Le Chatlier's Principle

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Upsetting the Balance

• Changing the condition so that forward and/or reverse rate changes will mean a system is no longer at equilibrium
• It will adjust and form a new equilibrium

Le Chatlier's principle

If a system at equilibrium is subject to a change in conditions, the system will adjust to re-establish equilibrium in such a way as to partially counteract the imposed change

Increase in concentration of a reactant

• If the concentration of reactant increases, thus rate of forward reaction increases
• According to Le Chatlier’s principle, the system will partially counteract this change to achieve equilibrium
• Thus, the reactants must be consumed, and thus the forward reaction is favoured
• Thus, the forward reaction is favoured, and equilibrium shifts right
• Note: the concentration of reactants is still partially higher
  • Initially, forward rate increases, reverse rate stays the same
  • Over time, forward rate decreases, reverse rate increases
    • Forward reaction rate is faster than reverse, thus more products are produced than are consumed
    • Thus, reverse rate will increase and forward rate will decrease, over time

Rate of reaction explanation

1. Initially, as the concentration of reactant increases, frequency of collisions and thus frequency of successful collisions increases, and thus rate of forward reaction increases.
2. The rate of reverse reaction remains constant
3. However, over time, more products are produced than are consumed in the reverse reaction, and thus product concentration increases over time, and thus the rate of reverse reaction increases over time
4. Conversely, more reactants are being consumed than are produced, and thus reactant concentration decreases over time, and thus the rate of forward reaction decreases over time
5. This continues until the rate of forward and reverse reactions are equal, and the system is in dynamic equilibrium

Change in concentration of a total solution (Dilution)

• If total concentration of solution decreases, the rate of all reactions decrease
  • Note: concentration of water in solution does not increase, do not ever say this
• According to Le Chatlier’s principle, the system will re-establish equilibrium by favouring the reaction that produces the most ions/particles, to partially counteract the decrease in total concentration
• Thus, the reaction that produces the most moles is favoured
  • If it is forward, equilibrium shifts right
  • If it is reverse, equilibrium shifts left

Change in pressure (by changing volume)

• If volume decreases, total partial pressure/concentration of all gases will increase
• According to Le Chatlier’s principle, the system will re-establish equilibrium by partially counteracting this increase in total partial pressure/concentration
• Thus, the reaction that produces the least moles of gas is favoured
  • If it is forward, equilibrium shifts right
  • If it is reverse, equilibrium shifts left

Rate of Reaction Explanation

1. Increase pressure (decrease volume)
2. Increases concentration of reactants and products
3. Forward and reverse rates increase
4. However, as the forward reaction requires a collision between more particles, it has a greater proportionate increase in frequency of successful collisions than the one with less particles, the rate of the forward reaction increases more than the rate of reverse
5. Thus, the rate of forward reaction is favoured. Thus over time, more products are produced than are consumed in the reverse reaction, and thus product concentration increases over time, and thus the rate of reverse reaction increases over time
6. Conversely, more reactants are being consumed than are produced, and thus reactant concentration decreases over time, and thus the rate of forward reaction decreases over time
7. This continues until the rate of forward and reverse reactions are equal, and the system is in dynamic equilibrium

Change in Temperature

• If temperature increases, the average kinetic energy of all particles increases
• According to Le Chatlier’s Principle, the system will re-establish equilibrium by partially counteracting this increase in temperature
• Thus, the endothermic (forward reaction) is favoured
• Equilibrium shifts right

Rate Explanation

1. Increase temperature
2. Both reaction rates increase
3. The higher activation energy of the forward (endothermic) reaction means the forward reaction has a greater proportionate increase in particles with collision energy sufficient to overcome activation energy, when compared to the reverse (exothermic) reaction
4. Thus, the rate of forward reaction is favoured. Thus, over time, more products are produced than are consumed, and thus product concentration and rate of reverse reaction increases over time
5. Conversely, more reactants are being consumed than are produced, and thus reactant concentration decreases over time, and thus the rate of forward reaction decreases over time
6. This continues until the rate of forward and reverse reactions are equal, and the system is in dynamic equilibrium
7. Equilibrium has shifted right, and is established at a net higher equilibrium rate

Catalyst

• Addition of a catalyst increases both rates of reaction. Both rates increase by same amount
• No favouring
• No change in equilibrium