A2/A-level Chemistry - Le Chatelier's Principle

Topic 3.1.6 - Chemical equilibria, Le Chatelier’s principle and K_c

July 11, 2021

In A-level Chemistry, Le Chatelier's Principle is introduced in topic 3.1.6
The principle describes the natural restoration of equilibrium when reaction condition changes
Like sitting on a seesaw, our goal is to keep the balance of both sides by adding or removing weight
Reaction could be influenced by change in concentrations, pressure, temperature, **catalyst (NO effect on the position of equilibrium)

Change in concentration

E.g. (reactant side) A + 2B ---> C (Product side)
If you add more A, concentration of A increases
System responses to decrease A back to near initial concentration
Meaning that the reaction shifts to product (left) side
Resulting in decrease in concentration for both A and B
Alternatively, if C is removed , the system responses by increasing concentration of C
Equilibrium shifts right and concentration of A and B decrease

Change in pressure

Only applies to reactions in gas phase
E.g. (reactant side) A(g) + 2B(g) ---> C(g) + D(g) (Product side)
the numbers of molecules on both sides of the equilibrium are different
In this case, no. of mole on left = 3 while no of mole on right = 2
If pressure increases, the reaction responds by decreasing it
As the right side has smaller no. of mole than the left, equilibrium shifts to product side, decreasing the pressure

to increase pressure, equilibrium shifts to direction with larger mole number
to decrease pressure, equilibrium shifts to direction with smaller mole number

Change in temperature

Depends on whether the reaction is endothermic or exothermic
If the forward reaction is exothermic, the back reaction would be endothermic, vice versa
If temperature decreases, system responds to increase the temperature
Equilibrium shifts to right side to produce more products as forward reaction releases heat
If temperature increases, system responds to decrease the temperature
Equilibrium shifts to left side to produce more reactants as backward reaction absorbs heat
Reversely for forward reaction being endothermic and backward reaction being exothermic

Addition of catalyst

Catalyst refers to any substance that increases the rate of a reaction without itself being consumed
E.g. transition metals, enzymes
It speeds up BOTH forward and back reaction by the same proportion
There is NO change/shift in the equilibrium position
The catalyst just increases the speed for the rates of forward and backward reaction to be equal, reaching the dynamic equilibrium
**Meaning that Le Chatelier's Principle DOES NOT apply to catalyst addition

Remarks:

The reactions are proceeding to achieve dynamic equilibirum
Remember in A-level Chemistry, dynamic equilibrium occurs when a reversible reaction takes place with the forward reaction rate equal to the backward reaction rate

Image references:

Darfted by Yoyo (Chemistry)