Le Chatelier's Principle

A system will remain at equilibrium as long as…

rate of forward and reverse reaction are equal

A system will no longer be at equilibrium if a change in what? How does it affect the reaction?

• reaction conditions or stress

• unequally affects the rates of the reactions

What is Le Chatelier’s Principle?

if a dynamic

equilibrium is disturbed by a change in the reaction conditions, the balance between the forward and reverse processes will shift to counteract the change and return the system to equilibrium

What are the stresses which can be applied to systems?

1. Change in Concentration

2. Change in Temp.

3. Change in Pressure

4. Addition of a Catalyst

How does changing Concentration affect reactions?

when you add a reactant/product, the system will shift the opposite way

• if you increase concentration of reactants, the system will increase the forward reaction (R → P) to create more products to return to equilibrium

How does changing Temperature affect reactions?

• does affect K

• Think of it as: the energy needs to stay the same so:

  • if the product is exothermic (R → P +energy), if you increase temp., it will increase the reverse reaction to use up that energy

How does changing Pressure affect reactions?

• remember that in gaseous form, molecules want to be as far away from each other as possible

• If you increase pressure/decrease volume, the reaction will shift to the side with less moles

• If you decrease pressure/increase volume, the reaction will shift to the side with more moles (to fill up the space)

How does adding a catalyst affect reactions?

• forward and reverse reactions pass through the same transition state

  • due to catalysts, both are lowered in E_a equally

• no position change in equilibrium

• no change in K

• speeds up time to reach equilibrium

• used in industrial processes