Equilibrium constant spontaneous change, prediction

Thermodynamics is used to predict whether reactants have a spontaneous tendency to change into products. This tendency is associated with a decrease in the free energy or Gibbs energy of the system (G) to a minimum. As a consequence, the thermodynamic criterion for spontaneous change at constant temperature and pressure is AG < 0. Under standard conditions (concentrations = 1 M, and P = 1 atm), the standard Gibbs energy variation (AG°) is related with the equilibrium constant (A) by equation 11 ... 

Given sufficient time, chemical substances in contact with each other tend to come to chemical equilibrium. Chemical equilibrium is the time-invariant, most stable state of a closed system (the. state of minimum Gibbs free energy). We study chemical equilibrium concepts so as to learn the direction of spontaneous change of chemical reactions in any system, especially for conditions of constant temperature and pressure. We want to be able to compute the hypothetical equilibrium stale of a system. We would like to predict the conditions for equilibrium in different systems and at different temperatures and pressures without having to measure them. 

In considering thermodynamic parameters, e.g., heat and work, we do not need to know the exact chemical pathway taken by the reactants in conversion to products. Using thermodynamics, we can obtain information about reactions that cannot be studied directly in living systems. Thermodynamics predicts, on the basis of the known energy levels of reactants and products, whether a reaction can be expected to occur spontaneously or how much energy must be supplied to drive the reaction in one direction or another. Such information is crucial in establishing reaction routes in metabolic pathways. Thermodynamics explains how equilibrium constants are related to changes in temperature. Thermodynamics also explains the basis for enzyme catalysis. 

Reaction quotient (Q) (12.3) Expression identical in form to the equilibrium constant, but in which the concentrations do not correspond to equilibrium values. Comparison of the reaction quotient to the equihbrium constant predicts the direction of spontaneous change. 

Two ways of predicting reaction direction are from the value of AG and from the relation of Q to K. These variables represent different aspects of the same phenomenon and are related to each other by AG = RT In Q/K. When Q = K, the system can release no more free energy. Beginning with Q at the standard state, the free energy change is AG°, and it is related to the equilibrium constant by AG° = -RT In K. For nonstandard conditions, AG has two components AG° and RT In O. Any nonequilibrium mixture of reactants and products moves spontaneously (AG < 0) toward the equilibrium mixture. A product-favored reaction has K > 1 and, thus, AG° < 0. 

A chemical reaction proceeding toward equilibrium is a spontaneous change. Recall that we can predict the net direction of the reaction—its spontaneous direction— by comparing the reaction quotient (0 with the equilibrium constant (K). But why is there a drive to attain equilibrium And what determines the value of the equilibrium constant And, most importantly, can we predict the direction of a spontaneous change in cases that are not as obvious as burning gasoline or falling books ... 

Equation 18.11 enables us to predict the spontaneity of a process using the change in enthalpy, the change in entropy, and the ab.solute temperature. At constant temperature and pressure, for processes that are spontaneous as written (in the forward direction), AG is negative. For processes that are not spontaneous as written but that are spontaneous in the reverse direction, AG is positive. For systems at equilibrium, AG is zero. 

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