Laboratory training

Experiment 1. Interaction between metals and halogenhydrides

Put Zinc granule into the first test-tube, small amount of Iron chips into the second and some Copper chips into the third. Add 5-7 drops of hydrochloric acid solution into each of them. Determine in which cases the reactions take place and the gas is evolved. Write the reactions.

Experiment 2. Halogens oxidative activity in free state

Put 3-4 drops of KBr or NaBr solution into the test-tube, add 5-6 drops of chloric water (Chlorine solution in H₂O, which includes free Chlorine in the form of Cl₂). Free evolved Bromine is extracted by shaking the mixture with 3-4 drops of benzol. Determine the color of the Bromine and write the reaction. Carry out similar test with KI and Bromine water in another test-tube.

Experiment 3. Reducing properties of I^- ions

Put 4-5 drops of H₂SO₄ into 2-3 drops of KI and add by dropping the solution of Sodium or Potassium Nitrite (KNO₂, NaNO₂) up to the change of mixture color. Prove the presence of free Iodine in the mixture adding some drops of starch. Write the equation.

Experiment 4. Chlorates as oxidazing agents

Add 2-3 drops of KClO₃ solution to the same volume of Iron (II) Sulfate solution and add 1-2 drops diluted sulfuric acid. Check the mixture for the presence of Fe³⁺ adding Potassium or Ammonium thiocyanate.

Experiment 5. The salts of hydrohalogen acids insoluble in water

Put 3-5 drops of solutions hydrochloric and hydroiodic acids salts (for example, NaCl and KI) into the test-tubes and add into each several drops of Pb(NO₃)₂. Determine the color of occurred precipitates, write the equations.

Chapter # 12. The chalcogens

Vocabulary

English	Українська назва	English	Українська назва
Chalcogen	Халькоген	Perhydrol	Пергідроль
Ozone	Озон	Rare element	Розсіяний елемент
Hydrogen Peroxide	Пероксид водню	“Bad egg gas”	Сірководень

Sourses:

1. Introduction in General, Organic and Biochemistry, 7^th Edition, by Morris Hein, Leo R. Best, Scott Pattison and Susan Arena, Brooks/Cole Publishing Co., 2001. (Chapter 10, pp. 289-294, Chapter 27, pp. 765-777);

2. http://www.chemsoc.org/viselements/pages/Visual Elements Group 16.html

3. http://www.hrw.com/science/mc/index/htm

1. General characteristics

The elements of Group 6A (or the main sub-group of VI group) are:

Symbol	Electron configuration
Oxygen O	[He]2s22p4
Sulfur S	[Ne]3s23p4
Selenium Se	[Ar]3d104s24p4
Tellurium Te	[Kr]4d105s25p4
Polonium Po	[Xe]4f145d106s2 6p4

Often this group of chemical elements (except the last Po) is named the Chalcogens.

Appearance

The first element of this sub-group, Oxygen, is the only gas, and is colorless and odorless. Sulfur is a pale yellow, brittle solid. Selenium can have either an amorphous or a crystalline structure; the amorphous form can be red or black, and the crystalline form can be red or gray. Tellurium is a silvery-white color with a metallic luster. Polonium is a naturally radioactive element.

Selenium and Tellurium are rare elements with few uses, and along with Polonium will not be considered further here.

General Reactivity

Oxygen and Sulfur are highly electronegative elements - the electronegativity of Oxygen is second only to that of Fluorine. Their general reactivity is therefore dominated by their ability to gain electrons.

There is a transition down the sub-group from non-metallic to more metallic properties, so that Oxygen is a non-metal and Tellurium a metalloid. All the elements except Polonium form M^2- ions.

There is a marked difference between Oxygen and the other members of the Group. This arises from:

• the small size of the O atom which enables it to form double bonds;

• its inability to expand its valence shell like the other elements as it has no accessible d-orbitals;

• its high electronegativity, which enables it to participate in Hydrogen-bonding.

Occurrence and Extraction

Oxygen occurs widely as the free element in the form of O₂, comprising 21% of the air by volume. It also occurs as O₃, ozone, at high altitudes in the ozone layer. In the combined form it is found in very many minerals, and also in water. Oxygen is obtained industrially by the fractional distillation of liquid air. It is stored under pressure in cylinders.

Sulfur is found as the free element and also as metal Sulfide ores and a number of Sulfates. Native sulfur is brought to the surface from underground deposits by the Frasch Process, which uses superheated water to melt the Sulfur and force it upwards.

Physical Properties

The covalent and ionic radii increase going down the sub-group, as electrons occupy shells with higher quantum numbers.

Oxygen occurs as two gaseous allotropes, O₂ (dioxygen or more commonly Oxygen) and O₃ (trioxygen or ozone). Oxygen is more common. It condenses to a pale blue paramagnetic liquid at -183⁰C. Ozone is a pale blue, pungent gas, which condenses to an inky-blue liquid at -112⁰C. The ozone layer in the upper atmosphere is an important shield against harmful UV radiation from the Sun.

Sulfur has several allotropes, the two main ones being rhombic and monoclinic sulfur. These both consist of S₈ molecules.