Breaking and making bonds 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 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

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 = - 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