- Chemical reactions involve breaking old bonds and forming new ones. Bond breaking requires energy and is endothermic, while bond formation releases energy and is exothermic. - The bond energies table lists the average energy required to break common types of bonds. However, bond energies can vary depending on neighboring bonds. - Reaction enthalpy change (ΔH) can be calculated by adding the bond energies of reactants and subtracting the bond energies of products. This was demonstrated for combustion reactions of ethanol and methoxymethane.

1. BOND ENERGY

2. BOND ENERGY All chemical reactions involve the breaking of old bonds followed by the making of new bonds. The heat absorbed or released in a reaction comes from the chemical bonds being broken or made respectively.

3. BOND ENERGY Bond breaking is endothermic . Bond formation is exothermic . C-H + energy  C H H H  H-H + energy The greater the bond energy is, the stronger the bond

4. BOND ENERGY Table of average bond energies:

5. BOND ENERGY The bond energies on the table are averages , because they may differ depending on adjacent bonds. i.e. the C-H bond requires 435kJ to break, but breaking all 4 bonds of CH 4 requires 1652kJ of energy, this the average is 413kJ/mol 435kJ/mol 1652kJ/mol 4 bonds = 413kJ/mol C H H H H

6. BOND ENERGY Ex 1. H 2(g) + Br 2(g)  2HBr (g) Use bond energies to find the Δ H° of a reaction: Bond energies 432kJ/mol 193kJ/mol 2 x - 363kJ/mol Add the bond energies together: There are 2 moles of H-Br Bond formation is exothermic Δ H° = 432kJ/mol + 193kJ/mol + 2 x (-363kJ/mol) Δ H° = -101kJ/mol .: Δ H° is -101kJ/mol

7. BOND ENERGY Use bond energies to find the Δ H° of a reaction: 432kJ/mol 193kJ/mol 363kJ/mol Δ H° = Σ bond energy of reactants – Σ bond energy of products Can be solved using this formula as well Δ H° = (432kJ/mol + 193 kJ/mol) – (2 x 363kJ/mol) Δ H° = -101kJ/mol .: Δ H° is -101kJ/mol Ex 1. H 2(g) + Br 2(g)  2HBr (g)

8. BOND ENERGY For more complex molecules… Ex 2. Calculate the enthalpy of combustion for ethanol using average bond energies. C 2 H 5 OH (l) + 3O 2(g)  2CO 2(g) + 3H 2 O (g) Draw the structures to determine the types and number of bonds

9. BOND ENERGY Ex 2. Calculate the enthalpy of combustion for ethanol using average bond energies.

10. BOND ENERGY Ex 2. Calculate the enthalpy of combustion for ethanol using average bond energies. The molar enthalpy of combustion of ethanol based on bond energies is -1052 kJ/mol. The accepted value is -1368 kJ/mol. There difference is due to the use of average bond energies. .: Δ H° is -1.40x10 3 kJ

11. BOND ENERGY Ex 3. Calculate the enthalpy of combustion for methoxy methane (CH 3 OCH 3 ) using average bond energies. CH 3 OCH 3(l) + 3O 2(g)  2CO 2(g) + 3H 2 O (g)

12. BOND ENERGY Ex 3. Calculate the enthalpy of combustion for methoxy methane (CH 3 OCH 3 ) using average bond energies.

13. BOND ENERGY Ex 3. Calculate the enthalpy of combustion for methoxy methane (CH 3 OCH 3 ) using average bond energies. .: Δ H° is -1.09x10 3 kJ

14. BOND ENERGY Bond dissociation energy ( D ) is also a measure of bond strength in a chemical bond. It is the change in enthalpy of a homolysis reaction at abolute zero (0 kelvin) where a molecule is broken down into two free radicals. Δ H = __kJ/mol It is not the same as average bond energy.