Section 4 d

Breaking and making bonds

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

bond bond energy kJ/mol

H - H 436 H – Cl 432C - H 413 N - H 391O - H 464 O = O 498C = O 746 N N 945Cl - Cl 243 Br - Br 190C – C 348 H - Br 366C = C 612

sample calculation

CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (g)

ΔH = bonds broken – bonds made = 2640 – 3338 = - 700 kJ

reactants products CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (g)

bonds broken bonds made(4 (C-H) + 2 (O=O)) (2 (C=O) + 4 (H-O))

(4 x 413) + ( 2 x 498) (2 x 746) + ( 4 x 464)

= 2648 kJ = 3338 kJ

Energy level diagram - exothermic

• The energy level diagram below includes the energy taken into break bonds and the energy released when making new ones

Same energy level diagram