2. Chlorine

Being the third period element, Сhlorine has the following electronic structure: 1s²2s²2p⁶3s²3p⁵. The following oxidation numbers are possible for Chlorine in the compounds on the basis of external energy level structure (3s²3p⁵) (see Chapter 2.2-2.3):

Ground (nonexcited) state ( 1-, 0, 1+ oxidation number, for example, in HCl, Cl₂, HClO compounds):

3s		3p				3d
↑↓		↑↓	↑↓	↑

The first excited state (3+ oxidation number, for example, in HClO₂ compound);

3s		3p				3d
↑↓		↑↓	↑	↑		↑

The second excited state ( 5+ oxidation number, for example, in HСlO₃ compound);

3s		3p				3d
↑↓		↑	↑	↑		↑	↑

The third excited state ( 7+ oxidation number, for example, in HСlO₄ compound).

3s		3p				3d
↑		↑	↑	↑		↑	↑	↑

The general characteristics of the most important Chlorine compounds are shown in Table 15.

Table 15. General characteristics of Chlorine compounds

Oxidation number	-1	0		+1	+3	+5	+7
Compounds	HCl, NaCl	Cl2		Cl2O, HClO, KClO	Cl2O3, HClO2, NaClO2	Cl2O5 HClO3 KClO3	Cl2O7 HClO4 NaClO4
Names of acids and salts	Hydrochloric, Chlorides	-		Hypochlorous, Hypochlorites, Chlorates (I)	Chlorous, Chlorites, Chlorates (III)	Chloric, Chlorates, Chlorates (V)	Perchloric, Perchlorates, Chlorates (VII)
Redox properties	Only reducing agent	Properties of oxidizing and reducing agent; oxidizing properties prevail		Properties of oxidizing and reducing agent; oxidizing properties prevail	Properties of oxidizing and reducing agent	Reducing properties surpasses oxidizing ones	Only oxidizing agent
Acid-Base properties	HCl - strong acid	-		HClO - weak acid; unstable in free state	HClO2 - acid of medium strength; unstable in free state	HClO3 - strong acid; unstable in free state	HClO4 - extrastrong acid; stable in free state
			Strengthening of acid properties

Chlorine (Cl₂) is the typical non-metal with brightly expressed features of oxidant. Due to high activity Chlorine in nature occurs only in bound state as Chlorides: NaCl - halite or rock salt, NaCl · KCl - sylvinite, KCl · MgCl₂ · 6H₂O - carnallite, MgCl₂ · 5H₂O - bischofite, etc. Chlorides are abundant in sea water.

In industry Chlorine is prepared by the electrolysis of concentrated solution of NaCl:

2NaCl + 2H₂O ↔ 2NaOH + Cl₂↑ + H₂↑

and in the laboratory - by influence of HCl on MnO₂ or KMnO₄:

2KMnO₄ + 16HCl = 5Cl₂ + 2MnCl₂ + 2KCl + 8H₂O

Mn7+ + 5  Mn2+	2 (reduction);
2Cl- - 2  Cl2	5 (oxidation).

Chlorine actively reacts almost with all periodic system elements. With the metals it forms the salts and with non-metals - halogenous anhydrides:

Mg + Cl₂ = MgCl₂;

2P + 3Cl₂ = 2PCl₃.

Halogenous anhydrides are hydrolyzed forming both nonmetal acids with the relevant oxidation numbers:

PCl₅ + 4H₂O = H₃PO₄ + 5HCl.

Chlorine does not react directly with Oxygen, Carbon and Nitrogen.

HCl (Hydrochloric aqua solution) is of practical value. In the reaction of the Halogens and Hydrogen composition there is the tendency of oxidative activity weakening from Fluorine and Iodine: Fluorine reacts with Hydrogen generating the explosion in the dark; Chlorine - in air or while heating with the explosion, Bromine and Iodine - in case of extensive heating, for example,

Cl₂ + H₂ = 2HCl.

The intensity of HF, HCl, HBr, HI acids increases. It is characterized by number of reaction typical for acids:

2HCl + 2Na = 2NaCl + H₂↑;

2HCl + SrO = SrCl₂ + H₂O;

2HCl + Ca(OH)₂ = CaCl₂ + 2H₂O.

Halogens form Oxygen compounds as well, mainly: oxides, acids and salts.

The basis for preparation of halogens oxygen-contained compounds is their interaction with water and alkalis. In cold the reaction is carried out in case of dynamic balance:

Cl₂ + H₂O ↔ HCl + HClO

Cl20 + 2  2Cl-	1 (reduction);
Cl20 - 2  2Cl+	1 (oxidation).

2Cl₂ + 2Ca(OH)₂ = CaCl₂ + Ca(ClO)₂ + 2H₂O.

On heating such mixture reacts with the formation of Chlorates:

3Cl₂ + 3H₂O 5HCl + HClO₃;

Cl20 + 2  2Cl-	5 (reduction);
Cl20 - 10  2Cl+5	1 (oxidation).

3Cl₂ + 6NaOH = 5NaCl + NaClO₃ + 3H₂O.

HClO, HBrO, HIO electrolytes are the less stable and the weakest among Chlorine oxyacids and other halogens.

Thus in air HClO is decayed with active (atomic) Oxygen:

HClO = HCl + .

Active Oxygen discolors many colorants and that is why Chlorine (1+) oxyacid salts are called bleaching: NaClO are used for bleach and wash; the mixture of Ca(ClO)₂ and CaCl₂ which is known as bleaching powder, is used in agriculture for disinfections of the facilities, warehouses, and animal corpses.

Sodium, Magnesium and Calcium chlorates are of great value for the agriculture. They are used as herbicides for wild grass control. In Redox reactions Cl^- is exclusively reducing agent, but Cl⁷⁺ - only the oxidizing one. Halogens with the medium oxidation numbers (Cl⁰, Cl⁺, Cl³⁺, Cl⁵⁺) may be mainly as the reducing agents, but also the oxidizing ones. Oxidative activity of the compounds is increased from Cl⁷⁺ to Cl¹⁺.

For example,

2KI + NaClO + H₂SO₄ = I₂ + NaCl + K₂SO₄ + H₂O

Cl+ + 2  Cl-	1 (reduction);
2I- - 2  I2	1 (oxidation).

PRACTICE PROBLEMS

1. How oxidative and reducing activity of the Halogens is changed in the group?

2. Which oxidative levels are typical for Fluorine and what is the difference between Fluorine and other halogens?

3. Write the formulas of four oxidative Chlorine acids and indicate the strongest acid and oxidizing agent.

4. Which chemical reactions should be carried out in order to receive bleaching powder, having KMnO₄, HCl, CaO and H₂O?

5. Write the equations according to the transformation series:

KCl → Cl₂ → KClO → KClO₃ → KCl;

NaF → CaF₂ → F₂ → HF → H₂[SiF₆].

6. Complete the equations and put the coefficients according to the schemes of electron redistribution:

1. Ni(OH)₂ + NaClO + H₂O → Ni(OH)₃ + …

2. KI + KIO₃ + H₂SO₄ →

3. Zn + KClO₃ + H₂SO₄ →

4. KMnO₄ + NaI + H₂O →