Spilling the “Tea” about Chemical Equilibrium and Le Châtelier’s Principle

Abstract

Tea has been implemented in various educational laboratories, often to demonstrate Le Châtelier’s principle. The color of tea can primarily be attributed to thearubigin species, and is an equilibrium between the lightly colored, weak acid reactant (TR) with a darkly colored anion (TR^–) and hydronium ion products. This system responds to the addition of an acid or base, thus allowing students to observe the response visually. Here, we have extended this concept to a chemical education lab experiment where students can quantitatively determine the chemical equilibrium constant of thearubigins in tea. This experiment allows students to gain experience with unit conversions, ICE tables, pH measurements, absorbance spectroscopy, calibration curves, and calculations of K_eq. Students measure the pH of tea to be 4.7–5.7 and by assumptions with respect to the concentrations of thearubigin species at equilibrium, students calculate an equilibrium constant (K_eq) of the aqueous thearubigin species of 3.6 × 10^–6 – 3.5 × 10^–5. Ultimately, students demonstrate qualitative and quantitative understanding of how applying Le Châtelier’s principle to an equilibrated system at a constant temperature does not alter the equilibrium constant.