Equilibrium (1) Applying Le Chatelier's Principle

Edexcel A level Chemistry (2017)

Topic 10: Equilibrium I:

Here is the third learning objective:

10/I/2. be able to predict and justify the qualitative effect of a change in temperature, concentration or pressure on a homogeneous system in equilibrium.

A homogenous system has all reactants and products in the same physical state.

e.g. esterification:

CH₃CH₂OH(l)    +    CH₃COOH(l)     ⇌    CH₃COOCH₂CH₃ (l)   +   H₂O (l)

Or

Ammonia formation:         N₂ (g) +   3H₂ (g)   ⇌  2NH₃ (g)

It was the French chemical engineer Henri Louis Le Chatelier who first noticed that changing the conditions of a system at equilibrium resulted in a negative feedback.

He put it something like this (but originally in French!!)

If a stress is applied to a system at equilibrium then the position of the equilibrium moves in such a way as to absorb the stress applied. 

And here is what the famous Linus Pauling had to say about Le Chatelier’s Principle:

The student (or the scientist) would be wise to refrain from using the mathematical equation unless he understands the theory that it represents, and can make a statement about the theory that does not consist just in reading the equation. It is fortunate that there is a general qualitative principle, called Le Chatelier's principle, that relates to all the applications of the principles of chemical equilibrium. When you have obtained a grasp of Le Chatelier's principle, you will be able to think about any problem of chemical equilibrium that arises, and, by use of a simple argument, to make a qualitative statement about it.… Some years after you have finished your college work, you may (unless you become a chemist or work in some closely related field) have forgotten all the mathematical equations relating to chemical equilibrium. I hope, however, that you will not have forgotten Le Chatelier's principle.

This is what the learning objective means by “qualitative effect” of changing conditions of a system at equilibrium.

There is more about Le Chatelier here on Wikipedia.

a) Changing the temperature of a system at equilibrium:

The formation of ammonia is exothermic in the forward direction (left to right as written here.)

         exothermic

N₂ (g) +   3H₂ (g)   ⇌  2NH₃ (g)  ΔH = –92kJ/mol

If the temperature at which the reaction is being carried out is increased then the position of equilibrium moves to the left.

The concentration of hydrogen and of nitrogen in the equilibrium mixture increase but the concentration of ammonia decreases. 

The position of equilibrium moves to oppose the applied change i.e. in an endothermic direction. 

Use this table to predict the effect of changes in temperature

Type of reaction	Exothermic forward reaction	Endothermic forward reaction
Applied change	Increase in the reaction temperature	Increase in the  reaction  temperature
Effect on the position of equilibrium	Moves to the left to favour the reactants	Moves to the right to favour the products.

The result is that in the Haber process for the production of ammonia the yield of ammonia falls away as the operating temperature of the ammonia converter increases. 

As you can see from this data table and graph of real data from the production process:

At 373K, ammonia yield is 98.4% at 200atm but raise the temperature to 773K and the yield falls away to 18.3%.

b) Changing the pressure of a system at equilibrium.

The formation of ammonia brings a decrease in the number of molecules in the reaction vessel since there are four moles of reactants that become two moles of product in the forward direction (left to right as written here.)

N₂ (g) +   3H₂ (g)   ⇌  2NH₃ (g)  ΔH = –92kJ/mol

four moles         ⟶          two moles

If the pressure at which the reaction is being carried out is increased then the position of equilibrium moves to the right.

The number of moles of hydrogen and of nitrogen in the equilibrium mixture decreases but the number of moles of ammonia increases. 

Remember Le Chaltelier’s principle:

If a stress is applied to a system at equilibrium then the position of the equilibrium moves in such a way as to absorb the stress applied. 

The position of equilibrium moves to oppose the applied change i.e. in the direction of less moles in the reaction vessel since this will reduce the pressure. 

Use this table to predict the effect of changes in pressure

Type of reaction	Forward reaction involves a decrease in the number of moles.	Forward reaction involves an increase in the number of moles.
Applied change	Increase the pressure in the reaction vessel.	Increase the pressure in the reaction vessel.
Effect on the position of equilibrium	Moves to the right to favour the products.	Moves to the left to favour the reactants.

The result is that in the Haber process for the production of ammonia the yield of ammonia increases as the operating pressure of the ammonia converter increases. 

As you can see from this data table and graph of real data from the production process:

At a constant 373K, ammonia yield is 90% at 10atm but raise the pressure to 400atm and the yield increases to 99.4%.

Qualification:

If there is no change in the number of moles in the reaction from reactant to product then changing the pressure has no effect on the position of equilibrium. 

So in this example increasing or decreasing the operating pressure has no effect on the position of equilibrium:

N₂(g)   +    O₂(g)   ⇌     2NO(g)

Provided of course the temperature remains constant.

c) Changing the concentration of a system at equilibrium

This change applies to systems in either the gaseous state or liquid/aqueous state.

It usually refers to the addition of new material to the reaction mixture or the removal of the product as it forms.

The esterification of ethanol by ethanoic acid:

CH₃CH₂OH    +    CH₃COOH     ⇌     CH₃COOCH₂CH₃    +   H₂O

This is a homogenous equilibrium mixture since reactants and products are both in the same liquid state.

Removing the ester as it forms will reduce the concentration of the ester in the reaction mixture.

The equilibrium will respond to this change.

Le Chatelier tells us that the response will be negative feedback i.e. the equilibrium will change to restore the state of affairs.

To restore the equilibrium state, the concentrations of the reactants will have to decrease and this will allow for the concentration of the products to increase.

They might even be restored to the position they were in before removal of product.

The effect works both ways.  Add reactant and there is a similar increase in the product concentration in the reaction mixture at equilibrium.

Note too that this effect happens even though the number of moles of reactant is the same as the number of moles of product in the overall reaction. 

And all these changes are happening when the other equilibrium conditions such as the reaction temperature remain constant. 

Use this table to predict the effect of changes in concentration

Type of reaction	Homogenous in liquid or gaseous state.	Homogenous in liquid or gaseous state.
Applied change	Remove product from the reaction mixture.	Add reactant to the reaction mixture.
Effect on the position of equilibrium	Moves to the right to favour the products.	Moves to the right to favour the products.
Conditions	Temperature and pressure constant	Temperature and pressure constant

This approach to manipulating an equilibrium reaction is commonly used in industrial processes to generate the product so that these chemical processes are not pure equilibria.

They never have time to establish an equilibrium state before stuff is either added or removed from the reaction vessel.