Advices For Studying Organic Chemistry

room temperature (298 K) the T∆S° term is 0.8 kJ mol–1.

4) The enthalpy change for the reaction and the free-energy change are almost

equal ⇒ ∆H° = – 102.5 kJ mol–1 and ∆G° = – 103.3 kJ mol–1.

5) In situation like this one it is often convenient to make predictions about whether

a reaction will proceed to completion on the basis of ∆H° rather than ∆G° since

∆H° values are readily obtained from bond dissociation energies.

10.5B ACTIVATION ENERGIES:

1. It is often convenient to estimate the reaction rates simply on energies of
   activation, Eact, rather than on free energies of activation, ∆G‡.


2. Eact and ∆G‡ are close related and both measure the difference in energy
   between the reactants and the transition state.

1. A low energy of activation ⇒ a reaction will take place rapidly.


2. A high energy of activation ⇒ a reaction will take place slowly.

3. The energy of activation for each step in chlorination:
   Chain Initiation
   Step 1 Cl 2 2 Cl• Eact = + 243 kJ mol–1

Chain Propagation
Step 2 CH 3 –H + •Cl CH 3 • + H–Cl Eact = + 16 kJ mol–1
Step 3 CH 3 • + Cl–Cl CH 3 –Cl + •Cl Eact = ~ 8 kJ mol–1

1) The energy of activation must be determined from other experimental data.

2) The energy of activation cannot be directly measured –– it is calculated.

4. Principles for estimating energy of activation:
   
   
   1. Any reaction in which bonds are broken will have an energy of activation
      greater than zero.
      i) This will be true even if a stronger bond is formed and the reaction is
      exothermic.
      ii) Bond formation and bond breaking do not occur simultaneously in the