5.6.1 Temperature Dependence
of the Rate Constant, k

1. 2.
Experimental setup Reaction begins
3. 4.
30 seconds later 60 seconds later
Animation of Experiment (MPEG Format, 454 k)
Animation of Experiment (GIF Format, 467 k)

Photo 1: Experimental setup: Two test tubes, one with 1 molar iron(III) chloride solution and the other with 1 molar sodium thiosulfate solution, are placed in a water bath using a glass beaker and heated to given temperature, here 40°C. When the given temperature is reached, the solution from one test tube is poured into the other, and the empty test tube is removed.  The same is done simultaneously at two different temperatures, at 20° higher and 20° lower. The final setup is as follows:

Left: 20°C
Middle: 40°C
Right: 60°C

Photo 2: Iron(III) ions are reduced by sulfates to iron(II) ions:

2 Fe3+ + 2 S2O32-® 2 Fe2+ + S4O62-

The reduction has a darkly colored intermediary complex, whose decay is observed here.
Photo 3: After 30 seconds, the solution in the 60°C water bad has become almost completely colorless. A slight coloration is still visible in the solution at 40°C.
Photo 4: After 60 seconds, the solution in the 40°C water bath has likewise turned colorless. The solution in the 20°C water bath shows the first signs of a reaction after more than 10 minutes.
The speed of all chemical reactions rises with an increase in temperature. The temperature dependence of the rate constant k can be described quantitatively using Arrhenius' equation:

k = A×exp (-Ea / RT)

A is a characteristic constant for the reaction under consideration and Ea is the activation energy of the reaction. As a rule of thumb, a temperature increase of 10° causes a the reaction speed to double.

Exercise 5.6.1:
Are there reactions whose speed is independent of temperature?