Iodine clock reaction

The iodide clock reaction is a chemistry experiment designed to illustrate the different rates at which chemical reactions occur. The experiment can be performed several different ways; each variation involves an iodine solution that is mixed with a reactive solution. When the solutions are combined, the liquid is initially colorless. After a few seconds, the solution spontaneously turns dark blue. The experiment is called a “clock” reaction because if the proper amounts of chemicals are used, the reaction will always occur after a predictable period of time. The reaction was first observed in 1886 by Swiss chemist Hans Heinrich Landolt. For that reason, it is sometimes referred to as the Landolt reaction.

Background

Chemical bonds are created through the interaction between electrons in the outer shell of an atom. Electrons from one atom can be shared with electrons in another atom, bonding those atoms together to form a molecule. A chemical reaction occurs when the chemical bonds between atoms in two or more substances are broken down and rearranged, resulting in the formation of a new chemical molecule or molecules. If an atom gains an electron during the process, that atom becomes negatively charged. Such atoms are referred to as anions and are renamed using the suffix -ide.

Iodine is a naturally occurring chemical element that plays an important role in regulating human metabolism. Iodine is necessary for maintaining a healthy thyroid gland, but in its elemental, or pure form, iodine is corrosive. In large doses, it can be toxic. Elemental iodine becomes safe for humans when it chemically bonds with another molecule, such as potassium or sodium. When this happens, it becomes known as iodide. For example, iodized salt is the chemical combination of iodine with the element sodium. Its chemical name is sodium iodide.

Overview

Chemical reactions occur at different speeds depending on the properties of the molecules involved and the conditions under which the reactions take place. Scientists and chemistry teachers often use the iodine clock reaction to demonstrate how different reactions occur at different rates. The experiment uses an iodine solution and a reactive agent that contains starch, a complex carbohydrate that can be found in foods such as potatoes, wheat, and corn. The reaction can be accomplished using several different substances, as long as those substances contain the proper base chemicals.

A common method of demonstrating the iodine clock reaction uses products that can be found in the home or products that are easily purchased. The experiment requires distilled water, a 1000-milligram vitamin C tablet, hydrogen peroxide, liquid laundry starch, and an iodine solution. Iodine is typically found in most drug stores as an antiseptic. It is not pure iodine but is dissolved in a solution of water and alcohol. Laundry starch is made from grain products such as corn, rice, or wheat. Although the elements needed for the experiment are relatively safe, iodine and hydrogen peroxide can cause stains that are difficult to remove, so they should be used with care.

A crushed vitamin C tablet should be dissolved in 2 ounces of water. To make the first solution, 1 teaspoon of this mixture is added to 1 teaspoon of iodine and 2 ounces of water. The second solution is made by mixing 2 ounces of water, 3 teaspoons of hydrogen peroxide, and 1/2 teaspoon of starch. The laundry starch contains sulfuric acid, which combines with the hydrogen peroxide to make a reacting agent.

The first solution is poured into the second solution and then transferred to an empty cup. The combined solutions should be poured back and forth several times between cups to mix them thoroughly. When the solutions are mixed, two forms of iodine are created: elemental iodine and an iodide formed by iodine bonding with the vitamin C. The resulting liquid will be clear at first; however, within a few seconds, it will suddenly turn a very dark blue.

This effect is caused by two separate chemical reactions occurring in the mixed solutions at the same time. In the first reaction, the hydrogen peroxide reacts with the iodide molecules from the vitamin C/iodine solution to produce elemental iodine. This form of iodine turns dark blue when exposed to starch. However, the liquid maintains its clear color because a second reaction consumes the elemental iodine as quickly as it is produced. This second reaction is caused by a chemical called sodium thiosulfate that is produced by the vitamin C.

This second reaction occurs much faster than the first reaction. As soon as elemental iodine is created by the first reaction, it is chemically changed by the second reaction. Because the second reaction is faster, the sodium thiosulfate is exhausted more quickly. When this happens, the second reaction suddenly stops, and the first reaction takes over. This allows the hydrogen peroxide to create elemental iodine without interference. The iodine reacts with the starch and turns dark blue.

If the proper amounts of chemical ingredients are used in the experiment, the color change will occur in a predicable amount of time. If the amount of hydrogen peroxide is increased, the first reaction will be sped up, and the time between color changes will be shorter. If more vitamin C is added, the second reaction will be extended and the color change will take longer.

Variations on the experiment can be done with other chemical ingredients; however, many of these ingredients are more dangerous to use or found primarily in professional environments such as chemistry labs. Chemists often use pure forms of the chemical ingredients to perform the experiment. For example, the experiment can be done directly with sulfuric acid and sodium thiosulfate, rather than getting those substances from starch and vitamin C tablets.

Bibliography

“Experiment 6: The Rate Laws of an Iodine Clock Reaction.” Community College of Baltimore County, faculty.ccbcmd.edu/~cyau/124%2006%20Iodine%20Clock%20Kinetics%202012.pdf. Accessed 18 Mar. 2019.

Harrington, Rebecca, and Dave Mosher. “Make a Clear Liquid Turn Black in Midair with this Bizarre Science Experiment.” Business Insider, 5 Dec. 2015, www.businessinsider.com/how-iodine-clock-experiment-works-ingredients-2015-12. Accessed 18 Mar. 2019.

Group, Edward. “How Are Iodide and Iodine Different?” Global Healing Center, 25 Feb. 2015, www.globalhealingcenter.com/natural-health/how-are-iodide-and-iodine-different/. Accessed 18 Mar. 2019.

“Iodine.” Office of Dietary Supplements, 17 Feb. 2016, ods.od.nih.gov/factsheets/Iodine-Consumer/. Accessed 18 Mar. 2019.

“Iodine Clock Reaction.” Discovery Express, 16 Feb. 2018, www.discoveryexpresskids.com/blog/iodine-clock-reaction9340827. Accessed 18 Mar. 2019.

“Iodine Clock Reaction.” Imagination Station, www.imaginationstationtoledo.org/educator/activities/iodine-clock-reaction. Accessed 18 Mar. 2019.

“Iodine Clock Reaction.” Nuffield Foundation and the Royal Society of Chemistry, October 2015, www.rsc.org/learn-chemistry/resource/res00000744/iodine-clock-reaction?cmpid=CMP00005152. Accessed 18 Mar. 2019.

“Iodine Clock Reaction.” Tennessee Technological University, 2019, www.cae.tntech.edu/~dcashman/demos/demo‗iodine-clock.html. Accessed 18 Mar. 2019.

"The Iodine Clock Reaction." LibreTexts, 31 Oct. 2024, chem.libretexts.org/Ancillary‗Materials/Laboratory‗Experiments/Wet‗Lab‗Experiments/MIT‗Labs/Lab‗3%3A‗Chemical‗Kinetics/1‗-‗The‗Iodine‗Clock‗Reaction. Accessed 20 Nov. 2024.