 |
|
|
|
|
|
|
|
When dealing with the effect of concentration changes it is important to realize that you cannot change the concentration of a pure solid, or a pure liquid.
For example, a glass of pure water always has a concentration of 100 % pure water. Hold on! Couldn't you add something to the water, some salt for example, so that then the water is less than 100 % water? Wouldn't that change the concentration of the water? Yes, of course, but then it is no longer pure water.
| You can decrease the concentration of a pure substance by mixing it with something else, but then it is a solution, not a pure substance any longer. Only solutions have a variable concentration. |
A saturated solution is an equilibrium. Adding more of the solid to a saturated solution has no effect. Since the added solid does not change the concentration, it will not cause a change in the solution.
Changing the concentration of a gas will have an effect that can be predicted by le Châtelier's principle. However this is usually dealt with as the effect of pressure on gases, rather than through the effect of concentration.
While we cannot change the concentration of the solid phase, it is certainly possible to change the concentration of the ions in solution. To examine the effect of concentration, consider the solubility equation for dissolving copper(II) sulfate in water:
CuSO4 (s) 
Cu2+ (aq) + SO42- (aq)
Now consider what will happen if we increase the concentration of the Cu2+ (aq) in this saturated solution. Square brackets are used to symbolize molar concentrations in chemistry, so we could state this question as, "Now consider what will happen if we increase [Cu2+ (aq)]?"
Use le Châtelier's principle by applying the following three steps: 1.Identify the stress. In this case the applied stress is an increase in concentration of Cu2+ (aq) 2. Identify how the system will respond to the stress. Remember that the system will always do the opposite of the applied stress. In this case, the system will try to decrease the [Cu2+ (aq)].
3. Identify how each substance will respond to this change, using the "see-saw" effect. In this case each substance on the right of the equilibrium sign will go down, and everything on the left side will go up. The reaction will shift to the left.
|
|||||||||||||||||||||||||||||||||
Cu2+ (aq) is blue so the solution will become darker blue.
The Cu2+ (aq) will end up at a higher concentration than it used to be
(after all, we added more of it to the solution). However, it won't go up as much as
it would have if there was no  .The
[SO42- (aq)] will decrease from its original amount in the solution
by |
|
|
|
|
Exactly the opposite will happen if we decrease the [Cu2+ (aq)].
| Use le Châtelier's principle by applying the following three steps: 1.Identify the stress. In this case the applied stress is a decrease in [Cu2+ (aq)] 2. Identify how the system will respond to the stress. Remember that the system will always do the opposite of the applied stress. In this case, the system will try to increase the [Cu2+ (aq)].
3. Identify how each substance will respond to this change, using the "see-saw" effect. In this case each substance on the right of the equilibrium sign will go up, and everything on the left side will go down. The reaction will shift to the right.
|
||||||||||||||||||||||||||||||
Cu2+ (aq) is blue so the solution will become lighter blue.
The Cu2+ (aq) will end up at a lower concentration than it used to be
(after all, we removed some of it from the solution). However, it won't go down as
much as it would have if there was no  .The [SO42- (aq)] will increase from its original
amount in the solution by |
|
|
|
|
Now think about what would happen if you added some more CuSO4 (s) to the container.
| Use le Châtelier's principle by applying the following
three steps: 1.Identify the stress. In this case there is no applied stress since [CuSO4 (s)] is fixed 2. Identify how the system will respond to the stress. Remember that the system will always do the opposite of the applied stress. In this case, the system will do nothing.
3. Identify how each substance will respond to this change, using the "see-saw" effect. In this case the reaction will do nothing.
|
||||||||||||||||||||||||||||||
Now think about this. Since adding solid CuSO4 has no effect on the reaction, how could we change the concentration of Cu2+ (aq) ions to demonstrate its effect, as described in the first two examples above? The answer is to add a different source. For example instead of adding CuSO4 (s), what about adding some Cu(NO3)2 (s)? This will work, since it will have its own separate solubility reaction.
| Cu(NO3)2 (s) | |
Cu2+ (aq) | + 2NO3- (aq) |
| CuSO4 (s) | |
Cu2+ (aq) | + SO42- (aq) |
There is an ion that is the same, or "common" between these two substances. Adding an ion from another source like this is called the common ion effect.
Check your
understanding of le Châtelier's principle with these questions.
Try these three experiments that demonstrate the effect of concentration and le Châtelier's principle:
Water softening is a good technological application of
concentration and le Châtelier's principle.
The formation of crystals illustrates le Châtelier's principle
and solubility.
Increase
Decrease
