Equilibrium Constants in Terms of Pressure, Kp

When the reactants and products in a chemical reaction are gases, we can formulate the equilibrium-constant expression in terms of partial pressures. When partial pressures in atmospheres are used in the expression, we denote the equilibrium constant Kp (where the subscript p stands for pressure). For the general reaction in Equation 15.7, we have 

A FIGURE 15.5 The same equilibrium mixture is produced regardless of the initial NO2 concentration. The 

What is the difference between the equilibrium constant K,. aixl the equilibrium constant Kp  

For a given reaction, the numerical value of fC . is generally different from the numerical value of Kp. We must therefore take care to indicate, via subscript c or p, which constant we are using. It is possible, however, to calculate one from the other using the ideal-gas equation = (Section 10.4) 

The usual units for n/V are mol/L, which equals molarity, M. For substance A in our generic reaction, we therefore see that 

When we substitute Equation 15.13 and like expressions for the other gaseous components of the reaction into Equation 15.11, we obtain a general expression relating 

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