What is an Equilibrium?

An equilibrium exists in a chemical (or physical) process when the forward reaction proceeds at the same rate as the reverse reaction. That is, the reaction is not nonspontaneous (reactant favored/will not work) or spontaneous (product favored/all reactants are consumed). The concentrations (molarities) of the reactants and products are nonzero and will not change over time. This is because the forward reaction proceeds at the same rate as the reverse reaction. An example of an equilibrium is shown below. Note that the completion arrow has been replaced with a two-headed equilibrium arrow:

The Equilibrium Expression

Consider the reaction shown above. The equilibrium expression for this reaction is shown below.

There are several things to take note of in an equilibrium expression. First, each compound in the original reaction is enclosed in brackets. When a compound is enclosed in brackets, it indicates that the concentration (molarity for solutions) of the species should be used there. Only gases and aqueous species are considered for the determination of K. Additionally, the product(s) are listed in the numerator of the expression and the reactants are in the denominator. This tells us that the equilibrium expression shows a relationship between the concentration of reactants and products. If the concentration of products are far greater than the reactants then K will be very large. This is indicative of a spontaneous reaction, one that is not an equilibrium. If the concentration of the reactants are far greater than the products, then the value of K will be very small. This is indicative of a nonspontaneous reaction, also one that is not at equilibrium. When the value of K = 1, then the reaction is at equilibrium. The reactions list page shows the relative spontaniety of reactions based on K.

Le Chatlier's Principle

Le Chatlier's principle states that when an equilbrium is stressed, the reaction will respond in a manner to reverse that stress. There are many changes in condition that qualify as a stress. When an equilibrium responds to a stress, it is said to shift in the direction (reactants/left or products/right) that will restore the equilibrium condition. They are discussed in greater detail below.

Stressing an Equilibrium - Pressure

States of matter are often important when considering the impact on an equilibrium. Changes in pressure radically impact gases, as described on the page dedicated to the gaseuos state of matter. When considering how an equilibrium will shift, first consider if the change in pressure will allow for an increase or decrease in volume. Boyle's Law tells us that an increasre in pressure decreases volume, and as a result there is less spaces for gas particles to roam. This will shift an equilbrium in the direction where fewer moles of gas are present. The opposite is true with a decrease in pressure. This results in an increase in volume which allows for more space for gas particles to roam. The equilibrium will shift in the direction of the fewer moles of gas. In the event that both sides of the reaction have equal numbers of moles of gases, a change in pressure has no effect on the equilibrium.

In the first reaction above, a change in pressure will have no impact on the equilbrium. This is because there are 2 moles of molecules (1 mol nitrogen and 1 mol oxygen on the left and 2 moles of nitrogen monoxide on the right) on both sides of the reaction. In the second reaction, there are 3 moles of molecules on the left (2 mol of nitrogen monoxide and 1 mol oxygen) and only 2 moles of molecules (all nitrogen dioxide) on the right. In this reaction, pressure makes a difference. An increase in pressure (decrease in volume) will shift the equilibrium to the right because there are fewer gas molecules which can more easily accommodate the reduced space. A decrease in pressure does the opposite, as the increase in volume will require more gas particles to restore the pressure and restore the equilibrium. This shifts the reaction to the left.

Stressing an Equilibrium - Temperature

In order to determine how temperature affects an equilibrium, one needs to know which side of the reaction generates heat. Since the side that generates heat will also raise the temperature of the system, an increase in temperature will shift the equilibrium away from the heat. A decrease in temperature will favor the side with the heat, as producing heat will help counter the reduction in temperature.

Summary of Stresses

Stress Favors/Shifts
Remove Quantity of a Reactant Left/Reactants
Remove Quantity of a Product Right/Products
Add Quantity of a Reactant Right/Products
Add Quantity of a Product Left/Reactants
Increase Pressure Favors side with fewer moles of gas
Decrease Pressure Favors side with lesser moles of gas
Increase Temperature Endothermic - products; Exothermic - reactants
Decrease Temperature Endothermic - reactants; Exothermic - products
Catalyst Does not change equilibrium