Chemistry Hydrogen Chloride and Oxoacids of Halogens

Topics Covered :

● Preparation of Hydrogen Chloride
● Properties of Hydrogen Chloride
● Uses of Hydrogen Chloride
● Oxoacids of Halogens

Hydrogen Chloride :

`=>` Glauber prepared this acid in `1648` by heating common salt with concentrated sulphuric acid.

`=>` Davy in `1810` showed that it is a compound of hydrogen and chlorine.

Preparation :

`=>` In laboratory, it is prepared by heating sodium chloride with concentrated sulphuric acid.

`color{red}(NaCl +H_2SO_4 overset(420 K)→ NaHSO_4 +HCl)`

`color{red}(NaHSO_4 +NaCl overset(823 K)→ Na_2SO_4 +HCl)`

`color{red}(HCl)` gas can be dried by passing through concentrated sulphuric acid.

Properties :

`=>` It is a colourless and pungent smelling gas.

`=>` It is easily liquefied to a colourless liquid (b.p. `189 K`) and freezes to a white crystalline solid (f.p. `159 K`).

`=>` It is extremely soluble in water and ionises as below :

`color{red}(HCl (g ) +H_2O (l) → H_3O^(+) (aq) +Cl^(-) (aq) \ \ \ \ \ K_a = 10^7)`

`=>` Its aqueous solution is called hydrochloric acid.

`=>` High value of dissociation constant `color{red}(K_a)` indicates that it is a strong acid in water.

`=>` It reacts with `color{red}(NH_3)` and gives white fumes of `color{red}(NH_4Cl)`.

`color{red}(NH_3 + HCl → NH_4Cl)`

`=>` When three parts of concentrated `color{red}(HCl)` and one part of concentrated `color{red}(HNO_3)` are mixed, aqua regia is formed which is used for dissolving noble metals, e.g., gold, platinum.

`color{red}(Au +4H^(+) + NO_3^(-) + 4Cl^(-) → Au Cl_4^(-) + NO +2H_2O)`

`color{red}(3Pt +16 H^(+) +4NO_3^(-) +18 Cl^(-) → 3Pt Cl_6^(2-) +4NO +8H_2O)`

`=>` Hydrochloric acid decomposes salts of weaker acids, e.g., carbonates, hydrogencarbonates, sulphites, etc.

`color{red}(Na_2CO_3+2HCl → 2NaCl +H_2O +CO_2)`

`color{red}(NaHCO_3 +HCl → NaCl +H_2O +CO_2)`

`color{red}(Na_2SO_3+2HCl → 2NaCl +H_2O +SO_2)`

Uses :

`color{green}("It is used")` :

● In the manufacture of chlorine, `color{red}(NH_4Cl)` and glucose (from corn starch)

● For extracting glue from bones and purifying bone black

● In medicine and as a laboratory reagent.

Q 3000691518

When `HCl` reacts with finely powdered iron, it forms ferrous chloride and not ferric chloride. Why?


Its reaction with iron produces `H_2`.
` Fe + 2HCl → FeCl_2 + H_2`
Liberation of hydrogen prevents the formation of ferric chloride.

Oxoacids of Halogens :

`=>` Due to high electronegativity and small size, fluorine forms only one oxoacid, `color{red}(HOF)` known as fluoric (I) acid or hypofluorous acid.

`=>` The other halogens form several oxoacids.

`=>` Most of them cannot be isolated in pure state.

`=>` They are stable only in aqueous solutions or in the form of their salts.

`=>` The oxoacids of halogens are given in Table 7.10 and their structures are given in Fig. 7.8.