Equilibrium Summary

• Equilibrium Definition: When the number of molecules leaving the liquid to vapor equals the number returning, equilibrium is attained and is dynamic in nature.
• Equilibrium Constant (Kc): Expressed as the concentration of products divided by reactants, each raised to the stoichiometric coefficient.
  • For reaction: aA + bB ⇌ cC + dD
  • Kc = [C]^c[D]^d / [A]^a[B]^b
• Kp: For gaseous reactions, Kp is expressed using partial pressures instead of concentrations.
• Le Chatelier's Principle: States that a change in temperature, pressure, or concentration will shift the equilibrium to counteract the change.
• Catalysts: Increase the rate of reaction without affecting the equilibrium composition.
• Electrolytes: Substances that conduct electricity in aqueous solutions, including acids, bases, and salts.
• Acid-Base Definitions:
  • Arrhenius: Acids produce H⁺ ions; bases produce OH⁻ ions.
  • Brönsted-Lowry: Acids are proton donors; bases are proton acceptors.
  • Lewis: Acids are electron pair acceptors; bases are electron pair donors.
• Ionization Constants: Expressions for weak acids (K) and weak bases (Kb) are developed using the Arrhenius definition.
• pH Scale: pH = -log[H⁺]; pOH = log[OH⁻]; pKa = -log[K]; pKb = -log[K]; pKw = -log[Kw].
• Solubility Product Constant (Ksp): Relates to the solubility of sparingly soluble salts and their equilibrium in solution.

Chemistry Notes

Suggested Activities for Students Regarding This Unit

• pH Determination: Use pH paper to determine the pH of:
  • Fresh juices of various vegetables and fruits
  • Soft drinks
  • Body fluids
  • Water samples
• Salt Solutions: Determine the pH of different salt solutions to identify if they are formed from strong/weak acids and bases.
• Buffer Solutions: Prepare buffer solutions by mixing sodium acetate and acetic acid, then determine their pH using pH paper.
• Indicators: Observe the colors of different indicators in solutions of varying pH.
• Acid-Base Titrations: Perform acid-base titrations using indicators.
• Common Ion Effect: Observe the common ion effect on the solubility of sparingly soluble salts.
• pH Meter Comparison: Measure pH with a pH meter and compare results with pH paper.

Exercises

Equilibrium and Kc Calculations

1. Vapor Pressure Change:
   • What is the initial effect of increasing the volume of a sealed container on vapor pressure?
   • How do rates of evaporation and condensation change initially?
   • What happens when equilibrium is restored?
2. Kc Calculation: For the equilibrium: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), given concentrations [SO₂]=0.60M, [O₂]=0.82M, [SO₃]=1.90M, calculate Kc.
3. Kp Calculation: At a total pressure of 10⁵Pa, iodine vapor contains 40% by volume of I atoms. Calculate Kₚ for the equilibrium I₂(g) ⇌ 2I(g).
4. Equilibrium Constant Expressions: Write the expression for Kc for the following reactions:
   • 2NOC1(g) ⇌ 2NO(g) + Cl₂(g)
   • 2Cu(NO₃)₂(s) ⇌ 2CuO(s) + 4NO₂(g) + O₂(g)
   • CH₂COOC₂H₅(aq) + H₂O(l) ⇌ CH₃COOH(aq) + C₂H₅OH(aq)
   • Fe³⁺(aq) + 3OH⁻(aq) ⇌ Fe(OH)₃(s)
   • I₂(s) + 5F₂ ⇌ 2IF₅
5. Kp to Kc Conversion: Find Kc from Kp for the following equilibria:
   • 2NOC1(g) ⇌ 2NO(g) + Cl₂(g); Kₚ = 1.8 X 10⁻² at 500 K
   • CaCO₃(s) ⇌ CaO(s) + CO₂(g); Kₚ = 167 at 1073 K

Important Concepts

Conjugate Acid-Base Pairs

• Definition: A conjugate acid-base pair consists of two species that transform into each other by the gain or loss of a proton (H⁺).
• Examples:
  • HNO₂ ↔ NO₂⁻
  • HF ↔ F⁻
  • H₂SO₄ ↔ HSO₄⁻

Lewis Acids and Bases

• Lewis Acids: BF₃, H⁺, NH₄⁺
• Lewis Bases: OH⁻, F⁻

pH Calculations

• Hydrogen Ion Concentration:
  • For a soft drink with [H⁺] = 3.8 X 10⁻³ M, calculate pH.
  • For vinegar with pH = 3.76, calculate [H⁺].

Ionization Constants

• Ionization Constants:
  • HF: 6.8 X 10⁻⁴
  • HCOOH: 1.8 X 10⁻⁴
  • HCN: 4.8 X 10⁻⁹
• Conjugate Bases: Calculate the ionization constants of the corresponding conjugate bases.

Common Mistakes and Exam Tips

Common Pitfalls

• Misunderstanding Equilibrium: Students often confuse dynamic equilibrium with static equilibrium. Remember that in dynamic equilibrium, the rates of the forward and reverse reactions are equal, but the reactions continue to occur.
• Incorrect Use of Equilibrium Constants: When calculating Kc or Kp, ensure you are using the correct concentrations or partial pressures and that they are raised to the power of their coefficients in the balanced equation.
• Ignoring Temperature Effects: Changes in temperature can affect the value of Kc or Kp. Be careful to note how temperature changes influence equilibrium positions.
• Assuming Complete Dissociation: In weak acids and bases, do not assume complete dissociation. Always consider the degree of ionization and use the appropriate equilibrium expressions.

Exam Tips

• Practice Writing Equilibrium Expressions: Be comfortable with writing Kc and Kp expressions for various reactions. This skill is crucial for solving equilibrium problems.
• Understand Le Chatelier's Principle: Be prepared to apply Le Chatelier's principle to predict how changes in concentration, pressure, or temperature will affect the position of equilibrium.
• Use pH Calculations Wisely: When calculating pH from hydrogen ion concentration, remember the formula pH = -log[H⁺]. Also, be familiar with how to calculate [H⁺] from pH.
• Check Units: Always ensure that your units are consistent, especially when dealing with concentrations and pressures in equilibrium calculations.
• Review Common Ion Effect: Understand how the presence of a common ion affects the solubility of salts and the ionization of weak acids and bases.