Chlorine and Hydrogen Chloride — Revision Notes

⚡ 30-Second Revision

Chlorine ($Cl_2$) — Greenish-yellow gas, pungent. Strong oxidizing agent.
Preparation — Lab: $MnO_2 + 4HCl \rightarrow MnCl_2 + Cl_2 + 2H_2O$. Ind: Deacon's ($4HCl + O_2 \xrightarrow{CuCl_2} 2Cl_2 + 2H_2O$), Electrolysis of brine ($2NaCl + 2H_2O \rightarrow 2NaOH + Cl_2 + H_2$).
Reactions

- With $H_2O$: $Cl_2 + H_2O \rightleftharpoons HCl + HOCl$. $HOCl \rightarrow HCl + [O]$ (bleaching). - With cold, dilute $NaOH$: $Cl_2 + 2NaOH \rightarrow NaCl + NaOCl + H_2O$. - With hot, conc. $NaOH$: $3Cl_2 + 6NaOH \rightarrow 5NaCl + NaClO_3 + 3H_2O$.

Bleaching — By oxidation (nascent oxygen), permanent, requires moisture.
Hydrogen Chloride (HCl) — Colorless gas, pungent. Highly soluble in water.
Preparation — Lab: $NaCl + H_2SO_4(conc) \xrightarrow{<200^circ C} NaHSO_4 + HCl$. Ind: $H_2 + Cl_2 \rightarrow 2HCl$.
Properties — Anhydrous gas is not acidic. Aqueous solution (hydrochloric acid) is a strong acid.
Reactions

- With metals: $Zn + 2HCl \rightarrow ZnCl_2 + H_2$. - With bases: $NaOH + HCl \rightarrow NaCl + H_2O$. - With carbonates: $Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2$. - With $NH_3$: $NH_3 + HCl \rightarrow NH_4Cl$ (white fumes).

Aqua Regia — 1:3 $HNO_3:HCl$, dissolves noble metals.

2-Minute Revision

For NEET, remember Chlorine ($Cl_2$) is a greenish-yellow gas, a strong oxidizer. Its industrial production via electrolysis of brine (chlor-alkali process) or Deacon's process (catalytic oxidation of HCl) and lab preparation from $MnO_2$ and HCl are crucial.

Key reactions include its disproportionation with alkalis: cold/dilute NaOH yields NaCl and NaOCl, while hot/concentrated NaOH gives NaCl and $NaClO_3$. Crucially, chlorine's permanent bleaching action requires moisture, as it's mediated by nascent oxygen from hypochlorous acid.

Hydrogen chloride (HCl) is a colorless gas, extremely soluble in water to form hydrochloric acid, a strong acid. Its lab preparation involves NaCl and concentrated $H_2SO_4$. Remember that anhydrous HCl gas is not acidic; acidity only manifests in aqueous solution.

Important reactions include its acidic properties with metals, bases, and carbonates, and its reaction with ammonia to form white fumes of $NH_4Cl$. Also, recall aqua regia (1:3 $HNO_3:HCl$) for dissolving noble metals.

Focus on balanced equations, reaction conditions, and the distinct properties of the gas vs. its aqueous solution.

5-Minute Revision

Let's consolidate the key aspects of Chlorine and Hydrogen Chloride for NEET. Chlorine ($Cl_2$), a greenish-yellow gas, is a highly reactive halogen. Its strong oxidizing power is due to its electron-deficient nature.

Industrially, it's obtained from the chlor-alkali process (electrolysis of brine: $2NaCl(aq) + 2H_2O(l) \rightarrow 2NaOH(aq) + Cl_2(g) + H_2(g)$) or Deacon's process ($4HCl(g) + O_2(g) \xrightarrow{CuCl_2, 723 K} 2Cl_2(g) + 2H_2O(g)$).

In the lab, $MnO_2$ reacts with concentrated HCl to yield $Cl_2$.

Chlorine's reactions are vital: it forms chlorides with metals (e.g., $2Fe + 3Cl_2 \rightarrow 2FeCl_3$). With water, it forms HCl and HOCl, the latter decomposing to nascent oxygen ($[O]$) responsible for its permanent bleaching action, which *requires moisture*. Its disproportionation with alkalis is critical: with cold, dilute NaOH, it gives $NaCl$ and $NaOCl$ (sodium hypochlorite); with hot, concentrated NaOH, it yields $NaCl$ and $NaClO_3$ (sodium chlorate).

Hydrogen chloride (HCl) is a colorless, pungent gas. Its lab preparation is from $NaCl$ and concentrated $H_2SO_4$ ($NaCl(s) + H_2SO_4(conc) \rightarrow NaHSO_4(s) + HCl(g)$). Industrially, it's from direct synthesis ($H_2 + Cl_2 \rightarrow 2HCl$).

Crucially, anhydrous HCl gas is not acidic; its acidity is only observed in aqueous solution (hydrochloric acid) due to ionization ($HCl(aq) + H_2O(l) \rightarrow H_3O^+(aq) + Cl^-(aq)$). Hydrochloric acid is a strong monoprotic acid, reacting with metals (e.

g., $Zn + 2HCl \rightarrow ZnCl_2 + H_2$), bases, carbonates (producing $CO_2$), and bicarbonates. A notable reaction is with ammonia, forming dense white fumes of ammonium chloride ($NH_3(g) + HCl(g) \rightarrow NH_4Cl(s)$).

Remember aqua regia, a 1:3 mixture of $HNO_3$ and $HCl$, capable of dissolving noble metals like gold due to the formation of nascent chlorine and complex ions. Focus on balanced equations, reaction conditions, and the distinct properties of the gaseous and aqueous forms of HCl.

Prelims Revision Notes

Chlorine ($Cl_2$)

Nature — Greenish-yellow gas, pungent, suffocating odor. Diatomic molecule.
Position — Group 17 (Halogen), Atomic No. 17, Electronic config: $[Ne]3s^23p^5$.
Oxidation States — Primarily -1, but also +1, +3, +5, +7 in oxyacids and oxides.
Preparation (Key Reactions)

* Lab: $MnO_2(s) + 4HCl(aq) \xrightarrow{\Delta} MnCl_2(aq) + Cl_2(g) + 2H_2O(l)$ * Industrial (Deacon's): $4HCl(g) + O_2(g) \xrightarrow{CuCl_2, 723 K} 2Cl_2(g) + 2H_2O(g)$ * Industrial (Electrolytic/Chlor-alkali): $2NaCl(aq) + 2H_2O(l) \xrightarrow{\text{electrolysis}} 2NaOH(aq) + Cl_2(g) + H_2(g)$

Chemical Properties

* Oxidizing Agent: Strong, gains electrons. * With Metals: Forms higher chlorides, e.g., $2Fe + 3Cl_2 \rightarrow 2FeCl_3$. * With Non-metals: $H_2 + Cl_2 \rightarrow 2HCl$. * With Water: $Cl_2 + H_2O \rightleftharpoons HCl + HOCl$.

(Chlorine water) * Bleaching Action: Due to nascent oxygen from $HOCl$ ($HOCl \rightarrow HCl + [O]$). Permanent. Requires moisture. Dry $Cl_2$ does not bleach. * With Alkalis (Disproportionation): * Cold, dilute NaOH: $Cl_2 + 2NaOH \rightarrow NaCl + NaOCl + H_2O$ (Sodium hypochlorite) * **Hot, conc.

NaOH**: $3Cl_2 + 6NaOH \rightarrow 5NaCl + NaClO_3 + 3H_2O$ (Sodium chlorate) * With Ammonia: Excess $NH_3 \rightarrow N_2 + NH_4Cl$. Excess $Cl_2 \rightarrow NCl_3 + HCl$.

Uses — Water purification, bleaching, manufacturing PVC, chloroform, DDT, $HCl$.

Hydrogen Chloride (HCl)

Nature — Colorless gas, pungent odor. Covalent molecule. Highly soluble in water.
Acidity — Anhydrous HCl gas is not acidic. Hydrochloric acid (aqueous solution) is a strong acid due to complete ionization ($HCl + H_2O \rightarrow H_3O^+ + Cl^-$).
Preparation (Key Reactions)

* Lab: $NaCl(s) + H_2SO_4(conc) \xrightarrow{<200^circ C} NaHSO_4(s) + HCl(g)$ * Industrial: $H_2(g) + Cl_2(g) \rightarrow 2HCl(g)$

Chemical Properties

* Acidic Reactions (with aqueous HCl): * With Metals (above H in reactivity): $Zn + 2HCl \rightarrow ZnCl_2 + H_2$. * With Bases: $NaOH + HCl \rightarrow NaCl + H_2O$. * With Carbonates/Bicarbonates: $Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2$.

* With Ammonia: $NH_3(g) + HCl(g) \rightarrow NH_4Cl(s)$ (dense white fumes). * Aqua Regia: 1 part conc. $HNO_3$ + 3 parts conc. $HCl$. Dissolves noble metals (Au, Pt) due to nascent chlorine and complex formation (e.

g., $H[AuCl_4]$).

Uses — Pickling steel, manufacturing chemicals, lab reagent, food industry.

Vyyuha Quick Recall

Chlorine Loves Oxidation, Reacts Intensely, Needs Everything Moist for Bleaching. Hydrogen Chloride Loves Acid, Soluble In Water, Dry Gas Not Acidic.