Comprehensive Chemistry Study Notes: Chlorine, Sulphur, Nitrogen, and Organic Compounds

Laboratory Preparation: Chlorine is produced by heating concentrated Hydrochloric acid ( $HCl$ ) with oxidizing agents like Manganese(IV) oxide ( $MnO_2$ ) or Potassium permanganate ( $KMnO_4$ ).
- $MnO_2(s) + 4HCl(aq) \rightarrow MnCl_2(aq) + 2H_2O(l) + Cl_2(g)$
- Purification: The gas is passed through water to remove trace $HCl$ gas and through concentrated Tetraoxosulphate(VI) acid ( $H_2SO_4$ ) to dry it.
- Physical Properties: Greenish-yellow gas with a choking smell, poisonous, and about $2.5$ times denser than air.
- Chemical Properties:
  - Reaction with Water: Forms a mixture of Hydrochloric acid ( $HCl$ ) and Oxochlorate(I) acid ( $HOCl$ ). Under sunlight, $HOCl$ decomposes to release oxygen.
  - Bleaching Agent: $HOCl$ is unstable and releases nascent oxygen ( $[O]$ ), which oxidizes dyes to a colorless form.
  - Reaction with Alkalis: With cold dilute $NaOH$ , it forms Sodium oxochlorate(I) ( $NaOCl$ ); with hot concentrated $NaOH$ , it forms Sodium trioxochlorate(V) ( $NaClO_3$ ).
- Tests for Chlorine:
  - Damp blue litmus paper turns red then bleaches white.
  - Moist starch-iodide paper turns blue-black due to displaced iodine.

Preparation: Prepared by the action of concentrated $H_2SO_4$ on a soluble chloride like $NaCl$ .
- $2NaCl(s) + H_2SO_4(aq) \rightarrow Na_2SO_4(aq) + 2HCl(g)$
Properties: Colourless, acidic, and much denser than air. Forms misty white fumes in moist air.
Tests:
- Forms dense white fumes of Ammonium chloride ( $NH_4Cl$ ) when brought near ammonia solution.
- Forms a white precipitate of Silver chloride ( $AgCl$ ) with Silver trioxonitrate(V) ( $AgNO_3$ ).

Occurrence: Found in underground deposits and combined as sulphides (e.g., Pyrite $FeS_2$ , Galena $PbS$ ).
Extraction (Frasch Process): Involves three concentric pipes. Super-heated water ( $170\,^{\circ}C$ , $10\,atm$ ) melts the sulphur, which is then forced up by hot compressed air ( $15\,atm$ ).
Allotropes:
- Crystalline: Rhombic (stable below $96\,^{\circ}C$ ) and Monoclinic (prepared by cooling molten sulphur).
- Non-crystalline: Plastic (soft, elastic, insoluble in $CS_2$ ) and Amorphous (precisely $\\delta$ -Sulphur).
Chemical Properties: Combines with metals to form sulphides (e.g., $Fe + S \rightarrow FeS$ ). Reacts with concentrated $H_2SO_4$ to produce $SO_2$ gas.

Hydrogen Sulphide ( $H_2S$ ): Prepared from Iron(II) sulphide ( $FeS$ ) and dilute acids. It has a rotten egg smell and acts as a powerful reducing agent.
- Test: Turns filter paper moistened with Lead(II) ethanoate black ( $PbS$ ).
Sulphur(IV) Oxide ( $SO_2$ ): Prepared by heating $Na_2SO_3$ with dilute acid or $Cu$ with concentrated $H_2SO_4$ .
- Bleaching: Bleaches by reduction in the presence of water; the effect is often temporary as dyes re-oxidize in air.
- Test: Decolorizes purple acidified $KMnO_4$ and turns orange acidified $K_2Cr_2O_7$ green.
Tetraoxosulphate(VI) Acid ( $H_2SO_4$ ):
- Contact Process:
  1. $S + O_2 \rightarrow SO_2$
  2. $2SO_2 + O_2 \rightleftharpoons 2SO_3$ (using $V_2O_5$ catalyst at $450\text{-}500\,^{\circ}C$ ).
  3. $SO_3$ is dissolved in conc. $H_2SO_4$ to form Oleum ( $H_2S_2O_7$ ), then diluted.
- Properties: Dehydrating agent (removes water elements from sugar or ethanol) and an oxidizing agent.

Industrial Preparation: Fractional distillation of liquid air ( $N_2$ distils at $-196\,^{\circ}C$ ; $O_2$ at $-183\,^{\circ}C$ ).
Laboratory Preparation: Thermal decomposition of Ammonium dioxonitrate(III) ( $NH_4NO_2$ ).
- $NH_4NO_2(s) \rightarrow N_2(g) + 2H_2O(g)$
Ammonia ( $NH_3$ ): Prepared by heating an ammonium salt with a non-volatile base ( $Ca(OH)_2$ ). Dried using Calcium oxide ( $CaO$ ).
- Haber Process: $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$ (Conditions: $450\,^{\circ}C$ , $200\,atm$ , $Fe$ catalyst).
Nitrogen(IV) Oxide ( $NO_2$ ): Reddish-brown, poisonous gas prepared by heating Lead(II) trioxonitrate(V) ( $Pb(NO_3)_2$ ). It is a mixed acid anhydride (forms both $HNO_2$ and $HNO_3$ in water).

Vital Concepts:
- Catenation: Ability of carbon to link into chains or rings.
- Homologous Series: Family of compounds with the same general formula, similar chemical properties, and structural patterns differing by a $-CH_2$ group ( $14\,g/mol$ ).
- Functional Group: Atom or group determining chemical properties (e.g., $-OH$ for alkanols).
- Isomerism: Compounds with the same molecular formula but different structural arrangements.
Geometric Isomerism: Occurs in alkenes (Cis-Trans) due to restricted rotation about the double bond.

Alkanes ( $C_nH_{2n+2}$ ): Saturated hydrocarbons; undergo substitution and combustion.
- Methane ( $CH_4$ ): Prepared by heating Sodium ethanoate ( $CH_3COONa$ ) with Soda-lime.
Alkenes ( $C_nH_{2n}$ ): Unsaturated (at least one $C=C$ ); undergo addition reactions.
- Ethene ( $C_2H_4$ ): Prepared by dehydrating Ethanol ( $C_2H_5OH$ ) with concentrated $H_2SO_4$ at $170\,^{\circ}C$ .
Alkynes ( $C_nH_{2n-2}$ ): Highly unsaturated (at least one $C \equiv C$ ).
- Ethyne ( $C_2H_2$ ): Prepared by the action of cold water on Calcium carbide ( $CaC_2$ ).
- Test for Unsaturation: Both alkenes and alkynes decolorize Bromine water or acidified $KMnO_4$ . Ethyne uniquely forms metal dicarbides (e.g., $Ag_2C_2$ ) with ammoniacal silver solutions.

Q: Why is soda-lime used instead of caustic soda when preparing methane?
A: Soda-lime does not readily attack glass and is not deliquescent.
Q: How do you test for $H_2S$ gas?
A: Use filter paper moistened with Lead(II) ethanoate; it will turn black.
Q: Why can't conc. $H_2SO_4$ dry ammonia?
A: Because they react to form Ammonium tetraoxosulphate(VI) ( $(NH_4)_2SO_4$ ).
Classwork/Assignment References: New School Chemistry by Osei Yaw Ababio (6th Edition) pages 369, 405, 426, 427, and 547.