[siddharth]

What is the fundamental cause of chemical equilibrium. If some reaction is energetically favourable , why not proceed entirely toward completion. I have some idea of how this relates to Energy barriers and the Maxwell-Boltzmann distribution of molecular energies, but the picture I have is muddled up and any clarity on the matter would be greatly appreciated. Would entropy considerations also play a role?

[Yggdrasil]

The reason a reaction does not run 100% toward the most energetically favorable side is entropy.  A system that is 100% product, 0% reactant has a lower entropy than a system that has some mix of products and reactants.

[rajajidwivedi]

main cause of the problem is that equilibrium is the result of mutual balance of two opposite thermodyanamic properties one in entahlpy and other is entropy. each favour different reactions one forward and other backward so a situation arises where system has minimum free energy and  and tends to attain it. if any thing pressure, temperature is changed then free energy increases and so le chatelier principle is used.