Group 17 Elements: The Halogen Family

ChemistryThe p-Block Elements: Group 15, 16, 17 And 18For JEE aspirants

HALOGEN FAMILY

The group 17 includes fluorine (F), chlorine (Cl), Bromine (Br), Iodine (I). Which are collectively called as halogens (from the Greek hals "salt", and gennan, "to form or to generate"), because they are literally salt formers. Name of the halogens can be found in nature in their elemental form. The last member of this group is Astatine (At) which is radioactive halogen.

Halogen

Occurrence

Due to very high reactive nature halogens are not found in free state and found in form of compounds.

(i) Fluorine occurs as : Cryolite; Na₃AlF₆

Fluorspar CaF₂

Fluorapatite; 3Ca₃(PO₄)₂CaF₂

(ii) Chlorine occurs as : Sodium chloride; NaCl

KCl, MgCl₂, CaCl₂

(iii) Bromine occurs as : Bromides of alkali metal

(iv) Iodine occurs as : Iodides of alkali metal

Sodium iodate; NaIO₃

Isolation

Fluorine

The electrolytic method is used in the preparation of elemental halogen. The method for fluorine was developed by Moissan in 1886.

A molten mixture of potassium fluoride and anhydrous hydrogen fluoride at $\sim$ 350 K using mild steel and carbon rod free from graphite as cathode and anode respectively to avoid the accidental explosive mixing of hydrogen and fluorine.

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Chlorine

It is produced industrially by the electrolysis of natural brine or aqueous solution of NaCl. In the laboratory chlorine can be prepared by the oxidation of HCl by MnO₂ or by KMnO₄.

$M n O_{2} (s) + 4 H C l (a q) M n C l_{2} (a q) + 2 H_{2} O (ℓ) + C l_{2} (g)$

$2 K M n O_{4} (s) + 16 H C l (a q) 2 K C l (a q) + 2 M n C l_{2} (a q) + 8 H_{2} O (ℓ) + C l_{2} (g)$

Bromine

It is commercially produced by the oxidation of bromide ions in natural brine with chlorine.

$2 B r (a q) + C l_{2} (g) B r_{2} (ℓ) + 2 C l^{-} (g)$

Iodine

It is manufactured from natural brines or sea weed by the oxidation of iodide ion with chlorine. Iodine is recorded from iodates by reduction with NaHSO₃.

$N a I O_{3} (a q) + 3 N a H S O_{3} (a q) N a I (a q) + 3 N a H S I O_{4} (a q)$

$N a I O_{3} (a q) + 5 N a I (a q) + 3 H_{2} S O_{4} (a q) 3 I_{2} (s) + 3 H_{2} O (ℓ) + 3 N a_{2} S O_{4} (a q)$

Illustration 1. Which one of the following molecules will form a linear polymeric structure to hydrogen bonding?

(A) HCl (B) HF

(C) H₂O (D) NH₃

Solution: (D)

CHEMICAL PROPERTIES IN HALOGEN

Among all the halogens fluorine is the most reactive and reactivity decreases down the group. The high reactivity of fluorine is due to high electronegativity and low bond dissociation energy among the group.

1. Oxidizing power

The halogens are strong oxidizing agents. This is due to their high electron affinities. The oxidizing power of the halogens decreases on going down the group from fluorine to astatine.

$F_{2} > C l_{2} > B r_{2} > I_{2}$

2. Hydrides

Halogens combine with hydrogen to form volatile hydrides of the formula HX. Due to hydrogen bonding in HF it is liquid but all other like HCl, HBr and HI are gases.

….H – F ….H – F …..H – F

Hydrogen bonding in HF

The bonding is covalent in nature in all the halides. All hydrogen halides acts as acid in their aqueous solution.

HF < HCl < HBr< HI

Order in acidity

H – F bond is strongest, while HI is weakest so can easily liberate the H⁺ ion hence more acidic than HF.

Reducing property of hydrogen halide also increases down the group due to decrease in H – X bond strength.

3. Halides

The halogens are very reactive and form compounds with most of the metals and non- metals except He, Ne and Ar. These halides may be simple or complex.

(i) With the alkali & alkaline metals like Na, Mg, K etc. halogens form ionic halides due to low IP of metals. Their ionic character decreases as the size of halogen atom increases.

(ii) With metals having high IP like Sn, Pb, Sb halogen forms covalent halides. Halides is in higher oxidation state is more covalent $(S n C l_{4}, P b C l_{4}, S b C l_{5})$ than the halides in lower oxidation state $(S n C l_{2}, P b C l_{2}, S b C l_{3})$

(iii) With non – metals like P, As, S, the halogens form covalent halides as $P F_{5}, A l C l_{3}, A s C l_{3}, S_{2} C l_{2}$ etc.

4. Oxides

Due to high electron affinity of both halogens and oxygen they do not combine directly with oxygen but their oxides with oxygen can be prepared indirectly.

\begin{align} 2{{F}_{2}}\,\,\,\,\,\,\,\,+\,\,\,\,\,2NaOH\,\,\,\,\,\,\,\,\xrightarrow{{}}2NaF+2O{{F}_{2}}+{{H}_{2}}O \\ \,\,\,\left( 2\%\,\,solution \right)Oxygen\,\,difluoride \\ \end{align}

\begin{align} 2C{{l}_{2}}+2HgO\xrightarrow{575\,K}HgO.HgC{{l}_{2}}+C{{l}_{2}}O \\ dichlorine\,\,oxide \\ \end{align}

The compounds of oxygen with fluorine are called as fluorides because fluorine is more electro negative than oxygen. Most of these compounds are endothermic and unstable and are likely to explode resulting in the formation of more stable products.

All the oxides are powerful oxidizing agents and decompose explosively when they are given mechanical shock or heat.

The stuructures of some molecules is as follows

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5. Oxyacids

Halogens form oxyacids in which fluorine forms only hydrofluorous acid (HOF). $C l_{2}, B r_{2} an d I_{2}$ form four series of acids with formulae HOX, HXO₂, HXO₃ and HXO₄.

Strength of these acids increases as the oxidation state of halogen atoms increases. And also their conjugate base becomes more stabilized due to resonance.

Acidic strength

$H C l O_{4} > H C l O_{3} > H C l O_{2} > H C l O$

Perhalic acid>Halic acid> Halous acid> Hypohalous acid

In particular acid the acidity decreases as the size of the halogen atom increases.

HOCl>HOBr>HOI

HOCl, HOBr and HOI are weak acids, formed due to disproportionations of the halogen water.

$X_{2} + H_{2} O H O X + H X (X = C l, B r, I)$

Salts of these acids known as hypohalites

eg. CaOCl₂ (bleaching powder)

The halic acids $H C l O_{3}$ and $H B r O_{3}$ are also known in solutions, but iodic acid $H I O_{3}$ exists as white solid. Thus we can say that the stability of these acids increase as increasing the size of halogen acid. The salts of these acids are called halates. eg. NaClO₃ is a powerful weed killer and $K C l O_{3}$ is used in fire works.

Perhalic acids forms perhalates as their salts.

BLEACHING POWDER $(C a O C l_{2} . H_{2} O)$

It is also called calcium chlorohypochlorite because it is considered as a mixed salt of hydrochloric acid and hypochlorus acid. It is represented as

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Method of preparation

$C a (O H)_{2} + C l_{2} C a (O C l) C l + H_{2} O$

This above reaction is carried out by two methods :

(i) Hasenclever's plant

(ii) Backmann's plant

Physical properties

It is pale yellow powder. It has a strong smell of chlorine. It is soluble in water but a clear solution is never formed due to the presence of impurities.

Chemical properties

(a) $6 C a O C l_{2} C a (C l O_{3})_{2} + 5 C a C l_{2}$

autooxidation

(b) $2 C a O C l_{2} C o C l_{2} 2 C a C l_{2} + O_{2}$

$H C l O H C l + O$

On account of the formation of nascent oxygen, it shows oxidising and bleaching properties.

(i) Oxidising properties

$C a O C l_{2} + H_{2} S C a C l_{2} + H_{2} O + S$

$C a O C l_{2} + 2 F e S O_{4} + H_{2} S O_{4} F e_{2} (S O_{4})_{3} + C a C l_{2} + H_{2} O$

$C a O C l_{2} + K N O_{2} C a C l_{2} + K N O_{3}$

$3 C a O C l_{2} + 2 N H_{3} 3 C a C l_{2} + 3 H_{2} O + N_{2}$

(ii) Bleaching action

$co l o u r e d ma tt er + O co l o u r l ess p r o d u c t$

(d) It loses its chlorine by the action of dilute acids (in excess) or carbon dioxide.

$C a O C l_{2} + 2 H C l C a C l_{2} + H_{2} O + C l_{2}$

$C a O C l_{2} + H_{2} S O_{4} C a S O_{4} + H_{2} O + C l_{2}$

$C a O C l_{2} + C O_{2} C a C O_{3} + C l_{2}$

The amount of chlorine obtained from a sample of bleaching powder by treatment with excess of dilute acids or carbon dioxide is called available chlorine.

(e) Bleaching powder converts acetone or ethyl alcohol into chloroform.

$C a O C l_{2} + H_{2} O C a (O H)_{2} + C l_{2}$

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Inter halogen compounds

When halogens combine with themselves form interhalogen compounds. These are divided into four types. i.e.

(i) $A X (C l F, B r F, B r C l, I C l, I B r)$

(ii) $A X_{3} (C l F_{3}, B r F_{3}, I C l_{3})$

(iii) $A X_{5} (B r F_{5}, I F_{5})$

(iv) $A X_{7} (I F_{7})$

$C l_{2} + F_{2} 500 K 2 C l F$

$C l_{2} + (e x cess) 3 F_{2} 500 K 2 C l F_{3}$

$C l F_{3} + F_{2} h ν C l F_{5}$

Characteristics of interhalogen compounds

(i) They are covalent compounds

(ii) They are more reactive than their parent halogens. It is because A – X bond is relatively weaker than X – X bond.

(iii) They are very good oxidizing property having compounds.

(iv) Their m.p and b. p. increase as the electronegativity difference between them increases.

(v) Chlorofluoro hydrocarbons are known as Freon and are used as refrigeraters.

For example, Freon – 11 is CCl₃F, Freon- 12 is $C C l_{2} F_{2}$ , Freon-13 is $C C l F_{3}$ etc.

POLYHALIDE IONS

These are charged species formed by the combination of halide ion with another halogen molecule. e.g. $I^{-}_{(a q)} + I_{2} ⇌ I_{3}^{-}_{(a q)}$

Some other examples of polyhalide anions are $I C l_{2}^{-}, I B r_{2}^{-}, I C l_{4}^{-}, B r F_{4}^{-}, I F_{6}^{-}$ etc. Beside that –ve ions polyhalide cations eg. $I_{3}^{+}$ are also possible.

Illustration 2. "Fluorine can replace only halide ion from its salt". Explain the statement.

Solution: Due to high electronegativity fluorine has more tendency to be reductional potential reduced hence acts as a good oxidizing agent.

Illustration 3. Although fluorine has less electron affinity than chlorine but having high oxidizing power. Explain.

Solution: In fact, $E^{0}$ values of halogens refer to the process $\frac{1}{2} X_{2} (g) + e^{-} + a q X^{-} (a q)$ which comprises of following steps:

(i) Dissociation:

$\frac{1}{2} X_{2} (g) + e^{-} + a q X^{-} (a q)$ ; $\frac{1}{2} Δ H_{D i ssoc ia t i o n} = + v e$

(ii) Accepting the electron

$X (g) + e^{-} X^{-} (g) Δ H = - v e$

(iii) Hydration of anion

$X^{-} (g) + a q X^{-} (a q); Δ H_{h y d r a t i o n} = - v e$

Now the net energy release is summation of $\frac{1}{2} Δ H_{D} + (- Δ H) + (- Δ H_{h y d r a t i o n})$

So here although in (ii) step EA is high for chlorine but net energy released is – 752 kJ for fluorine than – 708 kJ for chlorine.

Illustration 4. Why halogens are highly reactive?

Solution: Because of low Y – X bond dissociation energy.

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