[davidenarb]

Let's consider a general equilibrium A1 + B = C + D.  this equilibrium can also be established with a reactant A2: A2 + B = C + D

Let's say for example that the reactant A1 is more reactive than A2, so the equilibrium 1 is established more rapidly than the second.

Does this mean that the equilibrium  1 favors the products over the reactants ?

[Corribus]

It depends on what you mean by "more reactive". Kinetic and thermodynamic favorability are related, but knowing how one changes can't always tell you how the other will. For instance, if we define reactivity as relating to the magnitude of the activation energy, then we have two possibilities: (1) The ΔG of the reaction with A1 is the same as the DG of the reaction with A2, but the transition state energies are different. OR (2) the transition state energies are the same but the ΔGs are different (because the Gibbs energy of A1 is different than that of A2). Or it could be a combination of both. If the ΔGs are the same, then the equilibrium constants will be the same, and it's a purely kinetic issue. If the ΔGs are different, then the equilibrium constants will be different, and it is primarily a thermodynamic issue.

[zsinger]

See:  Gibbs free energy.
         -Zack

[phth]

And if the reaction is endergonic, the thermodynamic and kinetic products will switch places.