In this general chemistry laboratory exercise, the absorbance of [CoCl(iPrOH)3]+, where iPrOH is isopropyl alcohol, is used to determine the equilibrium constants for its reaction with methanol to form [CoCl(iPrOH)2(MeOH)3]+ in the temperature range ∼10 to 20 °C. The absorbance versus concentration calibration plot is determined by sequential additions of a concentrated [CoCl(iPrOH)3]Cl standard to iPrOH. The equilibrium mixture is cooled in a CaCl2–ice bath, and measurements of temperature and absorbance are made at various time intervals as the solution warms to room temperature. The variation in the equilibrium constant as a function of temperature is used to determine the Gibbs energy, enthalpy, and entropy.

The thermodynamic properties of weak acid ionization reactions are determined. The thermodynamic properties are corresponding values of the absolute temperature (T), the weak acid equilibrium constant (K a), the enthalpy of ionization (Δi H°), and the entropy of ionization (Δi S°). The enthalpy of ionization (Δi H°) is determined from the enthalpy of neutralization of HCl(aq) and the enthalpy of neutralization of the weak acid by application of Hess’s law; NaOH(aq) is the base. A datalogger and a temperature sensor (±0.01 °C sensitivity) are used to measure and plot the thermograms of the reactions. The calorimeter constant (C Cal) is determined by electrical heating of the post-reaction solution; procedure takes 10–20 s for each C Cal determination. With NaOH(aq) as the limiting reagent, the post-reaction solutions consist of a weak acid and corresponding conjugate base allowing K a to be determined from the measured pH of these solutions. The values of T, K a, and Δi H° are used to calculate Δi S° according to the equation: Δi G° = −RT ln K a = Δi H° – TΔi S°. The choice of H3PO4(aq), a triprotic weak acid, provides an opportunity for students to predict and explain expected trends in K a, Δi H°, and Δi S° prior to their determination. The multiconcept nature of this lab exercise makes it an ideal capstone laboratory exercise in general chemistry.