Question

The iodine clock reaction involves reacting an aqueous solution of potassium iodate with another solution containing sulfuric acid, sodium bisulfite, and starch. The reaction begins colourless but at a certain point the free triiodide concentration builds up, complexing with the starch molecules and producing a dark blue to almost black colour almost instantaneously. Which of the following options will cause the blue colour to appear faster?

Increasing pH

Increasing bisulfite concentration

Increasing starch concentration

Decreasing temperature

Increasing temperature

Answer 1

Answer : increasing temperature

In iodine clock reaction, the blue color is due to the formation of triiodide-starch complex.

IO3- + 3HSO3- -----> I- + 3HSO4-

The iodate reacts with bisulphate to form iodide.

IO3- + 5I- + 6H+ -----> 3I2 + 3H2O

The remaining iodate ions again react with iodide to give triiodine.

However the iodine again reacts with bisulphate to give iodide.

I2 + HSO3- + H2O ----> 2I- + HSO4- + 2H+

When bisulphate gets over, iodine remains as such and complexes with starch to give blue color.

Increasing pH would give less H+ ion which would slow down the reaction. Increase in bisulphate concentration makes bisulphate react for a longer time to change I2 into I- and this would slow down the reaction. Increasing starch concentration has no effect because the blue color would appear only if bisulphate gets over in the reaction.

Increasing the temperature would speed up the reaction and the blue color would appear faster.