Elementary Kinetics

Kinetic vs. Thermodynamic Stability

Remember from thermodynamics that spontaneous, or naturally occurring, reactions always proceed toward the state of lower energy. That is, they go energetically "downhill." Knowing that, we might assume that anytime there is a reaction where the products have less potential energy than the reactants (products are more thermodynamically stable), that reaction would proceed at a fast rate, like a skier heading down the slopes.

However, in reality, some thermodynamically favorable reactions proceed at a glacially slow pace, a phenomenon that thermodynamics just can’t explain. Luckily, our knowledge of kinetics comes to the rescue.

Take a look at this reaction diagram. It shows the energy of the molecules involved in the hypothetical spontaneous reaction as the reaction proceeds over time. Notice how, as for all spontaneous reactions, the DG is negative (the Gibbs Free Energy of products is less than that of the reactants).

Notice how in converting from reactants to products, the molecules need to temporarily become much more energetic in order to break and remake chemical bonds. This energy "barrier" is referred to as the "energy of activation" (E_a). If the reactants don’t have enough kinetic energy to get over the "hump," the reaction won’t proceed, even if it’s thermodynamically favorable.

Going back to our example of the skier, if she is to get to the bottom of the ski run, she will need to have enough speed coming into that hill to get up and over the final hump to the finish line.

We remember from collision theory that the individual reactant molecules have a distribution of energy levels. Some have a low amount of kinetic energy (are moving slowly), whereas others have a high amount of kinetic energy (are moving more quickly). Only those molecules that have enough kinetic energy to make it over the E_a "hump" will react to become products.

From this example, it makes sense that reactions with a lower E_a "hump" will proceed more quickly, because more molecules are likely to have enough energy to overcome that energy barrier. Likewise, reactions with a very tall E_a "hump" will proceed very slowly, because it is unlikely that many molecules will be energetic enough to make it up and over that hill. Thus, for a reaction to be kinetically favorable, it needs to have a low energy of activation.

An important example of a reaction that is thermodynamically favorable, but kinetically unfavorable is:

With its negative DG value, this reaction for the breakdown of ATP is thermodynamically favorable. Lots of energy is released when ATP is hydrolyzed. However, ATP is kinetically stable because the hydrolysis reaction has a high energy of activation. In other words, for this reaction to happen, ATP has to overcome a tall energy barrier. These properties make ATP the perfect "energy currency" for use in our bodies. Useful energy for biochemical reactions is safely trapped in the ATP molecule until released by an enzyme that needs the energy to "pay" for an energy-requiring chemical reaction. The enzyme grabs an ATP molecule and helps it over the E_a "hump," and then channels that energy from the ATP hydrolysis to the chemical reaction that needs it.