## Info

/I = 1 [0.001995 (22) + 0.0004938 (22) + 0.002( 1 ) + 0.0025( 1 ) + 0.0022(22)] = 0.023 Ans

Example 11.4 In Example 11.3, calculate the activity coefficient and the activity in mg/L of the bicarbonate ion.

Solution:

Y = 10 i+u4(^) _ io 1. {sp } _ y[sp ] _ 0.86 (0.0025) _ 0.00215 mg/L Ans

11.1.2 Equilibrium Constant as a Function of Temperature

The equilibrium constants given previously were at 25°C. To find the values of the equilibrium constants at other temperatures, the Van't Hoff equation is needed. According to this equation, the equilibrium constant K (Ksp for the solubility product constants) is related to temperature according to a derivative as follows:

dlnK _ AH0 dT RT2

T is the absolute temperature; AH0 is the standard enthalpy change, where the standard enthalpy change has been adopted as the change at 25°C at one atmosphere of pressure; and R is the universal gas constant.

The value of R depends upon the unit used for the other variables. Table 11.1 gives its various values and units, along with the units used for AH0 and T. By convention, the concentration units used in the calculation of K are in gmmols/L.

Enthalpy is heat released or absorbed in a chemical reaction at constant pressure. Table 11.2 shows values of interest in water stabilization. It is normally reported as enthalpy changes. There is no such thing as An absolute value of an enthalpy does not exist, only a change in enthalpy. Enthalpy is a heat exchange at constant pressure, so enthalpy changes are measured by allowing heat to transfer at constant pressure; the amount of heat measured during the process is the enthalpy change. Also, the table indicates enthalpy of formation. This means that the values in the table are the heat

TABLE 11.1 Values and Units of R

R Value