Sodium + Chlorine:
Pass the Salt, Please
Sodium is a silver-colored metal which is soft enough to cut with aknife. It is an extremely reactive metal, and is always found naturally inionic compounds, not in its pure metallic form. Pure sodium metal reactsviolently (and sometimes explosively) with water, producing sodium hydroxide,hydrogen gas, and heat:
2Na(s) + 2H2O(l) ——>2NaOH(aq) + H2(g)
Chlorine is apoisonous, yellow-green gas, with a very sharpodor, and was used in gas warfare during World War I.
Sodium and chlorine react with each other, however, to produce a substancethat is familiar to almost everyone in the world: sodium chloride,or table salt:
2Na(s) + Cl2(g) ——>2NaCl(s)
It is easy to see why this reaction takes place so readily when we look at iton an atomic level: sodium has one electron in its outermost (valence)shell, while chlorine has seven electrons in its valence shell. When asodium atom transfers an electron to a chlorine atom, forming a sodium cation(Na+) and a chloride anion (Cl-), both ions have completevalence shells, and are energetically more stable.
The reaction is extremely exothermic, producing a bright yellow light and agreat deal of heat energy.
In the following demonstrations, a 2.5 liter bottle is filled with chlorinegas. A coating of sand on the bottom of the bottle absorbs some of theheat energy produced during the reaction, and prevents it from breaking. Asmall piece of freshly-cut metallic sodium is placed in the flask, and then asmall amount of water is added, which reacts with the sodium and causes it tobecome hot. The hot sodium then reacts with the chlorine, producing abright yellow light, a great deal of heat energy, and fumes of sodium chloride,which deposits on the walls of the bottle.
In the first video clip, the sodium flares up almost immediately uponreaction with the water, and "burns out" quickly. (Don't blink,or you'll miss it.) In the second, water is added twice, to produce oneshort flash, followed by a much longer one. (This reaction can also be done with molten sodium, but I've never been braveenough to try that.)
Video Clip 1: REAL, 679 KB
Video Clip 2: REAL, 1.74 MB
 |  |
 |  |
 |  |
 |  |
 |  |
 |  |
!!! Hazards !!!
Do not expose sodium metal to water! The reaction of sodium and waterproduces hydrogen gas and heat, which is not a good combination! Sodiummust be stored under mineral oil, or some other high-molecular weighthydrocarbon.
Chlorine gas is toxic, and extremely irritating to the eyes and mucousmembranes.
This reaction evolves a great deal of heat energy; make sure that the flaskhas no cracks in it.
Procedures
Bassam Z. Shakhashiri, Chemical Demonstrations: A Handbook for Teachers of Chemistry, Volume 1. Madison: The University of Wisconsin Press, 1983, p. 61-63.
Lee R. Summerlin, Christie L. Borgford, and Julie B. Ealy, Chemical Demonstrations: A Sourcebook for Teachers, Volume 2, 2nd ed. Washington, D.C.: American Chemical Society, 1988, p. 56-57.
References
John Emsley, The Elements, 3rd ed. Oxford, Clarendon Press, 1998, p. 56-57, 194-195.
David L. Heiserman, Exploring Chemical Elements and their Compounds. New York: TAB Books, 1992, p. 43-48, 70-74.
Martha Windholz (ed.), The Merck Index, 10th ed. Rahway: Merck & Co., Inc., 1983.
