ABSTRACT
This is what might be termed a plausibility argument, in that what is known about the temperature dependence of the equilibrium constant, and the relationship between it and the reaction rate constants, makes the suggestion of the Arrhenius equation for the rate constants entirely reasonable and consistent with the known facts. Arrhenius’s subsequent contribution was that he felt that the temperature dependence of colliding atoms or molecules in gases and liquids was too small to give the observed values of activation energies. So he proposed an equilibrium between reactive molecules and the others based on the van’t Hoff equation (Laidler 1984). Of course, later experiments have shown that reaction rate constants do follow an Arrhenius equation and are indeed exponentially dependent on 1/T (there is often a mild temperature dependence of the preexponential term as well). Figure 4.2 shows the exponential temperature dependence for the second-order reaction rate constant (note the units for k) for the forward reaction of H g I g HI g2 2 2( ) ( ) ( )+ . In this case (NIST 2013),
k cm mol s e kJ mol RT
1 94 10= × ( )− − −. /
Note that the reaction rate increases by about 11 orders of magnitude over a span of only about 400°C. By measuring the rate of a reaction as a function of temperature, the two important parameters of the rate equation, Q and k0, can be determined.
