The H3PO4 Acid Ionization Reactions: A Capstone Multiconcept Thermodynamics General Chemistry Laboratory ExerciseClick to copy article linkArticle link copied!
Abstract
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 (Ka), the enthalpy of ionization (ΔiH°), and the entropy of ionization (ΔiS°). The enthalpy of ionization (ΔiH°) 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 (CCal) is determined by electrical heating of the post-reaction solution; procedure takes 10–20 s for each CCal determination. With NaOH(aq) as the limiting reagent, the post-reaction solutions consist of a weak acid and corresponding conjugate base allowing Ka to be determined from the measured pH of these solutions. The values of T, Ka, and ΔiH° are used to calculate ΔiS° according to the equation: ΔiG° = −RT ln Ka = ΔiH° – TΔiS°. The choice of H3PO4(aq), a triprotic weak acid, provides an opportunity for students to predict and explain expected trends in Ka, ΔiH°, and ΔiS° prior to their determination. The multiconcept nature of this lab exercise makes it an ideal capstone laboratory exercise in general chemistry.
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