl - Cl 1 1 x 243 = 243
total energy needed 1652 + 243 = 1895
Step 2:
work out the type and number of made bonds, and the energy released in kJ/mol
Expand to reveal answer
Bond made Number of bonds Energy released in kJ/mol
C - H 3 3 x 413 = 1239
C - Cl 1 1 x 346 = 346
H - Cl 1 1 x 432 = 432
total energy released 1239 + 346 + 432 = 2017
Step 3:
Overall energy change = breaking bonds total energy – making bonds total energy
Expand to reveal answer
Overall energy change = breaking bonds total energy – making bonds total energy
Overall energy change = 1895 - 2017
Overall energy change = – 122 kJ/mol
Step 4:
Explain if the reaction is an endothermic or an exothermic process.
Expand to reveal answer
Overall energy change = – 122 kJ/mol
Energy change is a negative number so energy has been lost to the surrounding, the reaction is exothermic.