Thermodynamics of the Ionization of Acetic Acid
Determining the thermodynamic parameters for the ionization of acetic acid in water to evaluate the feasibility of a large-scale production plant.
Project Overview
Acetic acid is widely used in the food, textile, and adhesive industries. The Board of Directors of the Beehive State Engineers requested an evaluation to determine whether to build a large acetic acid production plant.
Our objective was to determine the standard enthalpy, entropy, and Gibbs free energy for the ionization of acetic acid in water. By measuring the pH of a dilute solution across a specific temperature range, we tracked its dissociation and utilized the Van't Hoff equation to calculate these vital thermodynamic properties.
Target Thermodynamic Parameters
- ΔH° Enthalpy Change
- ΔS° Entropy Change
- ΔG° Gibbs Free Energy
Experimental Methodology
We measured the potential of Hydrogen (pH) of a 1 M acetic acid solution to obtain the dissociation constant (Kₐ). To minimize the variation of heat capacity across different temperatures, a tight 10 °C temperature range was utilized.
Procedure Steps
- Calibration: Calibrated the pH electrode using pH 3 and pH 7 buffer solutions.
- Initial Setup: Cooled the 1 M acetic acid solution to an initial temperature of 5 °C.
- Monitoring: Monitored the solution's temperature and pH as it naturally warmed to 15 °C.
- Data Collection: Recorded the pH value at every 0.5 °C increase using a digital thermometer and the pH electrode.
- Analysis: Utilized the pH data across three trials to calculate the equilibrium constant and generate a Van't Hoff plot.
pH electrode & thermometer]
Results & Thermodynamic Analysis
Calculated Values
Positive value indicates an endothermic reaction, requiring energy input.
Positive value indicates a significant increase in disorder during dissociation.
Positive value signifies the reaction is non-spontaneous under standard conditions (25°C, 1 atm).
Key Findings
The equilibrium constant (Kₐ) demonstrated a clear dependency on temperature. We generated a Van't Hoff plot mapping ln(Kₐ) against 1/T, resulting in a linear trendline with a negative slope, confirming the dissociation is endothermic.
At low temperatures, the positive unfavorable standard enthalpy dominates, resulting in a non-spontaneous reaction. However, because the standard entropy change is also positive, the entropic term will outweigh the enthalpic term as temperatures increase. Therefore, this process is entropically controlled.
Process Improvement
Minor deviations in pH readings were noted when compared to published literature. This discrepancy likely resulted from calibrating the pH instrument at room temperature but recording measurements between 5 °C and 15 °C. Future experimental trials must ensure the pH instrument is calibrated within the experimental temperature range.
Actionable Recommendations
For the proposed acetic acid production plant to be most efficient, a spontaneous reaction is desired so the reaction proceeds without continuous energy input.