Chemical Properties:

The entire halogens family is very reactive. Fluorine is the most reactive of all the halogens, joining directly with each and every other element apart from oxygen and some of the noble gases. Thus, it is as well termed as a super halogen. We have as well observed that the reactivity reduces from F₂ to I₂. In displacement reactions we will examine that fluorine shifts all the remaining three halogens from their salts, chlorine shifts bromine and iodine, whereas bromine displaces just iodine. Halogens react with one other to form inter-halogen compounds. Halogens, in general, react by most metals; however bromine and iodine don't react by some noble metals such as Ag and Au. Halogens as well react by numerous non-metals to form halides.

Halogens react by hydrogen to form hydrides which are termed as hydrogen halides. Fluorine and chlorine react by hydrogen with explosive violence; fluorine-hydrogen mixture explodes even in the dark. Chlorine-hydrogen mixture does so merely in the presence of light. This is termed as a photochemical reaction. It has been illustrated that this reaction begins by the formation of halogen free radicals. It is not surprising in view of lower X-X bond energy as compared to the H-H bond energy. Reactions of bromine and iodine with hydrogen are very slow, the latter being reversible.

C1-C1 → Cl + Cl initiation ............ (Step 1)

Cl + H-H → HCl + H propagation ............ (Step 2)

H + Cl-Cl → HCl + Cl propagation   ............ (Step 3)

Cl + H → HCl termination ............ (Step 4)

The above reactions are termed as Chain Reactions since after the initiation of the reaction, the propagation (Step 2) and (Step 3) are repeated in series till the reactants are consumed. The reaction gets terminated whenever free radicals begin combining with one other as in (Step 4).

Fluorine reacts strongly by water to form hydrofluoric acid and oxygen:

2F₂ + 2H₂O → 4HF + O₂

The other halogens are moderately soluble in water (Br₂ > C1₂ > I₂) and react partially to give a mixture of hydrohalic and hypohalous acids:

X₂ + H₂O ↔ HX + HOX

Halogens react by aqueous alkali, the reactivity decreasing from fluorine to iodine. Fluorine acts differently from other halogens. It reacts by alkalis violently making fluorides and fluorine oxides or oxygen as illustrated below:

2F₂ + 2NaOH → (in cold) → 2NaF + OF₂ + H₂O

2F₂ + 4NaOH → (in hot) → 4NaF + O₂ + H₂O

The other halogens react by alkalis in cold to provide hypohalites (XO~) and in hot to form halates (XO₃):

2NaOH + X2 → (in cold) → NaOX + NaX + H₂O

6NaOH + 3X2 → (in hot) →NaOXO₂ + 5Nax + 5H₂O

                                           (Here X = Cl, Br or I)

The halogen family react by hydrocarbons however reactivity reduces with the increase in atomic number. Fluorine is the most reactive and brings on the decomposition of hydrocarbons:

CH₄ + 2F₂ → C + 4HF

Chlorine and bromine replace hydrogen atoms, reaction with bromine being slower; iodine consists of little or no reaction:

CH₄ + 4X₂ → CX₄ + 4HX, Here X = Cl or Br

Some of the reactions of halogens are summarized in the table shown below.

Table: Reactions of Halogens

Basic Properties of Halogens:

In general, the fundamental or metallic nature of elements increases as we go down a group. Therefore, last member of each of the Groups 14, 15 and 16, that is, Pb, Bi and Po, correspondingly, is definitely metallic in character. However, this trend is not so well marked in the elements of Group 17 as very little is known regarding the last member of the group, astatine. There is, though, definite proof to exhibit the existence of halogen cations in media which are weakly nucleophilic.

Fluorine is the most electronegative element and consists of no fundamental properties. Chlorine exhibits a slight tendency to form cations. For illustration: ClF ionizes to form Cl⁺ and F¯ because of the higher electronegativity of fluorine. Bromine cation, Br⁺, exists in complexes like Br (pyridine)⁺ NO₃, electrolysis of ICN in pyridine solution provides iodine at the cathode.

This points out that ICN ionizes to I⁺ and CN^-. Most of the pyridine complexes, example: [I (pyridine)]+ NO₃, [I (pyridine)₂]⁺ClO₄ and I (pyridine)⁺ CH₃COO are acknowledged.

Iodine dissolves in oleum providing a bright blue solution which has been illustrated to encompass I₂ and I₃:

2I₂ + 6H₂S₂0₇ → 2I₃ + 2HS₃O₁₀ + 5H₂SO₄ + SO₂

3I₂ + 6H₂S₂0₇ → 2I₃ + 2H₂S₃O₁₀ + 5H₂SO₄ + SO₂

Electrical conductivity of the molten iodine is due to the presence of I₃⁺ + I₃^- species produced by the self ionization of iodine:

3I₂ ↔ I₃⁺ and I₃^-

The cations, Cl⁺ and Br₃⁺ are made up in the following reactions:

ClF + Cl₂ + AsF₅ → Cl₃AsF₆

O₂⁺.AsF₆ + 3/2 Br₂ → Br₃⁺.AsF₆^- + O₂

Compounds of Halogens:

Hydrogen Halides:

We are familiar that all the halogens combine by hydrogen and form volatile hydrides of the kind HX, which are as well termed as hydrogen halides. The reaction of fluorine with hydrogen is extremely violent while bromine and iodine react by hydrogen only at elevated temperatures and in the case of iodine the reaction doesn't carry on to completion:

H₂ + X₂ → 2HX, here X = Cl, Br and I

Hydrogen fluoride and hydrogen chloride are obtained via the action of concentrated sulphuric acid on fluorides and chlorides:

CaF₂ + H₂SO₄ → 2HF + CaSO₄

NaCl + H₂SO₄ → HCl + NaHSO₄

As concentrated sulphuric acid partly oxidizes HBr and HI to Br₂ and I₂, these are made up by the action of concentrated orthophosphoric acid on bromides and iodides:

NaBr + H₃PO₄ → HBr + NaH₂PO₄

NaI + H₃PO₄ → HI + NaH₂PO₄

Hydrogen bromide and hydrogen iodide are generally manufactured in the laboratory via the hydrolysis of PBr₃ and PI₃:

PBr₃ + 3H₂O → 3HBr + H₃PO₃

PI₃ + 3H₂O → 3HI + H₃PO₃

Under normal conditions HCl, HBr and HI are gases whereas HF is a liquid, because of strong hydrogen bonding. Their boiling points and melting exhibit a gradual increase in the order HCl < HBr < HI, however H-F doesn't follow the trend and has surprisingly higher values. This is due to the reason of strong hydrogen bonding in H-F molecules.

Hydrogen halides are mainly covalent compounds having varying degree of polarity of the H-X bond based on the electronegativity of the halogen atom. Therefore, H-F bond is most polar and the reducing order of polarity is H-F > H-Cl > H-Br > H-I as illustrated by the percent ionic character in such bonds

Some of the physical properties of the hydrogen halides are illustrated in the table shown below:

Table: Physical Properties of Hydrogen Halides

The thermal stability of hydrogen halides reduces from HF to HI. Therefore hydrogen fluoride is the most stable while hydrogen iodide is the least stable. This can be examined from their percentage dissociation data (table shown above). For illustration, as HF and HCl are not appreciably dissociated even at 1473 K, HBr is dissociated to the extent of around 0.5% and HI is dissociated to the extent of around 33% at 373 K.

Hydrohalic Acids: 

Aqueous solutions of hydrogen halides are termed as hydrohalic acids, namely, hydrofluoric, hydrochloric, hydrobromic and hydriodic acid. The hydrohalic acids form constant boiling point mixtures with water. The aqueous solutions of acids ionize as:

HX (aq) + H₂O → H₃O⁺ (aq) + X^- (aq)

Their acid strength obeys the order HI > HBr > HCl > HF. Acid strength is in general the tendency of HX (aq) to provide H₂O⁺ (aq) and H (aq). The enthalpy changes related with the dissociation of hydrohalic acids can be symbolized with the assistance of Bom-Haber cycle in its simplest form as represented below:

Fig: Bom-Haber cycle

The  acid  strength  based  on  the  sum  of  all  enthalpy  terms  for different phases  pointed out in the Born Haber cycle. Of these the most marked change is in the enthalpy of dissociation of H-X bond, that reduces in the order HF > HCl > HBr > HI and the enthalpy of hydration of X¯, that reduces from F¯ > Cl¯ >Br¯ > I¯. The net enthalpy change becomes more negative, that is, the reaction,

HX (aq) → H⁺ (aq) + X¯ (aq)

becomes more exothermic from HF to HI. As expected, acid strength differs in the reverse other, HI being the strongest acid and HF the weakest acid.

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