Hi Saul McShane,
That’s a bit of a complex question to answer!
The second law of thermodynamics basically states that everything tends to disorder. In other words for a reaction to proceed spontaneously the entropy must increase. Gibbs free energy is defined as:
G = H – TS
Where: G = Gibbs free energy, H=enthalpy, T=temperature and S=entropy
A reaction will proceed spontaneously if the Gibbs free energy is negative. An equilibrium will be formed if the Gibbs free energy is equal to zero and the reaction will not proceed if the Gibbs free energy is positive.
For many reactions the entropy is negative which would mean that the Gibbs free energy would be positive (because in the equation G=H -TS). In order to make the reaction proceed you must put in energy to increase the enthalpy (H) term.
The thermodynamics of electophilic aromatic substitution (EArS) are quite complicated. Benzene is very stable and has low Gibbs free energy. If this then reacts with an electrophile, the aromaticity is broken and this transition state is high in energy, but stabilised by resonance around the ring. A proton is then removed and the aromaticity of the benzene is regained and the energy is low once again.
In real terms people add catalysts to these reactions to lower the activation energy barrier and also look at using different reaction conditions like solvent and temperature to help the reaction to proceed. I hope that helps!