Le Châtelier’s Principle
• Le Châtelier’s Principle – chemical systems at equilibrium shift to restore equilibrium when a change occurs that disturbs the equilibrium
Factors disturb equilibrium
• Concentration, pressure and temperature or a combination of them
• Why would we care? Le Chatelier feeds the world
Le Châtelier’s Principle and changes in concentration
• Equilibrium shift – a change in concentrations of reactants and products in order to restore an equilibrium state
• What do you predict would happen if I added more SCN anion to the reaction?
• Or what about more FeSCN?
Explaining the effect of concentration on equilibria with collision theory
• Greater concentration of a reactant means higher probability of collision leading to higher probability in shift to product.
• It goes in both directions!
Applying the concentration change principle
• If you run a reaction in which you continually add a reactant and/or remove a product the equilibrium will continuously shift towards the product side of the equation
• Used by chemical engineers to design industrial processes for reversible reactions
• E.g. production of hydrogen gas uses high pressure and temperature to shift reaction to product
Le Châtelier and gas volume
• This is an extension of the concentration principle
• Assuming an ideal gas pressure is inversely proportional to volume and each gas exerts its own partial pressure
• Therefore changing the volume changes the pressure exerted by each gas
-Decreasing the volume will increase the pressure-This will favour a shift to the right since fewer chemicalentities will lead to a reduction in pressure
p. 444
Changing an equilibrium system without affecting equilibrium position
• Catalysts – can change the rate of forward and reverse reactions by lowering Ea but do not affect the equilibrium position (just how quickly you get there)
p. 445
• Inert Gas – adding an inert gas will increase the pressure thus increasing number of collisions but since it is inert the collisions will not cause reactions leaving the equilibrium intact
p. 445
• State of reactants – In reactions involving chemicals in different states adding or subtracting chemicals in other states will have not effect at equilibrium so long as they are not completely removed.