Structural Biochemistry/Thermodynamic Equilibrium

Equilibrium may be defined as lack of change, or a static condition. In thermodynamics, equilibrium not only implies the lack of change, it also refers to the absence of any tendency toward change on a macroscopic scale.

==Relationship between Equilibrium, Driving Force, and Resistance in a System==

A system of interest at equilibrium exists if and only if there is no net change in the surrounding conditions. In this case, the driving force can be viewed as negligible also since the absence of any tendency toward change also indicates the absence of any driving force. Thus, all forces are said to be in balance when equilibrium exists in a system. One thing that needs to be pointed out here is that whether a change actually occurs in a system not at equilibrium depends on two factors: the resistance as well as the driving force of the system.

Different Driving Forces Produce Different Kinds of Changes

Many kinds of changes might be encountered in a system; these different changes are caused by different driving forces. Driving forces may be classified by their physical properties, which include pressure, temperature, concentration, gradients in chemical potential, and so on.

For example, imbalance of mechanical forces, such as the pressure gradient on a piston, will tend to cause energy transfer in the form of work. Gradients in chemical potentials tend to cause substances to be transferred from one phase to another; temperature differences tend to cause the flow of heat in or out of the system. Nevertheless, the final results of the above listed tendencies are the same--equilibrium will be achieved in a system.

Equilibrium, Free Energy, and Reaction Direction

Thermodynamic equilibrium refes to a condition in which equilibrium exists with respect to P, T, and concentration. Equilibrium is established with respect to a given variable only if that variable does not change with time, and if it has the same value in all parts of the system and surroundings. The equilibrium with respect to concentration exists only if transport of all species across the boundary in both directions is possible. If the boundary is a movable wall that is not permeable to all species. The sign of free Energy (ΔG) can determine the direction of a spontaneous reaction. In addition, one can determine whether the reaction will proceed to the right or the left based on the reaction quotient (Q) and the equilibrium constant (K). As a result, Silberberg states that

If Q < K or Q/K <1 , then the reaction will head to the right

If Q > K or Q/K >1 , then the reaction will head to the left

If Q = K or Q/K =1 , then the reaction will be at equilibrium.

As a result, the equation that combines the equilibrium constant, free energy, and reaction quotient is ΔG= RT ln (Q/K)

Different Driving Forces Produce Different Kinds of Changes

Many kinds of changes might be encountered in a system; these different changes are caused by different driving forces. Driving forces may be classified by their physical properties, which include pressure, temperature, concentration, gradients in chemical potential, and so on.

For example, imbalance of mechanical forces, such as the pressure gradient on a piston, will tend to cause energy transfer in the form of work. Gradients in chemical potentials tend to cause substances to be transferred from one phase to another; temperature differences tend to cause the flow of heat in or out of the system. Nevertheless, the final results of the above listed tendencies are the same--equilibrium will be achieved in a system.

Effects of Chemical Reactions on Thermodynamic Equilibrium

In most of the applications of thermodynamics, the effects of chemical reactions are usually ignored. Since a system may stay in a long-term equilibrium if a chemical reaction is not initiated, it is convenient to not consider the effects that a chemical reaction might bring about to the system. Furthermore, equilibrium is mostly likely to be reached after a chemical reaction is carried out in most thermodynamic cases. Therefore, a purely physical process may be analyzed without regard to possible chemical reactions.

References

Smith, J.M. (2005). Introduction to Chemical Engineering Thermodynamics. McGraw Hill. ISBN 978-007-127055-7. {{cite book}}: Text "coauthors+ H.C. Van Ness, M.M. Abbott" ignored (help)

Silberberg, Martin S.(2010). Principles of General Chemistry (2nd Edition).McGraw Hill Publishing Company. ISBN978-0-07-351108-05