Chemical Equilibrium Review Lecture

Introduction

• Review aimed at Chemistry 40S students, also useful for AP Chemistry students.
• Excludes equilibrium constants calculated using pressures ($K_P$) as it's not covered in Chemistry 40S.

Key Concepts

Le Chatelier's Principle

• Addition of sulfuric acid increases hydrogen ion concentration, resulting in a forward shift in equilibrium.
• Example:
  • Potassium chromate (yellow) shifts to dichromate (orange) when $H^+$ is added.
  • Forward shift: concentration of $Cr_2O_7^{2-}$ increases, $CrO_4^{2-}$ decreases.

Reversible Reactions

• Can occur in both forward and reverse directions.
• Reaction quotient ($Q$) determines direction of shift:
  • $Q > K_c$: reverse shift.
  • $Q < K_c$: forward shift.
  • $Q = K_c$: equilibrium.

Dynamic Equilibrium

• Rate of forward reaction equals rate of reverse reaction.
• Established in a closed system where reactants/products cannot escape.

Equilibrium Constants ($K_c$)

• Large $K_c$: product favored, reaction nearly complete.
• $K_c$ only affected by temperature change.

True/False Questions Insights

1. Dynamic equilibrium requires equal forward and reverse reaction rates.
2. Closed systems necessary for equilibrium.
3. Equilibrium concentrations are not necessarily equal.
4. Large $K_c$ indicates a product-favored reaction.
5. Temperature is the only factor affecting $K_c$.

Calculations and Problem-Solving

• Using $K_c$ to determine shifts and calculate concentrations.
• Relationship between $K_c$ and reaction manipulation (e.g., reversing reactions, multiplying coefficients).

Le Chatelier's Principle Applications

• Predicting equilibrium shifts due to concentration, temperature, pressure changes.
• Example Calculations:
  • Effect of temperature on endothermic/exothermic reactions.
  • Volume change effect on gaseous reactions.

Exam Strategy

• Understand and apply reaction quotient $Q$.
• Utilize ice tables for organizing equilibrium data.
• Practice interpreting graphs and data for equilibrium shifts.

Example Problems

1. Equilibrium Shift with Added Reactants:

   • Increase in reactant concentration typically causes forward shift.
   • Example with nitrogen monoxide and oxygen.

2. Temperature Effect on $K_c$:

   • Decreased temperature in endothermic reactions decreases $K_c$.
   • Increase in exothermic reactions results in decreased $K_c$ with temperature.

3. Volume and Pressure Changes:

   • Decreasing volume increases concentration, affecting $Q$ and direction of shift.

4. Catalyst Impact:

   • Catalysts speed up reaction rate but do not affect equilibrium concentration or $K_c$.

Conclusion

• Emphasizes practice of equilibrium concepts and Le Chatelier's Principle.
• Useful for test preparation and understanding deeper chemical equilibrium topics.