fa01lec06 - Review of Chemical Thermodynamics 7.51...

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© RT Sauer 1999 1 Review of Chemical Thermodynamics 7.51 September 1999 If you haven’t had thermodynamics before, you’ll probably need to do some background reading. Possibilities include: Moore, W.J. (1972) Physical Chemistry , 4th edition, Prentice-Hall, Inc.; Eisenberg, DS & Crothers, DM (1979) Physical Chemistry with Applications to the Life Sciences , Addison-Wesley Publishing Co.; Tinoco, I, Sauer, K., & Wang, JC (1994) Physical Chemistry: Principles and Applications in Biological Sciences , 3rd Edition, Prentice-Hall, Inc., van Holde, KE (1985) Physical Biochemistry , Prentice-Hall, Inc. Thermodynamics allows us to predict how chemical reactions will change as a function of temperature and how changes in the structure of molecules might affect the equilibrium properties of a population of these molecules. There are four basic thermodynamic properties: G — Change in free energy between reactants and products; this measures the ability of the system to do work. Reactions with negative G’s proceed spontaneously and can be used to do work. Reactions with positive G’s require an input of energy for the reaction to proceed. H — Change in enthalpy between reactants and products; this is the heat given off or absorbed by a reaction at constant pressure. Reactions that absorb heat have positive H’s and those that produce heat have negative H’s. S — Change in entropy between reactants and products; entropy is a statistical measure of the number of states or accessible conformations. A positive S is an indication that the disorder or number of accessible of the system is increasing and vice versa. C p — Change in heat capacity between reactants and products; when a solution of molecules is heated, some of the thermal energy increases the kinetic energy of molecules, increasing the temperature, whereas some of the energy results in faster vibrations or rotation of the molecule. Heat capacity measures how much energy can be stored by a molecule in these internal vibrations or rotations. G provides a basic accounting function for chemical reactions. The free energy change for a reaction can be calculated from the equilibrium constant for that reaction using the equation shown below.
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© RT Sauer 1999 2 G ° = -RT ln K standard Gibb's free energy gas constant absolute temperature equilibrium constant G ° is a function of the equilibrium constant for the reaction, the gas constant R (1.98•10 -3 kcal/mol-deg), and the absolute temperature (in ° K). Remember that T( ° K) = T( ° C) + 273. Some politically correct biochemists use kJ/mol rather than kcal/mol. The conversion is relatively painless as 1 kcal/mol 4.2 kJ/mol and R = 8.3 •10 -3 kJ/mol-deg in these units. G ° is called the standard Gibb’s free energy, where the naught specifies a standard set of reaction conditions that include constant pressure (almost always 1 atm for biochemical reactions), a given temperature , and a set of standard-state concentrations . The temperature used in calculating G ° is that for which K eq for the reaction was measured.
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