The Equilibrium Mass Action Expression

We refer to this ratio as the reaction quotient, Q. Product concentrations appear in the numerator of the expression, and reactants appear in the denominator. Each concentration is raised to a power corresponding to the stoichiometric coefficient of that species in the balanced chemical equation for the equilihrium. At equilih-rium, however, this ratio becomes the equilibrium expression, and the corresponding value of Q is called the equilibrium constant, K. 

To determine the value of K for a particular reaction, we must measure the concentrations of the reactants and products at equilibrium. Note that, unlike rate expressions in chemical kinetics, the expression for the equilibrium constant is ahvays based directly on the reaction stoichiometry. The following Example Problem shows how to write an equilibrium expression. 

For the BMA process, the equilibrium below would be established if the products were not removed from the reaction vessel  

We use the definition of the equilibrium expression. Write the products in the numerator and the reactants in the denominator. The exponent for each concentration term is given by the corresponding stoichiometric coefficient fi-om the chemical equation. 

The only way we can assess an answer like this is to make sure that we used the definition of the equilibrium constant correctly. We have the products (HCN and H2) in the numerator and the reactants (NH3 and CH4) in the denominator, which is correct, and we have exponents to match the stoichiometric coefficients. So our answer is correct. 

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The Equilibrium (Mass Action) Expression Gas Phase Equilibria Kp vs. Kp Homogeneous and Heterogeneous Equilibria Numerical Importance of the Equilibrium Expression Mathematical Manipulation of Equilibrium Constants Reversing the Chemical Equation Adjusting the Stoichiometry of the Chemical Reaction Equilibrium Constants for a Series of Reactions Units and the Equilibrium Constant... 

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