• Le Chatelier’s Principle

• AP Chemistry

• Le Chatelier’s Principle

• Factors that Affect Equilibrium

• Concentration

• Temperature

• Pressure

–For gaseous systems only!

• The presence of a catalyst

• Concentration Changes

• Add more reactant è Shift to products

• Remove reactants è Shift to reactants

• Reaction Quotient

• The reaction quotient for an equilibrium system is the same as the equilibrium expression, but the concentrations are NOT at equilibrium!

• Changes in Concentration

Changes in concentration are best understood in terms of what would happen to “Q” if the concentrations were changed.

N2O4(g)  ó  2NO2(g)

• Temperature Changes

  Exothermic Reactions

• Consider heat as a product in exothermic reactions.

–Add heat è

  Shift to reactants

–Remove heat è

   Shift to products

• Temperature Changes

  Endothermic Reactions

• Consider heat as a reactant in endothermic reactions.

–Add heat è

  Shift to products

–Remove heat è

  Shift to reactants

• Pressure Changes

• Only affects equilibrium systems with unequal moles of gaseous reactants and products.

• N2(g) + 3H2(g) = 2NH3(g)

• Increase Pressure

–Stress of pressure is reduced by reducing the number of gas molecules in the container . . . . . .

• N2(g) + 3H2(g) = 2NH3(g)

• There are 4 molecules of reactants vs. 2 molecules of products.

–Thus, the reaction shifts to the product ammonia.

• PCl5(g) = PCl3(g) + Cl2(g)

• Decrease Pressure

–Stress of decreased pressure is reduced by increasing the number of gas molecules in the container.

• PCl5(g) = PCl3(g) + Cl2(g)

• There are two product gas molecules vs. one reactant gas molecule.

• Thus, the reaction shifts to the products.

• Presence of a Catalyst

• A Catalyst lowers the activation energy and increases the reaction rate.

• It will lower the forward and reverse reaction rates,

• Therefore, a catalyst has NO EFFECT on a system at equilibrium!

• It just gets you to equilibrium faster!

• Presence of an Inert Substance

• An inert substance is a substance that is not- reactive with any species in the equilibrium system.

• These will not affect the equilibrium system.

• If the substance does react with a species at equilibrium, then there will be a shift!

• Given:

• S8(g) + 12O2(g)  ó  8 SO3(g)   +  808 kcals

• What will happen when ……

• Oxygen gas is added?

• The reaction vessel is cooled?

• The size of the container is increased?

• Sulfur trioxide is removed?

• A catalyst is added to make it faster?

Given

  2NaHCO3(s)  ó Na2CO3 (s) +  H2O (g)   +  CO2(g)

• What will happen when . . . . . . .

• Carbon dioxide was removed?

• Sodium carbonate was added?

• Sodium bicarbonate was removed?

• Given

Ca5(PO4)3OH(s)  ó  5Ca2+(aq)  + 3PO43-(aq) +  OH- (aq)

• What will happen when. . . . . .

• Calcium ions are added?

• NaOH is added?

• 1 M HCl is added?

• Na3PO4(aq) is added?

…