Consider the following reaction where Kc = 6.50×10-3 at 298 K: 2NOBr(g) 2NO(g) + Br2(g) A reaction mixture was found to contain 9.83×10-2 moles of NOBr(g), 5.44×10-2 moles of NO(g), and 4.13×10-2 moles of Br2(g), in a 1.00 liter container. Is the reaction at equilibrium? If not, what direction must it run in order to reach equilibrium? The reaction quotient, Qc

This reaction isn't yet at an equilibrium. It must shift in the direction of the reactant (namely ) in order to reach an equilibrium.

For this mixture, the reaction quotient is .

Explanation:

A reversible reaction is at equilibrium if and only if its reaction quotient is equal to the equilibrium constant .

Start by calculating the equilibrium quotient of this reaction. Given the reaction:

.

Let , , and denote the concentration of the three species. The formula for the reaction quotient of this system will be:

.

(Note, that in this formula, both and are raised to a power of two. That corresponds to the coefficients in the balanced reaction.)

Calculate the reaction quotient given the concentration of each species:

.

(Note that the unit is ignored.)

Apparently, . Since and are not equal, this reaction is not at an equilibrium. If external factors like temperature stays the same,

Keep in mind that denotes a quotient. To reduce the value of a quotient, one may:

In , that means reducing the concentration of the products while increasing the concentration of the reactants. In other words, the system needs to shift in the direction of the reactants before it could reach an equilibrium.