AP Chemistry Unit 7.10 Reaction Quotient and Le Chateilier’s Principle

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AP Chemistry-Concise Summary Notes- All Topics

Reaction Quotient and Le Chateilier’s Principle

  • A disturbance to a system at equilibrium causes Q to differ from K
  • The reaction will “shift” to bring Q back into agreement with K

Effects of Changes to a System

1. Pressure (only affects gasses); there are three ways to change the pressure of a system with gas

    • Changing pressure may alter the equilibrium position, but it does not alter the equilibrium constant (K)

a. Effect of changing Pressure:

        • Increase pressure, the equilibrium will shift to the side with less moles of gas
        • Pressure decreases, equilibrium will shift to side to side with more moles of gas

b. Addition of an inert (unreactive) gas:

        • Increases the total pressure but has no effect on the equilibrium of the system or the concentrations or partial pressures of the reactants or products

c. Effect of changing Volume:

        • Increase volume: shift to side with more moles of gas
        • Decrease volume: shift to side with less moles of gas

d. Changing the volume of the system affects pressure (Boyle’s Law) → See letter a…

        • V increases → P decreases; V decreases → P increases
          • When volume of the container decreases, the system responds by reducing its own volume by decreasing the total number of gaseous molecules

2. Temperature: K will change depending on temp (treat energy as either a reactant or product)

  • Ex: Endothermic
    • Treat heat as reactant
    • Effect on K: Adding heat will shift in forward direction so K > 1
    • Forward direction → high temp & LP; Reverse direction → low temp & HP
  • Ex: Exothermic:
    • Treat heat as a product
    • Effect on K: Adding heat will shift in reverse reaction so K < 1
    • Forward direction → low temp & HP; Reverse direction → high temp and LP

3. Concentration: the system will shift away from the added component (or toward the removed component)

  • If question adds something in reaction, added component will likely react with smthn in the reaction
    • If add something that forms a precipitate (often OH-) → are taking reactant out of reaction
  • What if dilute the solution? (ex: add water vapor)
    • Dilute → all molarities (products and reactants) will decrease
    • Diluting will always cause a shift toward more aqueous species
      • If there are more reactants (denominator) than products → Q > K
      • If there are more products (numerator) than reactants → Q < K
  • What if increase the concentration of the solution?
    • Concentrate → all molarities (products and reactants) will increase
    • Concentrating will always cause a shift toward less aqueous species!
        • If there are more reactants (denominator) than products → Q < K
        • If there are more products (numerator) than reactants → Q > K
  • Changing the amount of liquid/solid or adding a catalyst will have NO SHIFT
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