Dark red iron[III] solution is rapidly reduced to colorless iron[II] by addition of tin[II] chloride solution, with the rate depending on concentration and temperature.
Four medium footed cylinders are prepared with 100 ml of ferric chloride solution 0.01M with potassium thiocyanate solution added to make the dark red complex. One of the solutions should be hot, so just the iron and thiocyanate solutions in that cylinder, with a 150ml beaker for the demonstrator to fill with hot water from the water cooler, right before the demonstration.
The cylinders are on the bench top in pairs, in front of a light box. Next to the pair of cylinders for comparing the effect of concentration on reaction rate are two 150ml beakers containing 100 ml of tin[II] chloride solution, 0.05M and 0.10M, marked 1X and 2X respectively.
The pair of cylinders for comparing the effect of temperature on reaction rate have next to them two 150ml beakers containing 100 ml tin[II] chloride, 0.05M, both marked 1X.
Swirl both of the iron[III] solutions in front of the light box, then add both beakers of tin[II] solution simultaneously. Put down the beakers and give the cylinders a quick swirl to assure mixing. The slow reaction (room temperature and 1X conc.) takes about a minute to go to completion. 2X conc. is about twice as fast, and the hot cylinder decolors in seconds.
A recent variation for Prof. Aspuru-Guzik and PS1 involved timing the addition of three concentrations of the tin[II]. The solutions were based on a stock solution of 50 g SnCl2 in 450 ml water and 50 ml conc. HCl. Small pieces of tin metal were added, and the stock solution allowed to settle covered overnight.
The tin[II] aliquots were 1X with 30 ml stock and 60 ml water, 2X with 60 ml stock and 30 ml water, and 3X at 90 ml stock. The iron[III] solution was at the standard concentration above. Times for 1X to fade to yellow was 60 seconds, 2X faded in 15 seconds, and 3X faded in 6 seconds. These times suggest a second order rate dependence on tin[II] concentration.