Biochemistry, 4th Edition P10. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | What Are the Basic Concepts of Thermodynamics 53 Free Energy Provides a Simple Criterion for Equilibrium An important question for chemists and particularly for biochemists is Will the reaction proceed in the direction written J. Willard Gibbs one of the founders of thermodynamics realized that the answer to this question lay in a comparison of the enthalpy change and the entropy change for a reaction at a given temperature. The Gibbs free energy G is defined as G H - TS For any process A- B at constant pressure and temperature the free energy change is given by AG AH - TAS If AG is equal to 0 the process is at equilibrium and there is no net flow either in the forward or reverse direction. When AG 0 AS AH Tand the enthalpic and en-tropic changes are exactly balanced. Any process with a nonzero A G proceeds spontaneously to a final state of lower free energy. If AG is negative the process proceeds spontaneously in the direction written. If AG is positive the reaction or process proceeds spontaneously in the reverse direction. The sign and value of AG do not allow us to determine how fast the process will go. If the process has a negative A G it is said to be exergonic whereas processes with positive AG values are endergonic. The Standard-State Free Energy Change The free energy change A G for any reaction depends upon the nature of the reactants and products but it is also affected by the conditions of the reaction including temperature pressure pH and the concentrations of the reactants and products. As explained earlier it is useful to define a standard state for such processes. If the free energy change for a reaction is sensitive to solution conditions what is the particular significance of the standardstate free energy change To answer this question consider a reaction between two reactants A and B to produce the products C and D. A B 4 C D The free energy change for non-standard-state concentrations is given by C D AG AG RTln rAlrp1 A
