1. Breaking and making bonds
Section 4 d
Breaking and making bonds

2. Bond energies
The table below shows the amount of energy needed to break the bond and the amount of energy released when that bond is made
bond
bond energy kJ/mol
H - H
436
H – Cl
432
C - H
413
N - H
391
O - H
464
O = O
498
C = O
746
N N
945
Cl - Cl
243
Br - Br
190
C – C
348
H - Br
366
C = C
612

3. CH4 (g) + 2O2 (g)  CO2 (g) + 2H2O (g)
sample calculation
CH4 (g) + 2O2 (g)  CO2 (g) + 2H2O (g)
ΔH = bonds broken – bonds made = 2640 – 3338 = kJ
reactants
products
CH4 (g) + 2O2 (g)
CO2 (g) + 2H2O (g)
bonds broken
bonds made
(4 (C-H) (O=O))
(2 (C=O) (H-O))
(4 x 413) + ( 2 x 498)
(2 x 746) + ( 4 x 464)
= 2648 kJ
= 3338 kJ

4. Energy level diagram - exothermic
The energy level diagram below includes the energy taken into break bonds and the energy released when making new ones

5. Same energy level diagram