1. Advanced Higher Chemistry Unit 2 Applications of Hess’s Law 2 Mean Bond Enthalpies

2. The mean bond enthalpy is defined as the energy required to break a covalent bond or bonds in the gaseous state H 2 (g)  2H(g)  H = +435 kJmol -1 Or it can be the energy released when 1 mole of a particular type of covalent bond is formed 2H(g)  H 2 (g)  H= – 435 kJmol -1

3. Since the precise energy of a bond is subject to variation due to the effects of neighbouring atoms or groups, data books always quote an average or mean. These are found on page 9 of Higher / Advanced Higher data book Bond HHHH O=O CCCCCHCHCOCO C=O mean B.E. (kJ mol -1 ) 432497346414365798

4. The idea of measuring and calculating the bond enthalpies for various bonds allows another application of Hess’s law It allows the enthalpy changes for a variety of chemical reactions or other bond enthalpies to be calculated that might otherwise be difficult. The values for these are in the data book and can be used as follows to calculate enthalpy changes:-

5. 1. The bond enthalpies for all of the bond breaking and bond making steps are calculated as a “profit and loss” account. 2. The total enthalpies for each side are added together i.e.  H =  bond breaking steps + bond making steps Note : it is helpful when doing these to draw any relevant molecules so as to count the number of bonds being broken and formed accurately.

6. e.g. 1 Calculate the enthalpy change for the reaction H 2(g) + Cl 2(g)  2HCl (g) Bonds BrokenBonds Made H  H 1 x 436 kJ Cl  Cl 1 x 243 kJ Total + 679 kJ 2H  Cl 2 x  431 kJ Total  862kJ

7. Hence the enthalpy change,  H = 679 + (  862) =  183kJ Since two moles of product are formed  H = 183  2 =  91.5kJmol -1

8. e.g. 2 Calculate the enthalpy change for the formation of ethene, C 2 H 4 Bonds BrokenBonds Made 2C (s) + 2H 2(g)  C 2 H 4(g) 2C (s)  2C (g) 2 x 715 kJ 2H  H 2 x 436 kJ Total + 2302 kJ C=C 1 x  607 kJ 4C  H 4 x  414 kJ Total  2263 kJ

9. Hence the enthalpy of formation of ethene = 2302 + (  2263) = + 39 kJmol -1