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

• 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

1. to increase pressure, equilibrium shifts to direction with larger mole number
2. 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

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Image references:

• Le Chatelier's Principle - https://www.nagwa.com/en/lessons/159121081945/
• ChemLibreTexts - Equilibrium when concentration/pressure/temperature change - https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Equilibria/Le_Chateliers_Principle/Le_Chatelier's_Principle_and_Dynamic_Equilbria

Darfted by Yoyo (Chemistry)