The 1827 Christmas Lectures
of Michael Faraday

Demonstration of Lecture #5 Today

Sulphur, Phosphorus, Carbon and their Acids

SULFUR

The beginning of this lecture will deal with sulfur. Sulfur is a well known combustible solid, yellow in color, that exists in at least six different allotropic forms. It can assume a negative electrical charge when it is rubbed or heated. It is not soluble in water, and has a density of 1.99 g/mL. The various molecular forms assumed are a function of temperature. At temperatures up to 95.5°C, there are sixteen S₈ rings to the unit cell. This is called rhombic sulfur. When the temperature range is 96°C to 119°C, the monoclinic molecular state is present consisting of six S₈ rings in a unit cell. From 119°C to 160°C sulfur becomes a yellow liquid as the S₈ ring opens and long spiral chain molecules are formed. The chains break from 160°C to 444°C as a viscous, amber fluid is formed. If the molten sulfur is poured into cold water, plastic sulfur is produced which will convert into the rhombic form on standing. At temperatures above 445°C, vaporous sulfur with a molecular formula varying from S₂ to S₁0 exists, with the S₂ form predominating.

Sulfur burns in air with a blue flame producing a particularly choking odor. It will burn more brilliantly in oxygen gas. Sulfur dioxide and sulfur trioxide are both formed as products of this combustion. When added to water, they become Arrhenius acids, both contributing hydrogen ions which combine with water to form hydronium ions. The resulting substances, sulfurous acid and sulfuric acid, differ in strength. Sulfurous acid is considered to be weak because it is only partially disassociated in water solution. The oxides may be classified as Lewis acids since they are both electron acceptors. Therefore, the oxides of sulfur can function as acids in the gaseous as well as the aqueous state.

Sulfurous and sulfuric acids react with bases to form a salt and water. Sulfur dioxide and sulfur trioxide react with bases to form salts. These reactions can be tracked with halochromatic natural dyes. Representative reactions are:

H₂SO₄ (aq) + 2 NH₄OH (aq) --> (NH₄)2SO₄ (aq) + 2 H₂O (l)
H₂SO₃ (aq) + 2 NH₄OH (aq) --> (NH₄)2SO₃ (aq) + 2 H₂O (l)
NH₄⁺ (aq) + H₂O (l) --> NH₃ (g) + H₃O⁺ (aq)
SO₃ (g) + NH₃ (g) --> H₂NSO₃H (s) (sulfamic acid)

Lab. 1-4: Prepare 3 different allotropes of sulfur; Compare the physical structures of these allotropes.

MATERIALS	ring-stand, ring, gauze, Bunsen burner, 6 inch test-tubes, test-tube rack, beakers (50, 100, 250 mL) watch glass, dropper, forceps, magnifying glass or stereoscopic microscope, filter paper, olive oil, powdered sulfur, distilled water.
PROCEDURES	Three mL olive oil in 50 mL beaker plus a small scoop of sulfur. Heat gently with stirring for 10-12 seconds. Place a few drops of the warm solution on a watch glass and set aside for 20 minutes. Add 2 inches of sulfur to test tube. Fold a piece of filter paper for filtering. Heat the tube slowly allowing sulfur to melt. Stop when sulfur is a pale yellow liquid. Do not overheat or allow sulfur to turn red. Quickly pour the melted sulfur into the paper funnel. Let cool. Carefully unfold the paper, filter and examine the sulfur. Be careful: Hot sulfur burns! Save crystals. To the same tube from Part B, add 2 inches of sulfur. Heat until sulfur begins to boil and turns red. Rapidly pour sulfur into a 250 ml beaker (3/4 full) of water. [For students] Examine all sulfur samples from Parts A - C with a magnifying glass or stereoscopic microscope. Describe and sketch each allotrope of sulfur.
HAZARDS & DISPOSAL	Take care not to overheat sulfur. Sulfur dioxide (SO2) is toxic. Take care not to burn hands. Dispose of suflur in trash can.
DISCUSSION	Part A produces orthorhombic crystals. Part B produces monoclinic crystals. Part C produces plastic sulfur. This demonstration can be used to enrich discussionsof crystals and allotropes.
Reference	Wilbraham, A., Staley D., Simpson, C., Matta, M. Addison-Wesley Chemistry Laboratory Manual, Addison-Wesley Publishing Company, Menlo Park, California, 1987, pp.293-296.

Lab. 5-10: Produce and test some properties of sulfur oxides.

MATERIALS	Flowers of sulfur, deflagration spoon, two wide mouth bottles with stoppers to fit, red cabbage extract, aqueous ammonia (1M), one long stem rose.
PROCEDURES	In bowl of a deflagration spoon, burn sulfur in a wide mouth bottle until the flame is extinguished. Stopper the bottle. Repeat with second bottle. Open stopper on one bottle and insert rose until bleached. Add 100 ml of water. Add 5 ml red cabbage extract. Add 1M ammonium hydroxide. Remark on color changes. To the second bottle, add 5 ml of red cabbage juice. Then, add the contents of this bottle to the first bottle. Remark on all color changes.
HAZARDS & DISPOSAL	The sulfur dioxide (SO2) is poisonous. Avoid breathing it. Disposal: down sink and trash in can. Residual sulfur and deflagrating spoon can be burned in air in a hood.
DISCUSSION	S + O2 --> SO2 SO2 + H2O--> H2SO3 H2SO3 <--> H+ + HSO3- H2SO3(aq) + 2 NH4OH(aq) --> (NH4)2SO3(aq) + 2 H2O [Draw structures and name them]. XOH + H+ <--> X+ + H2O [The indicator changes color: XOH is a different color from X+]

PHOSPHORUS

The next nonmetal to be considered is phosphorus. Phosphorus occurs as a soft solid that can be cut with a knife or as a powder. It has a density of 1.82 g/mL. There are three different types: red phosphorus, yellow or white phosphorus, and black phosphorus. Although phosphorus is slightly soluble in water, it is readily soluble in ether, oil, and carbon disulfide.

Chemically, phosphorus undergoes spontaneous combustion in air burning with a brilliant flame and yielding the toxic diphosphorus pentoxide. This diphosphorus pentoxide produces phosphoric acid when dissolved in water. If the combustion is incomplete, the product is diphosphorus trioxide which forms phosphorus acid in water.

Bone and teeth which contain calcium phosphate are sources of phosphorus. Phosphorus is usually obtained by rigorous heating of these substances and calcium phosphate mixed with sand together with carbon.

Another compound of phosphorus, produced by combining phosphorus and hydrogen, is phosphine gas. This gas is highly combustible and also highly toxic.

Lab. 22-24: Demonstrate the combustion of phosphorus to yield diphosphorus pentoxide; Show that diphosphorus pentoxide dissolves in water to yield an acid.

MATERIALS	Two test tubes (6 inch) with corks. Ring stand, clamp for test tube, deflagrating spoon, Bunsen burner, small quantity of red phosphorus, distilled water, solution of methyl orange indicator.
PROCEDURES	Fill the test tubes 1/3 full of distilled water. Add 3 drops of methyl orange to each tube. Cork one and set it aside as reference. Place the second tube in the clamp on the ring stand in the fume hood. Using the Bunsen burner, ignite a small piece of phosphorus on the deflagrating spoon. Immediately place the burning phosphorus just above the level of water in the test tube. When combustion is complete, cork the tube and shake. Compare the color with the reference tube.
HAZARDS & DISPOSAL	Phosphorus pentachloride is toxic and caustic. Do not expose any part of your body to it. Dispose of all liquids in sink. Make sure that all phosphorus on the spoon has been completely burned away.
DISCUSSION	P4(s) + 5 O2(g)--> P4O10(s) P4O10(s) + 6 H2O(l) --> 4 H3PO4(aq)
References	D.E.C. Corbridge, Phosphorus, An Outline of its Chemistry, Biochemistry, and Technology, Elsevier, New York, NY, 1990, pp 8,12, 179-180. The Staff of the Division of Chemical Engineering, Thad D. Farr, compiler, Phosphorus, Properties of the Element and Some of Its Compounds, U.S. Printing Office, Washington, D.C., 1950. Y. Abe, R. Ebisawa, D.E. Clark, L. Liottench, ``Photo-and Thermo-Coloring of Reduced Phosphate Glasses'', in Louis D. Quinn and John G. Verkade, eds., Phosphorus Chemistry, Proceedings of the 1981 International Conference, American Chemical Society, Washington, D.C., 1981, pp. 371-375.

CARBON

The third nonmetal considered in this lecture is carbon which is found in a variety of forms. Wood charcoal, coke, graphite, even diamond and the recently discovered buckminsterfullerene are all forms of carbon. Carbon is a major component of living things. When carbon is completely combusted, carbon dioxide is formed.

Carbon dioxide is a colorless, odorless, gas that extinguishes a flame. The presence of the gas can be detected by bubbling it through limewater. A precipitate of calcium carbonate appears. Carbon dioxide is also formed in the naturally observed phenomenon of fermentation.

There are also several compounds of carbon and hydrogen. One of these is methane, which is known as natural gas.

Lab. 34: Observe the combustion of paper.

MATERIALS	Paper and matches
PROCEDURE	Light end of paper with match.
HAZARDS & DISPOSAL	In garbage.
DISCUSSION	CmH2nOn + O2 --> CO2 + H2O A well known instance in common wood charcoal - burn wood & then when done flaming, quench it in water.

Lab. 36: Observe the decolorizing ability of charcoal.

MATERIALS	Grape juice and charcoal.
PROCEDURE	Place a spatula of charcoal in a glass of grape juice. Filter the solution.
HAZARDS & DISPOSAL	None
DISCUSSION	It absorbs colors and odors. It cleanses putrid substances.

Lab. 37: Demonstrate the combustion of charcoal in pure oxygen.

MATERIALS	Charcoal and bottles of oxygen.
PROCEDURE	Heat charcoal in a deflagrating spoon to redness, and lower deflagrating spoon into bottle of pure oxygen. Oxygen can be prepared by the decomposition of 3% H2O2. The catalyst MnO2 will be used to speed up the decomposition.
HAZARDS & DISPOSAL	flammability precaution
DISCUSSION	It absorbs much water and air but suffers no other change.

Lab. 38: Demonstrate the production of carbonic acid.

MATERIALS	Carbon dioxide and water.
PROCEDURE	Dissolve carbon dioxide in water to form carbonic acid.
HAZARDS & DISPOSAL	None
DISCUSSION	CO2 + H2O <--> H2CO3 H2CO3 <--> H+ + HCO3-

Lab. 40: Illustrate that carbon dioxide does not support combustion.

MATERIALS	Carbon dioxide and lighted taper.
PROCEDURE	Expose flaming taper to container of carbon dioxide.
HAZARDS & DISPOSAL	None
DISCUSSION	Bottles and jars of it. The gas is colorless and transparent - pungent odor when pure - extinguishes flame.

Lab. 41-42: Demonstrate the weight/density of carbon dioxide.

MATERIALS	Two jars of carbon dioxide.
PROCEDURE	Carbon dioxide remains in upright jar - taper - then pour it into another jar with a burning paper, dip it out. Include discussion of Grotto del Cane and 1986 Lake Nyos, Cameroon incident.
HAZARDS & DISPOSAL	Extinguish flame before discarding paper.
DISCUSSION	Compare with similar manipulation to hydrogen gas.

Lab. 43: Detect the presence of carbon dioxide using limewater.

MATERIALS	Calcium hydroxide (limewater) and carbon dioxide.
PROCEDURE	Add carbon dioxide and show precipitation of calcium carbonate.
HAZARDS & DISPOSAL	Calcium hydroxide is caustic.
DISCUSSION	Ca(OH)2(aq) + CO2(aq) --> CaCO3(s) + H2O The presence of carbon dioxide gas is easily ascertained by a taper or, more delicately, by limewater.

Lab. 52: Observe the oxidation of wax.

MATERIALS	A candle
PROCEDURE	Have the candle burning.
HAZARDS & DISPOSAL	Use caution around burning candle.

Lab. 55: Illustrate the nature of the flame about a coal fire or firewood.

MATERIALS	12 x 12 inch square of brown paper. Matches.
PROCEDURE	Roll up the paper, light one end, then the other, as described above.
HAZARDS & DISPOSAL	Slight fire hazard. Do over sink or fireproof board. Wear glasses.
DISCUSSION	Oxidation of paper and ``cracking'' of long chain hydrocarbons to produce volatile, inflammable gases. CnH2nOn + O2 --> H2O + CO2 + CO? + products CO + products + O2 --> H2O + CO2 The flame is merely the combustion of the gas produced within the wood or pole by the action of the heat of the lower part of the fire.

Authors

Andrew Peterson, Judy Phillips, Ruth Rand, Kay Reat, Debra Reynolds, Mary Rothermich, Roger Rumppe, Ely Jose (Jay) Salon.

Woodrow Wilson Leadership Program in Chemistry lpt@www.woodrow.org
The Woodrow Wilson National Fellowship Foundation webmaster@woodrow.org
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