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.