Question

Explain Hydracids (HCI, HBr and HI).

Answer 1

Preparation:-

1. By the direct combination of elements. 

\(H_{2} + Cl_{2} \overset{Pt}{ \longrightarrow} 2HCI \quad ; \quad H_2+Br_2 \longrightarrow 2HBr \quad ; \quad H_2+I_2 \overset{Pt,450^\circ C}{ \leftrightharpoons }2HI\) 

2. By heating a halide with acid (conc. H₂SO₄ in case of HCl ; and H₃PO₄ in case of HBr and HI). This constitutes the laboratory as well as commercial method. 

NaCl + H₂SO₄ → NaHSO₄ + HCl ; NaHSO₄ + NaCl → Na₂SO₄ + HCI 

Note. 

(1) HCl can't be dried over P₄O₁₀ or quicklime because they react with the gas chemically.

CaO + 2HCl → CaCl₂ + H₂O ; P₄O₁₀ + 3HCI → POCl₃ + 3HPO₃

Hence HCl gas is dried by passing through conc. H₂SO₄. 

(ii) HBr (and HI) cannot be prepared by heating bromide (and iodide) with conc. H₂SO₄ because HBr and HI are strong reducing agents and reduce H₂SO₄ to SO₂ and get themselves oxidised to bromine and iodine respectively.

KX + H₂SO₄  → KHSO₄ + HX 

H₂SO₄ + 2HX → SO₂ + X₂ + 2H₂O     (where X Br or I)

Hence HBr and HI are prepared by heating bromides and iodides respectively with conc.
mosphoric acid.

3KBr(3KI) + H₃PO₄ → К₃РО₄ + 3HBr (3HI)

3. HBr and HI are prepared by the reaction of phosphorus, halogen and water (laboratory method). 

P4 + 6Br₂(6I₂) → 4PBr₃(4PI₃)    (produced in situ)

PBг₃ (PI₃) + 3H₂O → 3HBr (3HI) + H₃PO₃

4. HCl, HBr and HI can also be prepared by passing H₂S or SO₂ through chlorine, bromine or odine aq. solution respectively.

(a) X₂ + H₂S → 2HX + S

(b) X2 + SO₂ + 2H₂O → 2HX + H₂SO₄ 

Properties:-

(i) Physical state. Except HF, other hydrogen halides are gases and their melting and boiling points increase with increase in atomic weight of the halogen. The anomalous behaviour of hydrogen fluoride (a liquid with b.p. of 19.5°C) is because of association of several HF molecules even in the vapour state through hydrogen bonding.

H - F --- H - F --- H - F ---

(ii) All are heavier than air.

(iii) All of them can be liquefied to colourless liquids.

(iv) Constant boiling mixtures. These are fairly soluble in water; HCl, HBr and HI form constant boiling mixture with water. The composition and boiling point of the constant boiling mixtures are given below.

Acid	HX%	b.p(°C)
HCI	22.24	110
HBr	48.00	126
HI	58.00	127

(v) Covalent nature. Hydrogen halides are undoubtedly covalety in the pure liquid dicated by their low melting and boiling points and poor electrical conductivity in the pure liquid state. However the bonds between H and X have a degree of polarity that decreases in the order:

H - F > H - CI > H - Br > H - I 

The above order is in accordance with the decreasing electronegativity of the halogens

F > C > Br > 1

(vi) Stability: The H - X bond strength decreases from HF to HI as is evident from the values of their heat of formation from HF to HI.

Thus HF is the most stable while Hl is the least stable. This is evident from their decomposition reaction: HF and HCl are stable upto 1200°C, HBr dissociates slightly and Hl dissociates considerably (20%) at 440°C. The decreasing stability of the hydrogen halides is also reflected in the decreasing values of dissociation energy of the H-X bond from H-F to H-1

(vii) Reducing property: Since stability of hydrogen halides decreases on moving from HF to HI, their reducing property increases on moving down the group. 

\(\underset{\longrightarrow}{HF, HCL, HBr, HI}\)

Decreasing stability and hence Increasing reducing power

Thus HF is not a reducing agent while HI is the strongest reducing agent. In fact HI is so strong reducing agent that its dilute solution reduces oxygen and itself oxidised to iodine. This explains wry the solution of HI acquires a brown colour on exposure to air.

\(4HI + O_{2} \longrightarrow 2H_2O + \underset{\text{brown colour}}{ I_2}\)

(vii) Acidic strength. When perfectly dry they have no action on litmus, but in aqueous solution they function as acids and hence turn blue litmus red. The relative strength of acids increases in the order given below which is in accordance with their dissociation constants, Ka.

\(\underset{ka}{} \quad\underset{7 \times 10^{-4}}{HF} < \underset{7 \times 10^{8}}{HCl} < \underset{7 \times 10^{10}}{HBr} < \underset{7 \times 10^{11}}{HI} \)

Thus HF is the weakest acid and HI is the strongest acid. However, note that this is contrary to the expectation on the basis of electronegativity difference ; HF having the most electronegative element (F) and having more ionic character should have the maximum tendency to release protons to water molecules and hence should have been the strongest acid. The anomalous behaviour is explained on the basis that the ionisation (ie. acidic character) of HX is not a

\(HX(aq) \longrightarrow H ^+ (aq) + X ^ - (aq)) ; \quad \triangleΗ\)

Answer 2

 (x) Action halogens. Chlorine is liberated from HCI only by fluorine, Br₂ is liberated from HBr by F₂ and Cl₂ (not by iodine), while I₂ is liberated from HI by F₂ Cl₂ and B₂

2HI + F₂(Cl₂, Br₂) → 2HF(HCl, HBr) + I₂

(xi) HCl, HBr and HI Ag⁺ ions to form precipitate of AgCl, AgBr and Agl respectively. Remember that AgF is soluble water.  

(a) AgCl is white, insoluble in HNO₃ but soluble in NH₄OH forming Ag(NH₃)₂ Cl complex.

(b) AgBr is pale yellow precipitate, insoluble in HNO₃ but sparingly soluble in NH₄OH.

(c) Agl is dark yellow precipitate, insoluble both in HNO₃ and NH₄OH 

(xil) Hydriodic acid reacts with copper sulphate liberating iodine formation of cupric iodide (this reaction is not given by HCI and HBr).

2CuSO₄ + 4HI → 2CuI₂ + 2H₂SO₄ 

\(\underset{\text{Cuprise iodine}}{2Cul_2} \longrightarrow \underset{\text{Cuprous iodide}}{Cu_2l_2 + I_2}\)

(xiii) Formation of aqua regia. A mixture of conc. HCl (3 parts) and conc. HNO₃ (1 part) is known as aqua regia. This is used for dissolving noble metals like Au and Pt as their chlorides.

Comparison of Hydracids (HCI, HBr and HI) 

	Property	HCI	HBr	HI
1.	Physical state**	Gas	Gas	Gas
2.	Stability	Stable	Unstable	Least stable
3.	Reducing power	Least	More than HCI	Maximum
4.	Strength of acid	Weak	Moderate	Strong
5.	Action of a mixture of MnO2 and H2SO4	Chlorine evolved gas is	Br2 gas (reddish brown vapour) is evolved. Turns starch paper yellow.	I2 gas (violet vapour) is evolved. Turns starch paper violet.
6.	Action with AgNO3 solution	White ppt. insoluble in HNO3 but soluble in NH4OH	Pale yellow ppt., insoluble in HNO3, sparingly soluble in NH4OH.	Yellow ppt. insoluble both in HNO3 and NH4OH
7.	Action of lead acetate solution	White ppt. of PbCl2 soluble in hot water	White ppt. of PbBr2 soluble in hot water	Yellow ppt. of Poly PbI2 soluble in hot water giving colourless solution
8.	Action of mercurous nitrate solution	White ppt. soluble in aqua regia
9.	Action of mercuric chloride solution			Scarlet ppt.
10.	Action of CuSO4 solution			Liberates I2
11.	Action of X2	Cl2 is liberated only by F2	Br2 is liberared by F2 and Cl2	I2 is liberated F2 by Cl2 and Br2 F2
12.	Confirmatory test	Heating with K2Cr2O7 + H2SO4 gives chromyl chloride (CrO2Cl2) which when passed fumes into Icad acetate solution gives a yellow ppt. of lead chromate.	When treated with chlorine water in presence orange of  CS2; orange colour is obtained in CS2 layer,	When treated with chlorine water in presence of CS2: violet  colour is obtained in CS2 layer.

Note the following order of properties among hydracids.

(i) HF > HI > HBr > HCl (boiling point) 

(ii) HI > HBr > HCl > HF (acid strength)

(iii) HF > HCl > HBr > HI (bond polarity)

(iv) HF > HCl > HBr > HI (dipole moment)

(v) HI > HBr > HCl > HF (bond length)

(vi) HF > HCI > HBr > HI (thermal stability) 

(vii) HI > HBr > HCl > HF (reducing power)

(viii) MF > MCl > MBr > MI (Ionic character of halides)

Uses of HCl.

(i) It is used in preparation of Cl₂ chlorides, aqua regia, etc.

(ii) It is a common laboratory reagent.

(iii) It is used in cleaning metal surfaces before soldering or electroplating.

(iv) It is also used in medicines.

Uses of HBr. 

(i) It is used as a laboratory reagent for preparing bromo derivatives unsaturated organic compounds.

(ii) Sod. and pot. bromides are used in medicines (sedatives).

Uses of HI

It is used as a reducing agent in organic chemistry.

Hydrofluoric acid, H₂F₂ or HF. Since hydrogen and fluorine combine with explosion even in the dark to form hydrofluoric acid, the reaction cannot be employed for the preparation of HF.

(i) Anhydrous hydrogen fluoride (HF) is obtained by heating dry potassium hydrogen fluoride.

\(KHF_2\overset{heat}{ \longrightarrow} KF + HF\)                                                (Lab. method) 

(ii) Industrially, the acid is prepared by heating fluorspar (CaF₂) with concentrated sulphuric acid.

\(CaF_2 + H_2SO_4 \longrightarrow CaSO_4 + 2HF\)

Hydrofluoric acid (aqueous hydrogen fluoride) is stored in gutta percha or wax bottles. It cannot be stored in glass or silica bottles as it attacks silicates as well as silica according to following reaction.

Na₂SiO₃ + 6HF → Na₂SiF₆ + 3H₂O

CaSiO₃ + 6HF → CaSiF₆ + 3H₂O

SiO₂ + 4HF → SiF₄ + 2H₂O 

SiF₄ + 2HF → H₂SiF₆

This action of hydrofluoric acid on silica and silicates is used for etching glass. The glass surface to be etched is coated with paraffin wax, the design is scratched on glass through wax coating. The whole surface is exposed to HF gas or 40% aq. solution when only the scratched portion of the glass is attacked.

Properties:-

(i) Hydrofluoric acid is colourless corrosive liquid with pungent smell and high b.p. due to hydrogen bonding.

(ii) The dry liquid does not attack metals under ordinary conditions (except potassium), but in presence of water it attacks glass and dissolves metals with the liberation of hydrogen gas.

(iii) It is a weak dibasic acid (due to strong H-F bond) and forms two series of salts, viz. NaHF₂ and NaF.