Many chemical applications of free energy are concerned with the approach to equilibrium. Applications often involve calculating the concentrations of products and reactants once equilibrium has been reached. Chemical reactions often run to completion unless the kinetics are so slow that the reaction does not have time to reach equilibrium. In living systems, cells and living organisms, biochemical reactions do not reach equilibrium. If they did the cell would die! The essence of biochemistry is that reactions are running at steady state. As products of one reaction are being produced, the next reaction in a series uses those products as reactants for the next transformation. In this section we will treat this type of situation as well as other types of applications in which equlibrium is not achieved. We can understand this type of situation by considering the equation:
ΔG = ΔGo + RT ln Q
When ΔG = 0 the process has reached equlibrium. However, the process can be at steady state with concentrations of reactants and products determined by the flow of reactants and products in a series of biochemical reactions. In this case ΔG is not equal to zero. In a living cell, the concentrations of reactants and products can force a reaction to proceed in the direction of products even when ΔGo > 0.
Protein folding and DNA hybridization are two examples of biological processes that do reach equilibrium, but where the unfolding or melting process occurs at readily accessible temperature. For example, proteins in a living cell have a temperature range of stability that is typically only within a few degrees of the folding temperature. In humans and warm-blodded animals proteins are normally stable just below body temperature. For example, body temperarture is approximately 310 K for humans. Many proteins begin to unfold at 315 K. In fact, this is why a fever can be harmful! Only a few degrees of increase in body temperature can have dramatic consequences. By the same reasoning, the fever can cause problems with the functioning of cells of invading organisms by the increase in temperature. The protein folding problem is a fascinating example of thermodynamics applied to the stability of a complex biological system.
DNA hybridization is an important issue both for understanding molecular biology and genetics, but also for biotechnology. Thermodynamics has been applied to the determination of DNA stability in order to predict the melting temperature of DNA oligonucleotides. This is of crucial important for the design of cloning and mutagenesis strategies by Polymerase Chain Reaction (PCR).
In Materials Science, non-equilibrium situations are common when one considers supersaturated solutions or polymer chemistry, to name two examples. In a supersaturated solution the concentration of solute is higher than can be supported at equilibrium. Usually, the solute remains in solution because of kinetics. The precipitation process may be slow or may require seeding. It is quite impressive to see a crystal grow within seconds when a seed crystal is added to a supersaturated solution of an inorganic salt. Supersaturated solutions are more common than you might think. The ocean is a supersaturated solution of calcium carbonate. This fact has important real world consequences since the formation of carbonate in the ocean is modulated by the uptake of CO2 from the atmosphere. We will discuss this in another part of the course where we consider the chemistry of atmospheric green house gases and their fate in the environment.
