Thermodynamics and Kinetics, Possibility and Actuality

摘要

A typical kinetic experiment, the persulfate–iodide clock reaction, is selected as an example to illustrate applications of thermodynamic possibility and kinetic actuality in chemical reactions. In the experiment, the strongest reducer, sodium thiosulfate Na2S2O3, cannot be directly oxidized by the strongest oxidizer ammonium persulfate (NH4)2S2O8, and this redox must be realized by an intermediate iodine I2 [presenting in I3- anion, a weaker oxidizer than (NH4)2S2O8]. After I2 molecules being reduced by Na2S2O3 molecules to form I- ions (a weaker reducer than S2O32-), the strongest oxidizer begins to oxidize the weaker reducer I- ions to form I2 molecules again, which is keeping on the redox circles until the stronger reducer Na2S2O3 has been exhausted completely. Why the strongest reducer was not being oxidized by the strongest oxidizer? In this paper we attempt to explain this situation based on the chemical theories of thermodynamics and kinetics.