Chem Lab Help: Equilibrium Systems

[Eshi]

Homework Statement

Explain what happened when the solution was added to water in terms of Le Chatelier's principle and ocument your explanation using observations. Think Carefully! write out the correct mass action expression before you answer--this is subtle!

Homework Equations

a. BiCl3(s) ↔ Bi^3+(aq) + 3Cl-(aq)
b. Bi^3+(aq) + Cl-(aq) + H2O(l) ↔ BiOCl(s) + 2H+(aq)
c. BiCl3(s) + H2O(l) ↔ BiOCl(s) + 2HCl(aq)

The Attempt at a Solution

Mass Action: 1* [HCl]^2 / 1 * 1
The 1's are because the solids and liquids are not counted in mass action expressions.

My observations were that HCl to the system got rid of the white precipitate of BiOCl making a clear solution of disolved BiCl3 in water. I observed that after adding water the precipitate reformed. My original theory was that the added concentration of H20 shifted the equation to the right because of the added concentration of H20, however, because H20 is not a part of the mass action expression, I changed my theory to be:

The adding of water causes the HCl to become dilute and less concentrated, shifted the equation to the right to counteract the change in the system.

Is this the right theory? I have heard a bunch of different ideas from my classmates, but I like my idea the best, but I'm not sure if this is the right answer.

 

[chemisttree]

Sounds good.

 

[Blueshift5]

Your explanation is on the right track, but there are a few things that can be clarified. First, let's define Le Chatelier's principle: it states that when a system at equilibrium is disturbed, the system will shift in a direction that minimizes the disturbance. In this case, the disturbance is the added water.

Using this principle, we can analyze each of the equations separately:
a. BiCl3(s) ↔ Bi^3+(aq) + 3Cl-(aq)
When water is added, the concentration of water increases, causing the system to shift to the right to counteract the increase in water. This means that more Bi^3+ and Cl- ions will be produced, leading to the formation of more BiCl3.

b. Bi^3+(aq) + Cl-(aq) + H2O(l) ↔ BiOCl(s) + 2H+(aq)
As you correctly stated, the addition of water causes the concentration of HCl to become more dilute. According to Le Chatelier's principle, the system will shift to the left to counteract this change. This means that more BiOCl will be formed, which is observed as the reformation of the white precipitate.

c. BiCl3(s) + H2O(l) ↔ BiOCl(s) + 2HCl(aq)
Similarly to equation b, the addition of water causes the concentration of HCl to become more dilute, leading to a shift to the left and the formation of more BiOCl.

In terms of the mass action expression, you are correct that solids and liquids are not included. However, it is important to note that the concentration of H2O remains constant throughout the reaction, so it does not affect the equilibrium constant. Thus, the mass action expression for each equation remains the same.

Overall, your explanation is correct, but it can be further supported by using Le Chatelier's principle to analyze each equation. Keep up the good work in your chemistry studies!