EO

Experiment 7: Solubility, Equilibrium and Thermodynamics - Solubility of Calcium Hydroxide

PURPOSE

To investigate how temperature affects the solubility of calcium hydroxide (Ca(OH)₂) and to determine its thermodynamic properties (ΔH°, ΔS°, and ΔG°) by measuring the solubility product (Ksp) at different temperatures through acid-base titration.

OBJECTIVES

Experimental Objectives

  • Prepare saturated Ca(OH)₂ solutions at low, room, and high temperatures.

  • Perform titrations using standardized HCl to determine OH⁻ concentration.

  • Calculate Ksp at each temperature from ion concentrations.

  • Use Ksp values to determine ΔG°, and create a ΔG° vs. T plot.

  • Use LINEST in Excel to derive ΔH° and ΔS° from the plot.

Learning Objectives

  • Understand the relationship between solubility, temperature, and thermodynamics.

  • Apply Le Chatelier’s principle to predict solubility changes.

  • Analyze dissolution using ΔG = ΔH - TΔS and ΔG = -RTlnK.

  • Interpret the signs and magnitudes of ΔH° and ΔS° in relation to solubility trends.

UNDERSTANDING THE BACKGROUND

  • Ca(OH)₂ Dissolution Reaction:
    Ca(OH)₂ (s) ⇌ Ca²⁺ (aq) + 2 OH⁻ (aq)
    The solubility of Ca(OH)₂ varies with temperature.

  • Solubility Product (Ksp):
    Ksp=[Ca2+][OH−]2K_{sp} = [Ca^{2+}][OH^-]^2Ksp​=[Ca2+][OH−]2

  • Thermodynamics Connection:

    • ΔG° = –RT ln K

    • ΔG° = ΔH° – TΔS°
      A plot of ΔG° vs. T yields a line with slope = –ΔS° and y-intercept = ΔH°.

  • Predicting Behavior:

    • If dissolution is exothermic, increasing temperature decreases solubility.

    • If endothermic, increasing temperature increases solubility.

    • Dissolution increases disorder (ΔS° > 0).

SUMMARY OF THE PROCEDUREl

  1. Preparation of Saturated Solutions:

    • Dissolve ~0.5 g of Ca(OH)₂ in 50 mL DI water at three temperatures: ice bath (~5–10°C), room temp, and boiling (~100°C).

    • Stir each solution for 5–10 min to allow saturation.

  2. Filtration:

    • Filter 40–50 mL of each solution using pre-wetted filter paper (with saturated solution, not water).

    • Keep filtered low-temp solution in ice bath to prevent solubility changes.

  3. Titration:

    • Titrate 10 mL of each filtered solution with <1 M standardized HCl using bromothymol blue.

    • End point: green (neutral). Indicator turns yellow (acid) beyond equivalence.

    • Perform 2–3 titrations per solution to ensure accuracy.

ANALYZING THE DATA

  1. Calculate [OH⁻] from the volume of HCl used and its concentration.

  2. Calculate [Ca²⁺] using the 1:2 molar ratio from the dissolution equation.

  3. Determine Ksp using Ksp=[Ca2+][OH−]2K_{sp} = [Ca^{2+}][OH^-]^2Ksp​=[Ca2+][OH−]2.

  4. Calculate ΔG° for each temperature:
    ΔG∘=−RTln⁡Ksp\Delta G^\circ = -RT \ln K_{sp}ΔG∘=−RTlnKsp​

  5. Graphical Analysis:

    • Plot ΔG° (y-axis) vs. Temperature in K (x-axis).

    • Use LINEST in Excel to determine slope (–ΔS°) and intercept (ΔH°).

    • Calculate ΔG° and Ksp at 25°C using derived values.

  6. Compare to Theoretical Values:

    • Use thermodynamic tables (e.g., Tro Table 18.2) to find literature values of ΔH°, ΔS°, and Ksp.

    • Calculate % error for your results.

  7. Discussion Points:

    • Did ΔH° and ΔS° match expected signs and values?

    • Explain how solubility changes with temperature and why titrating cold solutions must occur in an ice bath.

    • Assess the spontaneity of dissolution at different temperatures using ΔG°.