For the complex equilibrium of cobalt(II) chloride in hydrochloric acid, the equilibrium constant at the color-change temperature between the octahedral complex [CoCl(H2O)5]+ (pink) and tetrahedral complex [CoCl2(H2O)2] (deep blue) is inversely proportional to the initial concentration of hydrochloric acid. By measuring the color change temperatures for a series of solutions with different initial concentrations of hydrochloric acid, the temperature-dependent change in the equilibrium constant for the color-change-reaction step can be estimated. By plotting logarithmic equilibrium constant against reciprocal temperature according to the van’t Hoff equation, the value of DH2º for the color-change reaction step can be calculated from the slope of the van’t Hoff plot. The present simple experiment makes it possible for students to measure the temperature effect on the equilibrium shift in a quantitative way, which is then further extendable as an introductory experiment in chemical thermodynamics by the analysis of the temperature dependence of the equilibrium constant according to the van’t Hoff equation.