Here entropy is in J/kg-mol-K. The entropy for each gaseous species is
For solid and liquid species the last two terms are zero. Here (5® ) refers to the standard state entropy at temperature T. Typical values for entropy are listed in Tables 5-1 and 5-2.
When a chemical reaction is in equilibrium, an equilibrium constant has been devised which relates the partial pressures and the molar fractions of the species. For example, in the general reaction a\ + bB^cC + dT> (5-18)
a, b, c, and d are the stoichiometric molar concentration coefficients of the chemical molecules (or atoms) A, B, C, and D. The equilibrium constant K, when expressed as partial pressures, is a function of temperature.
Here po is the reference pressure. All pressures are in bars or 105Pa. When a + b = c + d, then Kp is independent of pressure. This condition is not valid for a reaction like Eq. 5-8. In this case the pressure increase will drive the equilibrium reaction into the direction of fewer moles and in the direction of absorbing heat if the temperature is increased. For Eq. 5-8 the hydrogen and oxygen equilibrium relation would be
The equilibrium constant can also be expressed as a function of the molar fractions rij because each partial pressure p„ is equal to the actual pressure p at which the reaction occurs multiplied by its molar fraction (pj = pnj). From Equation 5-19 the equilibrium constant K can also be expressed as c d / \ c+d—a—b
The equilibrium constant for the chemical formation of a given species from its elements is Kf. Typical values of Kf are shown in Tables 5-1 and 5-2. The free energy and the equilibrium constant for the formation of a particular species at standard conditions from its atomic elements are related, namely
Equations 5-19, 5-20, and 5-22 are often used together with mass balance and energy balance relations to solve the simultaneous equations; the equilibrium constant K is primarily used when chemical compounds are formed from their elements.
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