Le Chatelier's Principle

Learning Goals

Upon finishing this section you will be able to predict what will happen to the extent of a reaction when the reaction is perturbed by changing the temperature, changing the pressure, or changing the concentration of a reactant or product.

Synopsis

The guiding principle for these three possible perturbations is Le Chatelier's principle which states that "a change in any of the factors that determine the equilibrium conditions of a system will cause the system to change in such a manner as to reduce or counteract the effect of the change." Remember that K is only a function of temperature, so of these three changes only a temperature change will change K. So let's look at the possible ways in which the extent of the reaction can be shifted.

Suppose that we add some reactant. Some of the added reactant will be consumed, which will shift the reaction toward the product side
Suppose that we add some product. Some of the added product will be consumed, which will shift the reaction toward the reactant side.
Suppose that we increase the pressure by decreasing the volume. The pressure will have to decrease to return to equilibrium so the reaction will shift to the side that has the fewer gas molecules.
Suppose that we decrease the pressure by increasing the volume. The pressure will have to increase to return to equilibrium so the reaction will shift to the side that has the most gas molecules.
Suppose that we increase the temperature. To return to equilibrium, the reaction must consume some heat energy so the reaction will shift in the endothermic direction.
Suppose that we decrease the temperature. To return to equilibrium, the reaction must generate some heat energy so the reaction will shift in the exothermic direction.

We can now use these conclusions to predict the direction of the shift in equilibrium of a reaction caused by any of these changes.

Review Questions

For the reaction H₂(g) + I₂(g) --> 2 HI(g), the H_298.15 of the forward reaction is 26.36 kJ/mol. For the following changes, predict the direction of the equilibrium shift:
- Adding some hydrogen gas.
- Adding some HI gas
- Increasing the pressure
- Increasing the temperature.
Work Exercise 16.12 on page 776.

Section 16.7 Is There Life After Equilibrium?

Learning Goals

You should understand that not all reactions have their maximum usefulness to us after they attain the equilibrium state.

Synopsis

There are many nonequilibrium reactions that are of extreme importance to us. Such reactions include the chemical reactions occurring in a car battery, the biochemical reactions in plants and animals, the production of elements in stars, etc. The topic of nonequilibrium thermodynamics is extremely important in trying to understand the life process, and the topic is very complicated. Although we cannot embark on a discussion of such topics at this level, it is important to know that nonequilibrium processes exist all around (and in) us, and that they are extremely important to the functioning of our universe.