Predicting the Direction of a Reaction

We have already used equilibrium expressions to determine the value of an equilibrium constant using equilibrium concaitrations. In this section, we will learn how to use equilibrium expressions to predict the direction of a reaction and to calculate equilibrium concentrations. 

Oc is calculated using the initial concentrations of reactants and products. Similarly, Op can be calculated using the initial partial pressures of reactants and products. 

Remember that calculating Oc is just like calculating K products over reactants, each raised to the appropriate power—except that the concentrations we use to calculate Oc are the starting concentrations. To calculate we must use equilibrium concentrations. 

If we start an experiment with only reactants, we know that the reactant concentrations will decrease and the product concentrations will increase that is, the reaction must proceed in the forward direction in oidw for equilibrium to be established. Likewise, if we start an experiment with only products, we know that the product concentrations will decrease and the reactant concentrations will increase. In this case, the reaction must proceed in the reverse direction to achieve equilibrium. But often we must predict the direction in which a reaction will proceed when we start with a mixture of reactants and products. For this situation, we calculate the value of the reaction quotient, and compare it to the value of the equilibrium constant, Kc- 

The equilibrium constant, for the gaseous formation of hydrogen iodide from molecular hydrogen and molecular iodine. 

Student Annotation The comparison of 0 with K can refer either to Oc and IQ or Qp and Kp. 

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Predict the direction of a reaction, given K and the concentrations of reactants and products (Example 9.5). 

The entropy of reaction by itself, however, is not sufficient to predict the direction of a reaction. At 25° C, you know that H2O (1) is the stable phase, not H20(g). Moreover, the second reaction... 

The change in free energy, AG, can be used to predict the direction of a reaction at constant temperature and pressure. Consider the reaction ... 

AG° is predictive only under standard conditions Under standard conditions, AG° can be used to predict the direction a reaction proceeds because, under these conditions, AG° is equal to AG. However, AG° cannot predict the direction of a reaction under physiologic conditions, because it is composed solely of constants (R, T, and Keq) and is, therefore, not altered by changes in product or substrate concentrations. 

A more convenient function for predicting the direction of a reaction was discovered by Josiah Gibbs. He was the first to appreciate that in reactions occurring at equilib-rium and constant temperature, the change in entropy of the system is numerically equal to the change in enthalpy di-vided by the absolute temperature. This relationship is the one already presented in equation (6). The equation can be transposed to... 

We won t derive the equation for AG under nonstandard-state conditions. It s hardly surprising, however, that AG and Q should turn out to be related because both predict the direction of a reaction. Worked Example 17.8 shows how to use this equation. 

As we have used the basic concepts of thermodynamics in a chemical reaction to predict formation of new products and to predict the direction of a reaction, the same can... 

High levels of ADP formed in the myofibrils during contraction favor the reverse reaction namely, resynthesis of ATP - at the expense of creatine phosphate cleavage to creatine. This example shows that one must consider not only the standard free energy change but also the actual concentrations of all reactants and products when predicting the direction of a reaction in vivo. 

FIGURE 15.8 Predicting the direction of a reaction by comparing 0 and K at a given temperature. 

Predicting the Direction of a Reaction Calculating Equilibrium Concentrations... 

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