Le Chatlier's Principle
When a reaction reaches equilibrium, it will stay that way until something disturbs it. When a reaction is disturbed, the system will try to minimize whatever change was made to restore equilibrium. The concept that reactions can adjust when perturbed is called Le Chatlier's principle, and it is a valuable theory that allows us to manipulate chemical equations in a desirable way.
There are three different ways we can disturb a reaction: change the concentration of the species, change the volume, or change the temperature. Of these changes, the first two do not affect the equilibrium constant, and so we can predict their effects quantitatively; the third change does, so we cannot predict what the new concentrations will be. Note that only a volume reduction will affect an equation; adding an additional non-reactive substance, such as a noble gas, to increase the pressure will not cause a shift.
A very common demonstration of Le Chatlier's principle uses the reaction
2 NO2 (g) N2O4 (g)
ΔH = -57.2 kJ Kc = 170 at 298 K; Kc = 1300 at 273 K
because NO2 is a brown gas, while N2O4 is colorless. Therefore, this reaction gives easily visible results when disturbed.
Of the three changes listed above, we can adjust each two ways--increase and decrease:
- Remove product / add reactant: Removing a product or adding a reactant will cause the reaction to generate more product, adding more product to replace that lost or using up the excess reaction, so the reaction will shift to the right. In the example reaction, NO2 will be consumed and N2O4 generated, meaning the color of the mixture will lighten.
- Remove reactant / add product: Removing a reactant or adding a product will shift the reaction to the left as products are converted into reactants. In the reaction above, N2O4 will be decomposed into NO2, causing the mixture to darken.
- Increase volume: Increasing volume will cause the reaction to want to expand. Since two units of NO2 take up more volume than one unit of 2O4, the reaction will shift to the left, darkening the mixture.
- Decrease volume: Decreasing volume will cause the reaction to contract in volume, shifting to the right to consume two units of NO2, generating one of N2O4 and lightening the mixture.
- Increase temperature: Increasing temperature will cause the reaction to the left, because the reaction is exothermic and heat can be thought of as a product. N2O4 will be consumed, along with extra heat, to generate NO2 and darken the mixture.
- Decrease temperature: Decreasing temperature will cause this exothermic reaction to shift to the right, generating extra heat by converting NO2 into N2O4 and lightening the mixture.
You will have to evaluate the last four changes based on the characteristics of the reaction: do the products or reactants take up more volume? Is it endothermic or exothermic? By thinking of volume and heat as products or reactants, the changes caused by disturbing a reaction become much easier to visualize.
In the next page, we will revisit the concept of reaction rates in greater detail, allowing you to predict rates of reaction and analyze what factors affect these rates.