Le Chatelier's principle

So, as we know, the state of equilibrium is a state of chemical balance, where two opposite-direction reactions occur at equal rates. However, what happens if we add a stress to an equilibrium system. A stress is another way of saying " a change in conditions". Many conditions can change, like the concentrations of species, the temperature of the reaction system, the pressure of the system, and more, a handful of which will be covered here.

The principle that lets us analyze how a system at equilibrium will try to restore equilibrium when it's conditions change is known as Le Chatelier's Principle.

Remember that Le Chatelier's principle states that stressing a system at equilibrium will cause the system to do what it can to restore that equilibrium. Let's take the reaction below:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

The equilibrium constant for this is:

If I increase the concentration of ammonia, then the concentrations of hydrogen gas and nitrogen gas will also increase. Why so? Remember that we stressed the system's equilibrium by increasing the concentration of a product. This means that now Q>K. For the system to return to K, the concentration of the products at equilibrium will increase but so will the concentration of reactants to make the equilibrium constant the same.

In general, here are the rules of changing concentrations if you don't want to do it qualitatively with the equilibrium constant.

If you increase the concentrations of products, then the system will respond by increasing the concentration of reactants at equilibrium. If you decrease the concentrations of products, then the system will respond by also decreasing the concentration of reactants at equilibrium.

If you increase the concentrations of reactants, then the system will respond by increasing the concentration of products at equilibrium. If you decrease the concentrations of reactants, then the system will respond by also decreasing the concentration of products at equilibrium.

The equilibrium concentrations of species in a system are by no means dependent on the total pressure of the system. However, if one changes the volume of the system or adds an inert gas(a gas that doesn't react with anything), the partial pressures of the species in the system will change, causing the equilibrium to shift.

If you increase the volume of the system, the partial pressures of the involved species will decrease as the species will more dispersed throughout the system. What does this mean? This means that the equilibrium will shift to the side with more moles of gas. This is because if the volume of the system increases, the system will want to favor the reaction which produces more gas because more gas will take up more volume. In contrast, if you decrease the volume of the reaction system, the equilibrium will shift to the side with less moles of gas, because the system at a lower volume will favor the arrangement that allows for less moles of gas taking up space.

If you add an inert gas to the system, the total pressure won't change but since the inert gas has partial pressure, the partial pressures of the species that are actually in the reaction decrease. This means the equilibrium shifts to the side with more moles of gas. Note that this only happens if adding an inert gas changes the volume of the system.

Analyzing the effect that changing the temperature of a system depends on the change in enthalpy(ΔH). To simplify the concept of Le Chatelier's principle with respect to temperature, let's pretend the heat that either goes into or out of the system is part of the reaction. Obviously, it isn't but it lets us intuitively understand what we're analyzing. Let's use our good old equation from above. We know this reaction to be exothermic, meaning it produces heat.

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) + heat

Using this, we get a slightly better idea at what we're looking at. Now, Le Chatelier's Principle states that increases the temperature of the system will shift the equilibrium away from the heat. In this specific example, heat is a product so increasing the temperature of the system will shift the reaction left. If the reaction were endothermic(heat was a reactant and was being put into the system), the system's equilibrium will shift right.

So, here are the rules:

ΔH° is positive(endothermic, heat is a reactant)

Increasing the temperature shifts the equilibrium towards the products

Decreasing the temperature shifts the equilibrium towards the reactants

ΔH° is negative(exothermic, heat is a product)

Increasing the temperature shifts the equilibrium towards the reactants

Decreasing the temperature shifts the equilibrium towards the products

Note that for changes in concentration and pressure, the equilibrium constant stays the same.

However, changing the temperature of a system will change the numerical value of the equilibrium constant. Using the same rules above, the side that the equilibrium shifts to relates to how the equilibrium constant changes. If the change shifts the side to the reactants, the equilibrium constant decreases in value and if the change shifts to the side with products, the equilibrium constant increases in value.