Even though some reaction have a favorable change in Gibbs f

Even though some reaction have a favorable change in Gibbs free energy they may still proceed slowly. Why is this?

Solution

In all kinds of reactions, energy passess from one system to another (thermodynamic changes). According to second law of thermodynamics, a reaction would occur spontaneously only if it increases the entropy of the universe. Gibbs free energy (G) refers to the amount of usable energy of the system. Change in Gibbs free energy suggests reaction\'s energetics and spontaneity. (G=Gfinal–Ginitial). This change tells us the maximum usable energy which is released or absorbed when the inital products are changed to final products of the reaction. Gibbs free energy being positive or negative suggests whether the reacyion has occurred spontaneouly or not as spontaneous reaction favors the formation of products under given reaction conditions.

However, this formula works on assumptions like constant temperature and pressure during the reaction. These ideal conditions do not work for reactions occurring in living systems. Hence free energy is calculated as G=HTS, wherein H is the enthalpy change (measurement of bond energies, whether heat is released or absorbed), S is the entropy of the system (system becomes more disordered or ordered after the reaction) and T is the temperature of the reaction.

In living systems, the pace of a reaction depends on presence of variety of cofactors, complex ions in the cell, concentration of reactants, presence of inhibitors and also the varying temperature and pressure conditions which are difficult to predict! Also, the rate of a reaction depends on the path of the reactions between starting and final products. Some reactions may also have a mix of favorable and unfavorable conditions. More of unfavourable conditions/factors may slow down the reaction.