Le Chatelier's Principle and Equilibrium

Quiz and Lab Schedule

  • Monday Quiz: Covers KSP and KEQ.
  • Equilibrium Lab:
    • Visualizing equilibrium with color changes in three chemical reactions.
    • Manipulating reactions to shift between colors.
    • Pre-lab assignment due before Monday (read through for safety).
    • Some chemicals are corrosive, so safety precautions are important.
  • Post-Lab Topics: Collision theory and reaction rates.
  • Thursday Quiz: Covers equilibrium, collision theory, and reaction rates.
  • Final Labs: Labs four and five.
    • Lab partners can be chosen for these labs.
    • Iodine clock lab (famous for its sudden color change from clear to black).
  • The material is conceptual rather than mathematical (no significant figures or units).
    • Focus on relationships between concepts rather than calculations.
    • Questions will involve describing relationships.

Le Chatelier's Principle

  • When increasing something, the reaction shifts away from it to decrease it.
  • When decreasing something, the reaction shifts towards it to make more of it.
  • Applies to all reactions trying to maintain equilibrium.
  • Shifting changes ratios, driving the reaction forward or in reverse to restore equilibrium.

Example

  • If Br_2 is increased, the reaction shifts away from it.
  • If Br_2 is decreased, the reaction shifts towards it.

Conceptual vs. Mathematical

  • This section focuses on understanding the concepts rather than performing calculations.

Temperature as a Stressor

  • Endothermic Reactions: Absorb heat; heat is a reactant.
    • Heat goes in and is absorbed into the products.
  • Exothermic Reactions: Release heat; heat is a product.
    • Heat comes out and is associated with the products.

Representing Heat

  • Heat can be represented as the word "heat" in a chemical equation.
  • Heat can be described by a numerical value in joules (J) or kilojoules (kJ).
    • 1 kJ = 1000 J

Heat as a Reactant or Product

  • Heat is treated as a reactant or product when considering it as a stressor, even though it's technically a byproduct.

Endothermic Reactions

  • If heat is added (temperature increases), the reaction shifts to the right (forward).
    • More heat is absorbed.
  • If heat is decreased (temperature decreases), the reaction shifts to the left (reverse).
    • Less heat is absorbed.

Exothermic Reactions

  • If temperature increases, the reaction shifts to the left, favoring the reverse reaction and increasing absorption on the left-hand side.
  • If temperature decreases, the reaction shifts to the right, favoring the forward reaction and decreasing absorption.
  • Absorption always occurs on the opposite side of where heat is added or released.

Pressure and Volume

  • Review of the relationship between pressure, volume, and amount using the ideal gas law: PV = nRT
  • Pressure (P) is directly proportional to the amount of gas (n) and inversely proportional to volume (V).

Le Chatelier's Principle and Pressure/Volume

  • If pressure increases (volume decreases), the reaction shifts to the side with fewer moles of gas.
  • If pressure decreases (volume increases), the reaction shifts to the side with more moles of gas.
  • Important: Only gases are affected by pressure and volume changes at equilibrium.

Example

  • N2(g) + 3H2(g) \\rightleftharpoons 2NH_3(g) + heat
  • If pressure increases, the reaction shifts to the right (fewer moles of gas).
  • If pressure decreases, the reaction shifts to the left (more moles of gas).

Catalysts

  • Catalysts speed up chemical reactions by increasing both the forward and reverse reactions at the same rate.
  • Catalysts do not shift the equilibrium.
  • Adding a catalyst results in no shift in equilibrium.

Additional Notes

  • When considering volume and pressure, the entire system is affected.
  • When considering heat, only the side with "+ heat" is directly affected by the change in heat.
  • Changes in volume affect everything, while changes in heat only affect the side where "heat" is specified.