Reaction Quotient

Learning Goals

Now you will learn about the reaction quotient and how to compare its numerical value with that of the equilibrium constant so that you can predict if the reaction is at equilibrium, shifting to the right, or shifting to the left.

Synopsis

Remember that the equilibrium constant for the reaction aA + bB cC + dD is written in terms of the equilibrium concentrations of the reactants and products:

K_c = {[C]^c[D]^d / [A]^a[B]^b}_eq

The reaction quotient is written in exactly the same way, but the concentrations are not necessarily the equilibrium values. The concentrations of reactants and products are the measured values at some point in the progress of the reaction:

Q_c = [C]^c[D]^d / [A]^a[B]^b

So we will measure the concentrations of reactants and products and then we can calculate Q_c. If Q_c is equal to Kc, then obviously the reaction is at equilibrium. If Q_c is greater than K_c, then the numerator is larger than it will be at equilibrium, which means that the reaction is in the process of shifting toward the reactants. If Q_c is less than K_c then that means that the numerator is less than it will be at equilibrium so the reaction is in the process of shifting to the products.

Review Questions

The equilibrium constant for the reaction 2 SO₂ + O₂ --> 2 SO₃ is 2.8 x 10² at 1000 K. We measure 0.565 mol of SO₂, 0.187 mol of O₂, and 0.633 mol of SO₃ into a 2.5 liter container at 1000 K. Will the corresponding reaction be at equilibrium, shifting toward the products, or shifting toward the reactants?
Work problem 18 at the end of this chapter.