Scientific Tutor

How do you relate moles and concentration to Le Chatelier’s Principle?

Lets start off talking about moles (concentration) since it is a little easier to think about. Let us use my favorite chemical equation as an example and explain how we analyze it with Le Chatelier’s Principle.

200kJ + N_2(g) + 3 H_2(g) <—-> 2 NH_3(g)

If we talk about moles or concentration first there are a few ways we can phrase a question. We can increase the concentration of one chemical in the equation or decrease the concentration of one chemical in the equation. Lets start out with an increase the concentration of one. If we increase the concentration of N₂ then the options are we can either shift toward the products (right) or we can shift toward the reactants (left). In this case if we add N₂ then we shift toward the products (right). So if we add or increase the concentration on one side then the equilibrium will cause a shift away from that side to compensate for the new materials or chemicals coming in. This also means that the concentration of NH₃ will increase in our example, and the concentrations of N₂ and H₂ will decrease after the N₂ is added. I will give another example explanation. If we increase the concentration of NH₃ then the equilibrium will cause a shift toward the reactants (left). After to the NH₃ is added there will be a decrease in the NH₃ concentration and an increase in the N₂ and H₂ concentration.

Now lets try decreasing the concentration of something. If I decrease the concentration of H₂ then the equilibrium will cause a shift toward the reactants (left). So if you take away from the concentration of one side then the equilibrium causes a shift toward that side. Another way to say it is the equilibrium causes a replacement of what you take away. After the H₂ is removed the shift causes the NH₃ to decrease in concentration while the H₂ and N₂ increase in concentration. I tend to think of this example as a person holding a party and they put chips out in a bowl for everyone to take. As people take the chips (as the concentration decreases) you shift the chips from the bag to the bowl so that you can refill it. One more example in a chemical equations to try and drive the concept home. If you decrease the concentration of NH₃ then the equilibrium will cause a shift toward the products (right). After remove the NH₃ it will cause the N₂ and H₂ to decrease in concentration and the NH₃ to increase in concentration. Unfortunately, Le Chatelier’s Principle takes a long time to explain and is fairly complicated in text. For this reason I made videos on the explanation. In the videos it also explains an alternative way to think about concentration changes.

VIDEO explaining concentration for Le Chatelier’s Principle.

Remember the state of the molecules still affect Le Chatelier’s Principle. If a molecule is a solid or a liquid then it has no effect (no shift) in Le Chatelier’s Principle.

Examples: A) Which way will the equilibrium shift when we increase the concentration of CO₂?

B) Which way will the equilibrium shift when we decrease the concentration of CO₂?

2 C₄H_10(l) + 13 O_2(g) <—-> 8 CO_2(g) + 10 H₂O_(g)

A) Shift Left (toward reactants) B) Shift Right (toward products)

4 CO_2(g) <—-> C_4(s) + 4 O_2(g)

A) Shift Right (towards products) B) Shift Left (towards reactants)

Example: Which way will the equilibrium shift when we take away (decrease the concentration) of C₄?

6 C_4(s) + 12 O_2(g) + 24 H_2(g) <—-> 4 C₆H₁₂O_6(s)

Answer: No shift / no change (because C₄ is a solid)

PRACTICE PROBLEMS: Which way will the equilibrium shift when we change the concentration of the molecule mentioned above the equation?

Increase NaI

MgBr_2(aq) + 2 NaI_(aq) <——> MgI_2(aq) + 2 NaBr_(aq)

Answer: Shift Right

Decrease Ag₃PO₄

6 Ag⁺_(s) + Ca₃(PO₄)_2(s) <—-> 3 Ca²⁺_(aq) + 2 Ag₃PO_4(s)

Answer: No Shift (because Ag₃PO₄ is a solid)

Decrease O₂

H₂SO_4(aq) <—–> H_2(aq) + S_(s) + 2 O_2(aq)

Answer: Shift Right

Increase CO₂

C_4(s) + 4 O_2(g) <—-> 4 CO_2(g)

Answer: Shift Left