Chem – Le Chatelier’s Principle: Moles or Concentration

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 + N2(g) + 3 H2(g) <—-> 2 NH3(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.

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 C4H10(l) + 13 O2(g) <—-> 8 CO2(g) + 10 H2O(g)

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

4 CO2(g) <—-> C4(s) + 4 O2(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 C4(s) + 12 O2(g) + 24 H2(g) <—-> 4 C6H12O6(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

MgBr2(aq) + 2 NaI(aq) <——> MgI2(aq) + 2 NaBr(aq)

Decrease Ag3PO4

6 Ag+(s) + Ca3(PO4)2(s) <—-> 3 Ca2+(aq) + 2 Ag3PO4(s)

Answer: No Shift (because Ag3PO4 is a solid)

Decrease O2

H2SO4(aq) <—–> H2(aq) + S(s) + 2 O2(aq)  