Bonds and reaction energies

1. Bonds and reaction energies

2. Bonds • Energy is needed to break a bond between two atoms (endothermic) • Making new bonds releases energy (exothermic) • Total reaction energy = energy of bonds broken – energy of bonds made

3. Or • Etotal = Ereactants – Eproducts • Unit : kilojoules per mole • kJ per mole • kJ mol-1

4. H-H = 436, Br-Br = 193, H-Br = 368 e.g. • H2 + Br2 2HBr • broken: made: • H-H H-Br x 2 • Br-Br Total = (436 + 193) – (368 + 368) = -107  exothermic

5. Steps: • Obtain balanced equation • Draw molecules for reactant and product • Count each bond • Total each bond • Final calculation

6. H-H = 436 O=O = 500 H-O = 464 e.g. • 2H2 + O2 2H2O • broken: made: • H-H (twice) H-O x 4 • O=O Total = (436 *2 + 500) – (464 x 4) = 872 + 500 – 1856 = - 484  exothermic ( For one water it is -242 kJ mol-1)

7. H-H = 436 I-I = 151 H-I = 297 Try this: • H2 + I2 2HI • Think – what bonds are in the reactant molecules, what bonds are in the product molecules • Broken: Made: • H-H, I-I H-I x 2 • Total = (436 + 151) – (297 *2) • = -7 kJ mol-1 for the reaction as shown • (-3.5 kJ mol-1 for each mole of HI)

8. C-H = 414 O=O = 500 H-O = 464 C=O = 805 Try this: • CH4 + 2O2 CO2 +2H2O • Think – what bonds are broken,what bonds are made. • Broken: Made: • C-H x4, O=O x2 C=O x2, H-O x 4 • Total = (1656 +1000) – (1610 + 1856) • = 2656 – 3466 • = - 810 kJ mol-1

9. C-H = 414 O=O = 500 H-O = 464 C=O = 805 C=C = 613 Try this: • C2H4 + 3O2 2CO2 +2H2O • Think – what bonds are broken,what bonds are made. • Broken: Made: • C-H x4, O=O x3, C=C C=O x4, H-O x 4 • Total = (1656 +1500 + 613) – (3220 + 1856) • = 3769 – 5076 • = - 1307 kJ mol-1