パソコンと自作投げ込み式吸光度計による反応速度の測定(II) : フェノールフタレインの平衡移動反応についての反応速度解析

抄録

Phenolphthalein changes to colorless from red in strong alkali. This is because a quinoid type diavalent anion (red) becomes a trivalent anion (colorless). The change in the red color of this reaction could be analyzed by the combination of a personal computer and a throwing type colorimeter, as I mentioned in my former article. If the preparation for this experiment is done in advance, all experiments are completed in two-period school hours. We can get the rate of this reaction, concentration dependence, temperature dependence, activated energy, equilibrium constant and so on. The following four results were found. First, the reaction rate became faster in proportion to the concentration of phenolphthalein, and increased about 1.7 times by a temperature rise of 10℃. Second, the activated energy seen for the direct reaction was 51kJ/mol, and the activated energy seen for the inverse reaction was 97kJ/mol. Third, the heat of this reaction was +46kJ/mol, and this reaction was a exothermic reaction. Fourth, the equilibrium was moved to the left by the temperature rise of the solution, and the equilibrium constant decreased.