Thermodynamics of the [CoCl(ⁱPrOH)₃]⁺ to [CoCl(ⁱPrOH)₂(MeOH)₃]⁺ Reaction: A General Chemistry Laboratory Exercise

Abstract

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.

Exploring Chemical Equilibrium for Alcohol-Based Cobalt Complexation through Visualization of Color Change and UV–vis Spectroscopy

“…In order to observe the reverse equilibrium (eq ), students prepared Table solutions and analyzed the full spectrum on the LabQuest. The absorbance values and the given equilibrium constant expression, derived from eq , were used to determine the equilibrium constant for each solution in order to quantify and compare both directions of the cobalt complex equilibrium. For solution 3, representing the coexistence of two different complex species (and thus two distinct ε) in solution, students were asked to plot absorbance A against wavelength λ.…”

“…To improve comprehension of Le Châtelier’s principle, laboratory exercises often visualize concepts such as equilibrium using observable phenomena. For instance, color change in cobalt (Co) complexation , is an ideal identifier of reaction progress because it is both qualitatively distinguishable (when significant color change occurs) and quantifiable via instrumentation. UV–visible spectrophotometers are commonly used instruments for probing color-change equilibria in laboratory environments, and such equipment holds continued relevance in academia and industry to this day .…”

“…Several undergraduate laboratories and demonstrations ,,− have used Co complexation reactions for teaching equilibrium. Some experiments demonstrated the vivid color change by shifting the chemical equilibrium of Co complexes from pink octahedral to blue tetrahedral complexes by using HCl as a ligand. ,, Other work investigated the blue-to-pink transition by changing temperature in an alcohol-based Co complexation where the temperature effects on thermodynamic parameters were the focus . However, no prior work has used alcohol ligands to shift the chemical equilibrium from pink to blue Co complexes via alcohol concentration.…”

“…1,5,9 Other work investigated the blue-to-pink transition by changing temperature in an alcohol-based Co complexation where the temperature effects on thermodynamic parameters were the focus. 2 However, no prior work has used alcohol ligands to shift the chemical equilibrium from pink to blue Co complexes via alcohol concentration. This experiment allows a range of colors, from pink to blue, to be clearly visible as the reaction shifts from an alcohol-based pink complex to a blue complex.…”

Determination of Thermodynamic Values (∆S°, ∆H°, and ∆G°) from the Dissociation of a Weak Acid

“…[5][6][7][8][9][10][11][12][13] However, many of these analyses require instrumentation that may not be widely available to all laboratories, or require more sophisticated procedures that are unsuitable for an introductory audience. In the interest of bringing this concept to a broader group of students, an alternate methodology was developed to perform the analysis based on just the natural dissociation of the acid as determined by pH and temperature measurement.…”