Le Chatelier’s Principle ( AP Chemistry )

  • 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?


  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?

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