Response of Selected Properties

Thermodynamics is particularly useful in reducing the amoxmt of experimental data needed for determining how properties respond to changes of state. Such changes could be illustrated using many thermodynamic properties however, we will confine the discussion here to two important classes of quantities standard-state fugacities ( 12.5.1) and yields from chemical reactions ( 12.5.2). 

1 we defined a standard state to be a well-defined state of a real or hypothetical pure substance therefore, we need only consider how the standard-state fugadty / responds to changes in temperature and pressure. General expressions for the temperature and pressure derivatives of f° can be written immediately from (4.3.13) and U-3.14) (or see Table 6.2) hence. 

The pressure effect in (12.5.1) can arise only when we use FFF 2 in FFF 3-5, f° is always evaluated at the standard-state pressure P°. Further, on integrating (12.5.1) over a change of pressure, we obtain the Poynting factor, which appears in FFF 3. So our emphasis here is on how temperature affects f°. In 10.2 we identified two common classes of standard states those based on the pure component and those based on infinitely dilute solutions. We consider those two choices here. 

Pure-component standard states. When we take the standard state to be based on a pure component, then / = /p j-e i the derivatives in (12.5.1) and (12.5.2) become 

Since v/RT 0, f° must always increase with an isothermal increase in pressure, no matter which pure-component standard state we use. 

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