Henri LeChatelier's Principle

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The Effect of Heat on the Position of the Equilibrium and the Value of the Keq

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In 1888, LeChatelier gave a succinct statement of the principle he had announced 4 years prior. It is:

Every change of one of the factors of an equilibrium occasions a rearrangement of the system in such a direction that the factor in question experiences a change in a sense opposite to the original change.

Before going on to a more recent restatement, let's examine some important points gleaned from LeChatelier's words.

1. For LeChatelier's Principle to work, the chemical system MUST be at equilibrium first. Remember this means that there is no change in any of the concentrations of the substances involved.

2. Some type of change is involved in a factor that affects equilibrium. There are several factors and they are discussed below. The changes will involve increasing or decreasing something which affects the equilibrium.

3. In response to this change, the old equilibrium will rearrange itself. This means changing the old equilibrium concentrations to a new set of values without changing the Keq value. However, we will find that the Keq does change in value when the change involves temperature.

4. The last point is about the word "opposite." By this we mean that if something is increased, the induced change will be for something to decrease. Of course, the reverse case is also true.

Two examples of modern definitions are:

If a stress is applied to a system at equilibrium, then the system readjusts, if possible, to reduce the stress.

If a system at equilibrium is subjected to a stress, the equilibrium will shift in attempt to reduce the stress.

The four points discussed above are incorporated into each definition, it's just that sometimes it is implied, like the idea of "opposite." Most modern defnitions are very similar to these two in their wording.

Before going into a mess of examples, here are the factors that affect a chemical reaction at equilibrium.

Changes in:

1) concentration change
a) up
b) down
2) temperature
a) up
b) down
3) pressure
a) lower the pressure by increasing the volume
b) raise the pressure by decreasing the volume
c) raise the total pressure by adding an inert (non-reacting) gas
4) adding a catalyst

Please keep in mind that the above changes would be imposed on a system already at equilibrium.


Example #1: Which way will the equilibrium shift if more H2 is added to this reaction at equilibrium:

N2 + 3H2 ⇌ 2NH3

Answer:

The H2 amount goes up (by adding it), therefore according to LeChatelier's Principle, the reaction will try and use up the added H2. It does so by shifting the position of equilibrium to the right. This makes more NH3 by using up N2 and H2

Example #2: Using the same reaction in Example #1, which way will the equilibrium shift if some NH3 is removed from the reaction when it is at equilibrium.

Answer:

According to LeChatelier's Principle, the chemical system will attempt to replace the lost NH3. The stress was to remove NH3, so the opposite is to replace it. The equilibrium position will shift to the right in order to replace some of the lost NH3.

Example #3: Which way will the equilibrium shift if the system temperature goes up (heat is added):
2 SO2 + O2 ⇌ 2 SO3 + heat

Answer:

Even though heat is not a chemical substance, for the purposes of LeChatelier's Principle, you can treat it as if it has physical existence. Since heat is added, the reaction will shift to try and use up some of the added heat. In order to do this, the reaction must shift to the left.

Note that, since heat is not part of the equilibrium expression, the value of Keq would change when the chemical system is heated or cooled.


Example #4: Using the same reaction, which way will the equilibrium shift if heat is removed (that is, the temperature goes down).

Answer:

The reaction will attempt to do the opposite of what the stress was. Since the stress was to remove heat, the reaction will shift to the right to generate more heat (replacing only a part of what was lost).

Once again, the value of Keq would change, whereas in the other examples, its value WOULD NOT change.


Pressure Changes and their Effect on Equilibrium.

Remember that a pressure change potentially affecting the position of equilibrium can be accomplished three different ways.

(1) Increasing the volume of the reaction container will reduce the pressure.
(2) Reducing the volume of the reaction container will send the pressure up.
(3) Introducing an inert gas like argon into the reaction container will increase the total pressure.

Important point: the volume changes would affect ONLY the substances in a gaseous state. Liquid and solids would be unaffected by any volume changes. Since the below examples are all 100% gas phase, the position of the equilibrium will be changed, except in one particular circumstance, demonstrated in example #6.


Example #5: The container holding the following reaction (already at equilibrium) has its volume suddenly reduced by half. Which way will the equilibrium shift to compensate?

PCl3 + Cl2 ⇌ PCl5

Answer:

Since the volume went down, this means the pressure went up. The reaction will try to lessen the pressure by shifting to the side with the lesser number of gas molecules. This means a shift to the right because for every PCl5 molecule made, two molecules are used up. The lesser the total number of gas phase molecules in the container, the lesser the pressure.

Example #6: The container holding the following reaction (already at equilibrium) has its volume suddenly increased. Which way will the equilibrium shift to compensate?

H2 + Cl2 ⇌ 2HCl

Answer:

Neither side is favored over the other since both sides have the same number of total molecules (two). No matter which way the reaction shift, the total number of molecules would remain unchanged.

In cases like this, where there is an equal number of molecules on each side, the equilibrium would remain unchanged by the change in pressure (in either direction).

Example #6 is a favorite question to ask on a test. All other examples used would involve a change in the number of gas molecules, therefore showing a shift in the position of the equilibrium. In the above example, the number of gas molecules is the same on each side, so no shift.

You have been warned!


Example #7: The system below is already at equilibrium when some neon is added to the system. What happens to the position of the equilibrium? Does it shift right, left, or no change?

H2 + Cl2 ⇌ 2HCl

Answer:

The neon DOES NOT participate in the chemical reactions (forward and reverse) which make the equilibrium. Therefore, the presence of the inert gas has NO EFFECT on the position of the equilibrium. The [H2], [Cl2] and [HCl] would all remain unchanged.

Example #8: The system below is already at equilibrium when a catalyst is added to the system. What happens to the position of the equilibrium? Does it shift right, left, or no change?

PCl3 + Cl2 ⇌ PCl5

Answer:

There will be no change in the equilibrium. BOTH (with emphasis on both) the forward and the reverse reactions are speeded up. A catalyst just gets you to equilibrium faster, it doesn't affect the final position of equilibrium like changing the concentration would.

Example #9: Which way would the equilibrium shift if the pressure is increased?

N2 + 3H2 ⇌ 2NH3

Answer:

You should think of the pressure being increased by reducing the volume, not by adding an inert substance or heating the system up.

The position of the equilibrium would shift to the side with the lesser number of molecules. The shift would be to the right.


Go to more LeChatelier problems

The Effect of Heat on the Position of the Equilibrium and the Value of the Keq

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