Chemistry: Disturbing Equilibria: Le Châtelier's Principle

Disturbing Equilibria: Le Chtelier's Principle

Back in the late 1800s, the French chemist Henri Le Chtelier came up with a rule about equilibria that we still use extensively today, to the chagrin of many chemistry students.

Le Chtelier's Principle states that if you change the conditions of an equilibrium, the equilibrium will shift in a way that minimizes the effects of whatever it is you did.

In other words, equilibria are like obnoxious little kids. For example, if you yell at a little kid, the kid will change his behavior to minimize your yelling. Likewise, if you change the conditions of an equilibrium, it will change in a way that partially undoes whatever it is you did to it in the first place.

Change in Concentration

Let's say that you're doing a reaction with the equation A + B ⇔ C. Le Chtelier's principle says that if we change the concentration, the position of the equilibrium will also change.

For example, if the process explained earlier is at equilibrium, we can disturb the equilibrium by adding a bunch of compound A to the reaction. To minimize the effects of the added compound, the equilibrium will shift in a way that will decrease the amount of compound A—namely, it will produce more of compound C. Likewise, by adding more of compound C, the equilibrium will be pushed toward the left, making more of A and B.

This phenomenon is often seen when an ionic compound dissolves. Let's see an example:

• CaSO_4(s) ⇔ Ca⁺²_(aq) + SO₄^-2_(aq)

The K_sp value for calcium sulfate is 2.4 × 10^-5. Doing the math we learned earlier in this section, we can easily find the equilibrium concentration of the calcium ion:

• K_sp = 2.4 × 10^-5 = [x][x]
• x = 4.9 × 10^-3 M

However, what would happen if we added 1.0 M Li₂SO₄? Because the concentration of the sulfate ion would be increased by 1.0 M, the quantity of the calcium ion would now be:

• K_sp = 2.4 × 10^-5 = [x M][x + 1.0 M]

where the concentration of the sulfate ion would be (x + 0.10 M) to compensate for the added sulfate ion. Because the quantity of sulfate ion likely to dissolve is very small compared with the quantity we've added, we'll eliminate the "x" from this term to give us:

• 2.4 × 10^-5 = (x M)(1.0 M)
• x = 2.4 × 10^-5 M.

As you can see, the quantity of calcium ion has been greatly reduced by the addition of 1.0 M sulfate ion.

The reduced solubility of one compound caused by adding an additional quantity of one of its ions to solution is referred to as the common ion effect.

Change in Pressure

Gaseous equilibria can be changed by altering the pressure of the gases. For example, let's say that we're doing the reaction A_(g) + B_(g) ⇔ C_(g). In this reaction, two moles of gas are combining to make one mole of gas.

If we increase the pressure of this mixture of gases by squishing it into a smaller area, the pressure of each of the gases will increase. Le Chtelier's principle states that equilibria tend to want to decrease the effects of any changes, so the equilibrium will shift in a way that reduces the pressure of the system. The only way to accomplish this is to have fewer moles of gas present; as a result, the reaction will shift toward products to reduce the overall pressure.

Change in Temperature

Some reactions naturally give off energy (they're exothermic) and some reactions require energy to take place (they're endothermic). If we think of the energy in an exothermic reaction as being a product, it will have the form:

• A ⇔ B + energy.

Because endothermic reactions require energy to take place, we can think of energy as a reagent:

• A + energy ⇔ B

When we increase the temperature of a chemical reaction, what we're really doing is adding energy to it. Because we can think of energy as being either a reactant or product, the addition of extra energy will disrupt the equilibrium. For exothermic reactions, the addition of energy will push the reaction to the left toward reactants. For endothermic reactions, the addition of energy will push the reaction to the right toward the products.