Chemistry of Arsenic

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Natural Sources

Arsenic is found in the earth’s crust, most commonly in the form of FeAsS, or Iron Arsenide Sulfide. Arsenic can also be found in the atmosphere as arsenic trioxide dusts, a by-product of industrial smelting operations. Arsenic is usually very toxic, especially in the cases of inorganic arsenic compounds such as arsenic trichloride and arsenous acid. Arsenic compounds have also found their way into rivers and wells, contaminating the drinking water of millions of people. Due to numerous reactions that can occur with Arsenic, this element can be encountered in various phases.

Reactions

With hydrogen:  Arsenic trioxide can be reduced by hydrogen gas to form arsine. Arsine, AsH₃, is a very toxic gas that is denser than air. When arsine is heated, it breaks down to arsenic and hydrogen.

• Reaction of arsenic trioxide with hydrogen gas to form water and arsine:

With oxygen:  Solid arsenic is oxidized when exposed to oxygen. The surface of the metalloid becomes black. When heated in oxygen gas, arsenic binds with oxygen to form tetraarsenic decaoxide, or arsenious oxide.

• Reaction of arsenic with oxygen gas to form arsenious oxide:

With halogens:  Arsenic reacts well with halogens with the exception of astatine. Arsenic pentafluoride is a highly toxic colorless gas. When liquid arsenic trichloride is cooled with free chlorine molecule, arsenic pentachloride can be formed. Solid arsenic with bromine gas forms a light yellow solid called arsenic tribromide.

• Reaction of arsenious oxide with hydrochloric acid to form water and arsenic trichloride:

• Reaction of arsenic trichloride with Cl2 to form arsenic pentachloride:

• Reaction of solid arsenic with bromine gas:

With water: Pure arsenic is insoluble in water; however, many arsenic compounds dissolve easily in water. Arsenic in the form of arsenic trisulfide, and arsenic acid are examples of arsenic compounds that have found their ways into water supplies.

• Reaction of arsenic trisulfide with water to form hydrogen sulfide and arsenious acid:

                                                                  (Sample of solid Arsenic)

Isotopes of Arsenic

The atomic mass of Arsenic listed on the periodic table is 74.92 u. Arsenic has over 33 different isotopes, ranging from 60 to 92 in atomic mass. However, only one of these isotopes is stable, which is ⁷⁵As. Therefore, Arsenic is a monoisotpoic element. Like the rest of the nitrogen group, Arsenic has five valence electrons. In group five, Arsenic is most closely related to antimony, both being metalloids. Since nitrogen and phosphorus are non-metal, they share very little commonalities with arsenic and antimony. As with bismuth being a P-block metal, they too share very little similarities.

Old Usage

Arsenic in the 1800s was commonly used in the form of copper arsenite, a green compound that was used as a pigment to color wallpaper and paint.  This caused a lot of health problems. As mold and bacteria fed on the wallpaper and paint, they released trimethylarsine, C₃H₉As, which became airborne and inhaled. This would lead to arsenic poisoning and in some cases, death. It was substituted by copper carbonate by the end of the 1800s as the primary green pigment.

Since Arsenic is toxic to most organisms, it is praised as an herbicide, and pesticide. For example, copper arsenate, Cu₃(AsO₄)₂, mixed with chromium is a pesticide used to treat wood. As of 2004, CCA, chromium copper arsenate, has been put into restricted use by the EPA. In the 1900s copper acetoarsenite, or Paris Green, was used as an insecticide and fungicide in many agricultural farmland. Agent blue is one of the herbicides used during the Vietnam War by the US to destroy Vietnam rice farm. It is a blend of cacodylic acid, (CH3)2AsO2H, and sodium cacodylic, an arsenic salt.

Modern Usage

Since arsenic is a semi-metal, arsenic in the form of gallium arsenide is now used as a semiconductor for transistors and other electronic devices. It is used to make LED, light emitting diodes, and in some cases, it can substitute silicon in integrated circuits.

                                                   (Various Colored LED Lights)

Problems

1. What is Gallium Arsenide now used for in the electronic industry? 

2. Complete and balance: 2As + 5F₂ --> ?

3. True or false: Arsenic is more like antimony than phosphorus. 

4. True or False: Arsenic trioxide is a gas. 

5. Complete and balance: As(s) + H₂O(l) --> ? 

Solutions

Highlight to see: (Easy-Medium-Hard)
1. Gallium arsenide is now used for transistors. (H)
2. 2As+5F₂->2AsF₅ (E)
3_. True, both arsenic and antimony are semi-metals. (M)
4. False, arsenic trioxide is a solid. It can, however, be disperse in the air as dust. (M)
5. No reaction. Solid arsenic does not react well with water. (M)

References

1. Jones, Harry. Principles of inorganic chemistry. New York: The MacMillan Company, 1903. Print.
2. Mellor, Joseph. Modern inorganic chemistry. New York: Longmans, Green, and Company, 1912. Print.
3. Young, Jay A. “Arsenic (III) Chloride”   J. Chem. Educ., 2006, 83 (2), p 207

Outside Links and Online References

• http://en.wikipedia.org/wiki/Arsenic
• http://www.logitech.uk.com/gallium_arsenide.asp
• http://www.greenfacts.org/en/arsenic...rsenic-2.htm#1
• http://en.wikipedia.org/wiki/Arsenic_triselenide
• http://en.wikipedia.org/wiki/Arsenic_trioxide
• http://www.pesticide.org/hhg/arsenicinpesticides.html
• http://www.landscaper.net/agent.htm
• http://cameochemicals.noaa.gov/chemical/4468
• Image of Solid Arsenic: http://upload.wikimedia.org/wikipedia/commons/7/7b/Arsen_1a.jpg
• Image of LED Lights: http://upload.wikimedia.org/wikipedia/commons/c/cb/RBG-LED.jpg
• Electron Diagram: http://upload.wikimedia.org/wikipedia/commons/8/8d/Electron_shell_033_Arsenic.svg

Contributors

• Freddie Chak
• Jonathan Molina
• Tiffany Lui, University of California, Davis

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