Chemical clock

In experiments that are so similar to tricks, colorless solutions were painted in one or another color, and this happened immediately, as if by magic. Indeed, chemical reactions proceed very quickly and, as a rule, begin immediately after mixing the reagents. However, there are exceptions to this rule. The reaction mixture may remain colorless for a while, and then instantly turn color. Want - in five seconds, want - in ten; you yourself can set the "chemical clock" to the required time.

Want - in five seconds, want - in ten; you yourself can set the chemical clock to the required time

Prepare two solutions. The composition of the first: 3,9 g of potassium iodate KIO3 per liter of water. The composition of the second: 1 g of sodium sulfite Na2SO3, 0,94 g of concentrated sulfuric acid (carefully!) And a little, a few milliliters of starch paste - also per liter of water. Both solutions are colorless and transparent.

Measure out 100 ml of both solutions and quickly, preferably with stirring, add the second to the first. It is more convenient to put the experience together - let your friend immediately start counting the time using a stopwatch or a clock with a second hand. After six to eight seconds (the exact time depends on the temperature), the liquid will instantly turn dark blue, almost black.

Now measure again 100 ml of the second solution, and dilute 50 ml of the first with water exactly twice. With a stopwatch in hand, you will be convinced that the time elapsed from the moment the solutions are drained until they are stained will also double.

Finally, mix 100 ml of the second solution with 25 ml of the first, diluted four times with water, that is, to the same 100 ml. The "chemical clock" will run four times longer than in the first experiment.

This experiment demonstrates one of the fundamental chemical laws - the law of mass action, according to which the reaction rate is proportional to the concentrations of the reacting substances. But here's the question: why do the solutions color instantly after a pause, and not evenly and gradually, as you might expect?

Sulfuric acid in solution displaces iodate and sulfite ions from their salts. In this case, hydroiodic acid HI is formed in the solution, but it does not live long and immediately interacts with iodic acid HIO3. As a result, free iodine is released, it is it that gives a color reaction with starch.

If everything went like this, then the solution would darken gradually, as iodine was released. However, another process is going on in parallel: sulfurous acid H2SO3 reacts with free iodine and hydroiodic acid is again formed. This reaction proceeds faster than the previous one, and iodine, not having time to color the starch, is again reduced to IO3-.

It turns out that the color should not appear at all? Please note: during the reaction, sulfurous acid is continuously consumed, and as soon as all of it turns into sulfuric, iodine will no longer interfere with reacting with starch. And then the solution will instantly color throughout the entire volume.

By diluting the solution by half and four times, you decreased the concentration of potassium iodate, and the reaction rate decreased proportionally.

The explanation seems to have taken longer than the experiment with the chemical clock itself...