Question

can you determine the rate of reaction with only the entropy (delta G of reaction)?

Answer 1

No, we cannot determine the rate of reaction with only the entropy change () or a change in free energy () known to us. A Chemist needs other parameters to qualitatively or quantitatively make reasonable assumptions about how fast the reaction will proceed. These parameters, on a basic level, are introduced in the collision model for the rates of reactions, According to this model, the rate of reaction (or reaction kinetics) is dependent on three main parameters, viz., collision frequency (z), activation energy (Ea), and the probability (or orientation) factor (p). These factors, in turn, are dependent on the reaction conditions, which include concentrations of reactants or products, temperature, and pressure. For example, if the reactants are in higher concentrations, there will be a greater number of collisions between them, which will also increase the number of collisions that occur with enough energy (greater than or equal to Ea) or with the correct orientation. Eventually, the rate of reaction will increase with increasing reactant concentration.

On the other hand, thermodynamic quantities like change in free energy () are only concerned with whether a reaction will occur under given conditions or not. It is not concerned with the rate or speed with which the reactants will be converted to products. is related to entropy change () and enthalpy change () through the Gibbs free energy equation whose simple representation is written below:

In the above equation, if , T, and have such values that the value of comes out to be negative or less than zero, then the process or a chemical reaction can theoretically occur spontaneously in the specified direction, i.e. from reactants to products. Contrarily, if is greater than zero, the chemical reaction will not occur in the specified direction. In fact, then the reverse reaction can theoretically occur spontaneously. These predictions will be based on the initial and final states but will not give any information about the time required for the process.

A Chemist needs information from both thermodynamics and kinetics to fully explain the chemical reaction. Thermodynamics cannot predict the rate of reaction but the reverse is not true. That is, if a chemical reaction is taking place with a rate having a positive magnitude, then it is obvious that its is less than zero or it is spontaneous under those conditions.