transition state.
iii) Bond formation lags behind, and its energy is not all available for bond
breaking.
2) Activation energies of endothermic reactions that involve both bond
formation and bond rupture will be greater than the heat of reaction, ∆H°.CH 3 –H + •Cl CH 3 • + H–Cl ∆H° = + 4 kJ mol–1
(∆H° = 435) (∆H° = 431) Eact = + 16 kJ mol–1
CH 3 –H + •Br CH 3 • + H–Br ∆H° = + 69 kJ mol–1
(∆H° = 435) (∆H° = 366) Eact = + 78 kJ mol–1
i) This energy released in bond formation is less than that required for bond
breaking for the above two reactions ⇒ they are endothermic reactions.Figure 10.3 Potential energy diagrams (a) for the reaction of a chlorine atom with
methane and (b) for the reaction of a bromine atom with methane.
3) The energy of activation of a gas-phase reaction where bonds are broken
homolytically but no bonds are formed is equal to ∆H°.
i) This rule only applies to radical reactions taking place in the gas phase.Cl–Cl 2 •Cl ∆H° = + 243 kJ mol–1
(∆H° = 243) Eact = + 243 kJ mol–1