Pre-Lab Lecture Notes: Experiment 6 - Equilibrium

Instructor: Dr. Sansom

Key Objectives:

• Determine the value of the equilibrium constant (K) for a reaction.
• Explain why K is constant at a given temperature and describe how K changes at different temperatures.
• Relate changes in K to changes in ΔH (enthalpy), ΔS (entropy), and ΔG (Gibbs free energy).
• Use Le Chatelier's principle to predict system changes with stress (adding/removing reactant or temperature change).
• Develop critical thinking skills by making claims about reaction direction and supporting them with evidence and reasoning.

Key Concepts:

Dynamic Equilibrium:

• At equilibrium, reactant and product concentrations are constant but not necessarily equal.
• Forward and reverse reaction rates are equal.
• Dynamic equilibrium: reaction continues at the molecular level even though macroscopic changes are not observable.

Equilibrium Constant (K):

• Expression: ( K_q = \frac{[products]^{coefficients}}{[reactants]^{coefficients}} )
• Indicates reaction extent:
  • ( K < 1 ): Reactants favored.
  • ( K > 1 ): Products favored.
  • Intermediate K: Mix of both reactants and products.

Temperature Dependence:

• Reaction coordinate diagram: Raising temperature affects forward and reverse reaction rates differently.
• Heating increases both rates, but may shift equilibrium.
• Equilibrium shifts will indicate if the reaction is endothermic or exothermic.

Equations & Thermodynamics:

• ΔG = ΔH - TΔS and ΔG = -RT ln K
• Combination results in ln K = -ΔH/RT + ΔS/R
• Graph of ln K vs. 1/T:
  • Exothermic: Negative ΔH, positive slope.
  • Endothermic: Positive ΔH, negative slope.
• Entropy (ΔS):
  • Negative ΔS: Products more ordered, negative intercept.
  • Positive ΔS: Products more disordered, positive intercept.

Predictions:

• Reaction: Iron + Thiocyanate ions → Complex ion (FeSCN²⁺)
  • Likely exothermic (bond formation), negative ΔH, positive slope.
  • Likely negative ΔS (increased order), negative intercept.

Le Chatelier's Principle:

• Equilibrium system stressed by changing reactant quantities.
• Observations:
  • Shift right = More product (red color).
  • Shift left = More reactant (yellow color).

Lab Safety & Precautions:

• Wear long pants, closed-toed shoes, goggles, gloves, and lab coat.
• Handle iron(III) nitrate with care (corrosive and oxidizing).
• Avoid skin/eye contact and flames/sparks.
• Distinguish chemicals by concentration; use correct solution for each experiment part.

Final Notes:

• Carefully read solution labels and follow TA instructions.
• Ensure correct data collection to obtain meaningful results.

End of Lecture.