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Let we have a reaction that starts from some reactants to produce some products. When the activation energy is provided to the reactants they will be atomized and produce a gas of individual atoms. The gas contain the whole atoms that can either form again the reactants, or form the products, or instead form the elemental molecules of themselves from which the reactants or products can be produced in other reactions. Now that the gas of all individual atoms is common for all these different reactions my question is what conditions decide which reaction to happen?

Can I say the reaction happens in the direction at which the Gibbs free enthalpy decreases more rapidly? Or do thermodynamic conditions like temperature, pressure and especially concentration of the species also play a role, independent of the variation in the Gibbs function?

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The rate for a reaction is dependent upon the rate constant and, depending upon what type of reaction it is, the concentrations of the reactants. The rate constant can be further broken down into an entropic factor (how likely are the reactants to collide in an orientation that will produce a reaction) and an energy factor (the activation energy).

Assuming for the moment that the reactant concentrations are such that they will not control the direction of reaction, and that the entropic factors don't differ dramatically for one pathway over another (that's usually the case for similar reactions), then the first reaction to happen will be the one involving the reaction with the lowest activation energy. This will lead to the kinetically controlled product.

If the back (reverse) reaction from the kinetically controlled product also has a low enough activation energy such that it happens rapidly under our reaction conditions, then essentially we are setting up an equilibrium. In this case, thermodynamics will eventually control the product distribution and we will eventually wind up with the product that has the lowest free energy, the thermodynamically controlled product.

Many reactions first produce the kinetically controlled product, but with time, the thermodynamic product becomes the major product.

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  • $\begingroup$ thank you @ron, so you mean not the slope of change in the Gibbs free energy but the magnitude of the activation energy is the prime cause that determines the direction at the beginning, but if we wait for a long time eventually it is the magnitude of the Gibbs free energy that determines which product to be the major products? That is, the slope of change in the Gibbs energy is not really very important, neither in the rate of reaction nor in the final result achieved, right? $\endgroup$
    – topology
    Sep 6 '14 at 7:04
  • $\begingroup$ @topology Yes, that is correct. $\endgroup$
    – ron
    Sep 6 '14 at 13:17

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