Comprehensive Study Notes: p-Block Elements (Groups 15 to 18)

Group 15 (Nitrogen group) elements

• Introduction: This unit covers the p-block groups: pnictogens (Group 15), chalcogens (Group 16), halogens (Group 17), and inert gases (Group 18).
• Occurrence:   - Nitrogen: Approximately $78 \,$ of the earth's atmosphere is dinitrogen ($N_2$) gas. It is found in the earth's crust as sodium nitrate (Chile saltpetre) and potassium nitrate (Indian saltpetre).   - Phosphorus: The 11th most abundant element; exists as phosphate minerals (fluroapatite, chloroapatite, and hydroxyapatite).   - Arsenic, Antimony, and Bismuth: Present as sulphides and are not very abundant.
• Physical Properties of Group 15 Elements:   - Nitrogen: Gas at $293\,K$; Atomic Number: $7$; Isotopes: $^{14}N$, $^{15}N$; Atomic Mass: $14\,g\,mol^{-1}$; Configuration: $[He]2s^2 2p^3$; Atomic radius: 1.55\,\text{}; Density: $1.14 \times 10^{-3}\,g\,cm^{-3}$; MP: $63\,K$; BP: $77\,K$.   - Phosphorus: Solid; Atomic Number: $15$; Isotopes: $^{31}P$; Atomic Mass: $30.97\,g\,mol^{-1}$; Configuration: $[Ne]3s^2 3p^3$; Atomic radius: 1.80\,\text{}; Density (white): $1.82\,g\,cm^{-3}$; MP: $317\,K$; BP: $554\,K$.   - Arsenic: Solid; Atomic Number: $33$; Isotopes: $^{75}As$; Atomic Mass: $74.92\,g\,mol^{-1}$; Configuration: $[Ar]3d^{10} 4s^2 4p^3$; Atomic radius: 1.85\,\text{}; Density: $5.75\,g\,cm^{-3}$; MP: Sublimes at $889\,K$; BP: $889\,K$.   - Antimony: Solid; Atomic Number: $51$; Isotopes: $^{121}Sb$; Atomic Mass: $121.76\,g\,mol^{-1}$; Configuration: $[Kr]4d^{10} 5s^2 5p^3$; Atomic radius: 2.06\,\text{}; Density: $6.68\,g\,cm^{-3}$; MP: $904\,K$; BP: $1860\,K$.   - Bismuth: Solid; Atomic Number: $83$; Isotopes: $^{209}Bi$; Atomic Mass: $209.98\,g\,mol^{-1}$; Configuration: $[Xe]4f^{14} 5d^{10} 6s^2 6p^3$; Atomic radius: 2.07\,\text{}; Density: $9.79\,g\,cm^{-3}$; MP: $544\,K$; BP: $1837\,K$.

Nitrogen ($N_2$)

• Preparation:   - Industrial: Separated from liquid air by fractional distillation.   - Thermal Decomposition: Pure nitrogen is obtained by heating sodium azide at about $575\,K$:     - $2NaN_3 \rightarrow 2Na + 3N_2$   - Oxidation of Ammonia: Using bromine water:     - $8NH_3 + 3Br_2 \rightarrow 6NH_4Br + N_2$
• Properties:   - Inert Character: Nitrogen is relatively inert due to high bond energy ($225\,cal\,mol^{-1}$ or $946\,kJ\,mol^{-1}$).   - Isotopic Composition: Terrestrial nitrogen contains $14.5 \,$ of $N^{14}$ and $0.4 \,$ of $N^{15}$; the latter is used for isotopic labelling.   - Reactivity: Only reacts with Lithium at room temperature: $6Li + N_2 \rightarrow 2Li_3N$.   - Other Reactions: Combines with Group 2 metals and Thorium at elevated temperatures to form ionic nitrides (e.g., $3Ca + N_2 \xrightarrow{\text{red hot}} Ca_3N_2$, $2B + N_2 \xrightarrow{\text{bright red hot}} 2BN$).   - Haber's Process: Direct reaction with hydrogen to synthesize ammonia: $N_2 + 3H_2 \rightleftharpoons 2NH_3$ ($\Delta H_r = -46.2\,kJ\,mol^{-1}$). Reacts with oxygen at high temperatures ($3473\,K$) yielding only $4.4 \,$ nitrous oxide: $2N_2 + O_2 → 2N_2O$.
• Uses:   - Manufacture of ammonia, nitric acid, and calcium cyanamide.   - Liquid nitrogen: Used in cryosurgery and biological preservation.

Ammonia ($NH_3$)

• Preparation:   - Urea Hydrolysis: $NH_2CONH_2 + H_2O \rightarrow 2NH_3 + CO_2$   - Laboratory: Heating an ammonium salt with a base: $NH_4^+ + OH^- \rightarrow NH_3 + H_2O$ or $2NH_4Cl + CaO \rightarrow CaCl_2 + 2NH_3 + H_2O$.   - Metal Nitrides: Heating magnesium nitride with water: $Mg_3N_2 + 6H_2O \rightarrow 3Mg(OH)_2 + 2NH_3$.   - Industrial (Haber Process): $N_2$ and $H_2$ passed over iron catalyst (with $K_2O$ and $Al_2O_3$ promoters) at $750\,K$ and $200\,atm$ pressure.
• Properties:   - Physical: Pungent smelling gas, lighter than air. Liquefies at $9\,atm$. BP: $-38.4^\circ C$, FP: $-77^\circ C$. Highly associated via hydrogen bonding.   - Solubility: Extremely soluble in water (702 volumes in 1 volume water at $20^\circ C$, $760\,mm$ pressure). Forms hydrates ($NH_3 \cdot H_2O$ and $2NH_3 \cdot H_2O$).   - Dielectric Constant: High, making it a good ionizing solvent.   - Chemical Properties:     - Thermal Action: Decomposes above $500^\circ C$ into elements: $2NH_3 \rightarrow N_2 + 3H_2$.     - Reaction with Air/Oxygen: Burns in free oxygen with yellowish flame ($4NH_3 + 3O_2 \rightarrow 2N_2 + 6H_2O$). In the presence of Platinum (Ostwald's process): $4NH_3 + 5O_2 \rightarrow 4NO + 6H_2O$.     - Reducing Agent: Reduces metal oxides (e.g., $3PbO + 2NH_3 \rightarrow 3Pb + N_2 + 3H_2O$).     - Reaction with Chlorine:       - Excess ammonia: $8NH_3 + 3Cl_2 \rightarrow N_2 + 6NH_4Cl$       - Excess chlorine: $2NH_3 + 6Cl_2 \rightarrow 2NCl_3 + 6HCl$ (forms explosive nitrogen trichloride).     - Coordination Compounds: Forms complexes such as $[CaCl_2 \cdot 8NH_3]$, $[Cu(NH_3)_4]^{2+}$ (deep blue color), and $[Ag(NH_3)_2]^+$.
• Structure: Pyramidal shape ($sp^3$ hybridization with one lone pair). $N-H$ distance: 1.016\,\text{}, $H-H$ distance: 1.645\,\text{}, Bond angle: $107^\circ$.

Nitric Acid ($HNO_3$)

• Preparation:   - Laboratory: Heating $KNO_3$ or $NaNO_3$ with concentrated $H_2SO_4$: $KNO_3 + H_2SO_4 \rightarrow KHSO_4 + HNO_3$.   - Industrial (Ostwald's Process):     1. Catalytic oxidation of ammonia to $NO$ at $1275\,K$ using Platinum gauze.     2. Oxidation of $NO$ to $NO_2$: $2NO + O_2 \rightarrow 2NO_2$.     3. $NO_2$ absorption in water: $3NO_2 + H_2O \rightarrow 2HNO_3 + NO$.
• Properties:   - Colorless liquid, BP: $86^\circ C$. Forms a constant boiling mixture ($98 \,$, BP: $120.5^\circ C$). Pure acid turns yellow on standing due to decomposition into $NO_2$.   - Chemical Actions: Acts as an acid (e.g., $ZnO + 2HNO_3 \rightarrow Zn(NO_3)_2 + H_2O$), an oxidizing agent, and a nitrating agent.
• Oxidizing Agent Actions:   - Non-metals: Carbon oxidized to $CO_2$, Sulphur to $H_2SO_4$, Phosphorus to $H_3PO_4$, Iodine to $HIO_3$.   - Metals: Reacts with all metals except gold, platinum, rhodium, iridium, and tantalum. Metals like $Al, Fe, Co, Ni,$ and $Cr$ become passive in concentrated acid because of oxide layer formation.
• Steps of Metal Reactions:   1. Primary reaction: Metal nitrate formation and nascent hydrogen release ($M + HNO_3 \rightarrow MNO_3 + (H)$).   2. Secondary reaction: Preparation of reduction products ($HNO_2$, $NH_2OH$, $NH_3$, $H_2N_2O_2$).   3. Tertiary reaction: Decomposition or reaction of secondary products (formation of $NO$, $N_2O_3$, $N_2O$, $NO_2$).
• Copper and Magnesium Examples:   - Copper with Dilute acid: $3Cu + 8HNO_3 \rightarrow 3Cu(NO_3)_2 + 2NO + 4H_2O$.   - Copper with Conc. acid: $Cu + 4HNO_3 \rightarrow Cu(NO_3)_2 + 2NO_2 + 2H_2O$.   - Magnesium with acid (giving $NH_4NO_3$): $4Mg + 10HNO_3 \rightarrow 4Mg(NO_3)_2 + NH_4NO_3 + 3H_2O$.   - Magnesium with diluted acid (giving $N_2O$): $4Mg + 10HNO_3 \rightarrow 4Mg(NO_3)_2 + N_2O + 5H_2O$.
• Uses: Oxidizing agent, preparation of aquaregia, photography ($AgNO_3$), and gunpowder ($NaNO_3$).

Oxides and Oxoacids of Nitrogen

• Oxides Table:   - Nitrous oxide ($N_2O$): State +1; Colourless gas, neutral; Structure: linear ($N \equiv N \rightarrow O$).   - Nitric oxide ($NO$): State +2; Colourless gas, neutral.   - Dinitrogen trioxide ($N_2O_3$): State +3; Blue solid, acidic.   - Nitrogen dioxide ($NO_2$): State +4; Brown gas, acidic.   - Dinitrogen tetraoxide ($N_2O_4$): State +4; Colourless solid/liquid, acidic.   - Dinitrogen pentoxide ($N_2O_5$): State +5; Colourless solid, acidic.
• Oxoacids Table:   - Hyponitrous acid ($H_2N_2O_2$): State +1; Prep: $Ag_2N_2O_2 + 2HCl \rightarrow 2AgCl + H_2N_2O_2$.   - Nitrous acid ($HNO_2$): State +3; Prep: $Ba(NO_2)_2 + H_2SO_4 \rightarrow 2HNO_2 + BaSO_4$.   - Pernitrous acid ($HOONO$): State +3; Prep: $H_2O_2 + HONO \rightarrow HOONO + H_2O$.   - Nitric acid ($HNO_3$): State +5.   - Pernitric acid ($HNO_4$): State +5; Prep: $H_2O_2 + N_2O_5 \rightarrow HNO_4 + HNO_3$.

Phosphorus Allotropes and Properties

• Allotropes:   - White Phosphorus: Colourless/pale yellow; poisonous; garlic smell; phosphorescence (glows in dark). Low ignition temperature; undergoes spontaneous combustion at room temperature.   - Red Phosphorus: Formed by heating white phosphorus at $420^\circ C$ without air/light. Non-poisonous, no phosphorescence, stable polymeric structure.   - Other Forms: Black, scarlet, and violet phosphorus.
• Chemical Properties:   - Reaction with Oxygen: White phosphorus catches fire to give $P_4O_{10}$. Phosphorus trioxide ($P_4O_6$) and pentoxide ($P_4O_{10}$) are produced.   - Reaction with Chlorine: Phosphorus reacts to form trichloride ($PCl_3$) and pentachloride ($PCl_5$).

Phosphine ($PH_3$)

• Preparation:   - Hydrolysis of metallic phosphides: $Ca_3P_2 + 6H_2O \rightarrow 2PH_3 + 3Ca(OH)_2$.   - Disproportionation: Heating phosphorous acid: $4H_3PO_3 \rightarrow 3H_3PO_4 + PH_3$.   - Pure sample: Heating phosphonium iodide with caustic soda: $PH_4I + NaOH → PH_3 + NaI + H_2O$.
• Properties: Colourless, poisonous gas with rotten fish smell. Boiling point: $188\,K$. Weakly basic; forms phosphonium salts (e.g., $PH_4I$).
• Structure: Pyramidal shape ($sp^3$); bond angle reduced to $93.5^e$ due to lone pair.
• Uses: Producing smoke screens; Holmes signal (using $CaC_2$ and $Ca_3P_2$) for naval signals.

Phosphorus Chlorides

• Phosphorus trichloride ($PCl_3$):   - Preparation: Passing chlorine over white phosphorus or using thionyl chloride ($P_4 + 8SOCI_2 \rightarrow 4PCl_3 + 4SO_2 + 2S_2Cl_2$).   - Hydrolysis: Reacts with water to form phosphorous acid ($H_3PO_3$).   - Structure: Pyramidal; bond angle $100^\circ$; $P-Cl$ bond length $204\,pm$.
• Phosphorus pentachloride ($PCl_5$):   - Preparation: $PCl_3 + Cl_2 \rightarrow PCl_5$.   - Reactivity: Decomposes on heating ($PCl_5 \rightleftharpoons PCl_3 + Cl_2$). Hydrolyzes to phosphoryl chloride ($POCl_3$) then orthophosphoric acid ($H_3PO_4$).   - Uses: Chlorinating agent; replaces hydroxyl groups with chlorine.

Oxides and Oxoacids of Phosphorus

• Oxides Structure:   - Phosphorus trioxide ($P_4O_6$): P atoms at tetrahedron corners, O atoms along edges. $P-O$ distance is $165.6\,pm$.   - Phosphorus pentoxide ($P_4O_{10}$): Additional coordinate bonds with terminal oxygen atoms. Terminal $P-O$ bond length: $143\,pm$.
• Oxoacids Preparation Table:   - Hypophosphorous acid ($H_3PO_2$): state +1; prep: $P_4 + 6H_2O \rightarrow 3H_3PO_2 + PH_3$.   - Orthophosphorous acid ($H_3PO_3$): state +3; prep: $P_4O_6 + 6H_2O \rightarrow 4H_3PO_3$.   - Hypophosphoric acid ($H_4P_2O_6$): state +4; prep: $2P + 2O_2 + 2H_2O \rightarrow H_4P_2O_6$.   - Orthophosphoric acid ($H_3PO_4$): state +5; prep: $P_4O_{10} + 6H_2O \rightarrow 4H_3PO_4$.   - Pyrophosphoric acid ($H_4P_2O_7$): state +5; prep: $2H_3PO_4 \rightarrow H_4P_2O_7 + H_2O$.

Group 16 (Chalcogens/Oxygen Group)

• Occurrence: Oxygen is the most abundant element ($46.6\,$ of earth crust by weight). Sulphur exists as sulphates (gypsum, epsom) and sulphides (galena, zinc blende).
• Physical Properties Table (O, S, Se, Te, Po):   - Oxygen is gas ($BP: 90\,K$), others are solids. Radii increase from 1.52\,\text{} (O) to 1.97\,\text{} (Po).
• Oxygen ($O_2$):   - Industrial Preparation: Fractional distillation of liquefied air.   - Lab Preparation: Decomposition of $H_2O_2$ (with $MnO_2$) or thermal decomposition of oxides/oxoanions (e.g., $2KClO_3 \rightarrow 2KCl + 3O_2$).   - Properties: Diatomic, paramagnetic. Forms strong hydrogen bonds.
• Ozone ($O_3$):   - Preparation: Electric discharge through oxygen ($20,000\,V$ converts about $10\,$ to ozone).   - Structure: Bent shape, symmetrical with delocalised bonding.   - Property: Powerful oxidising agent (e.g., oxidises $KI$ to $I_2$). Used for organic oxidations.
• Allotropes of Sulphur:   - Rhombic (̑-sulphur): Thermodynamically stable at ordinary temp; yellow crystals ($S_8$).   - Monoclinic (̒-sulphur): Stable between $96^\circ C - 119^\circ C$. Long needle crystals.   - Plastic (̓-sulphur): Rubbery yellow ribbon formed by pouring molten sulphur into cold water.

Sulphur Dioxide ($SO_2$)

• Preparation: Burning sulphur in air ($S + O_2 → SO_2$) or roasting sulphide ores (e.g., $2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2$). Lab: $Cu + 2H_2SO_4 \rightarrow CuSO_4 + SO_2 + 2H_2O$.
• Properties: Colourless gas, suffocating odour, heavy. Soluble in water (forms sulphurous acid $H_2SO_3$).
• Chemical properties:   - Oxidising: Oxidises $H_2S$ to $S$ and $Mg$ to $MgO$.   - Reducing: Reduces $Cl_2$ to $HCl$, $KMnO_4$ to $Mn^{2+}$, and dichromate to $Cr^{3+}$.   - Bleaching action: Temporary bleaching via reduction ($SO_2 + 2H_2O \rightarrow H_2SO_4 + 2(H)$). Re-oxidises in air.
• Structure: $sp^2$ hybridization; bent shape; $p\pi-d\pi$ overlapping for double bond.

Sulphuric Acid ($H_2SO_4$)

• Manufacture (Contact Process):   1. OSulphur dioxide production (burning sulfur or iron pyrites).   2. Oxidation to $SO_3$ using catalyst ($V_2O_5$ or platinised asbestos).   3. $SO_3$ absorption in conc. acid to form oleum ($H_2S_2O_7$).   4. Dilution of oleum to desired concentration (96% pure acid).
• Properties: Viscous, colourless liquid. High BP and viscosity due to hydrogen bonding. Strong affinity for water (dehydrating agent: chars sugar to carbon).
• Reactions: Strong dibasic acid; forms sulphates and bisulphates. Oxidises carbon, sulphur, phosphorus, bromides, and iodides.
• Tests: Dilute acid/sulphate solution gives white precipitate with $BaCl_2$ ($BaSO_4$) or lead acetate ($PbSO_4$).
• Oxoacids of Sulphur: Includes sulphurous ($H_2SO_3$), sulphuric ($H_2SO_4$), thiosulphuric ($H_2S_2O_3$), dithionous ($H_2S_2O_4$), disulphuric (pyrosulphuric $H_2S_2O_7$), peroxomonosulphuric (Caro's acid $H_2SO_5$), and peroxodisulphuric (Marshall's acid $H_2S_2O_8$).

Group 17 (Halogens)

• Occurrence: Highly reactive; fluorine from fluorspar ($CaF_2$) or cryolite. Chlorine from sea water ($NaCl$).
• Chlorine ($Cl_2$):   - Preparation:     - $4NaCl + MnO_2 + 4H_2SO_4 \rightarrow Cl_2 + MnCl_2 + 4NaHSO_4 + 2H_2O$.     - Oxidation of $HCl$ by $PbO_2, MnO_2, KMnO_4,$ or $K_2Cr_2O_7$.     - Action of acids on bleaching powder ($CaOCl_2$).   - Manufacture: Electrolytic process (brine electrolysis) and Deacon's process (oxidation of $HCl$ air at $723\,K$ over $Cu_2Cl_2$ catalyst).   - Properties: Greenish-yellow gas, pungent. Heavy ($2.5$ times air). Soluble in water (chlorine water).   - Chemical actions: Reacts with metals/non-metals. Affinity for hydrogen (burns in turpentine leaving carbon). Permanent bleaching action via nascent oxygen: $H_2O + Cl_2 \rightarrow HCl + HOCl \rightarrow 2HCl + (O)$.
• Hydrochloric Acid ($HCl$): Prepared by action of salts with conc. acid. Aquaregia (3 parts conc. $HCl$ and 1 part conc. $HNO_3$) dissolves gold and platinum.
• Hydrogen Halides (HX): Bond dissociation enthalpy decreases from $HF$ ($562\,kJ\,mol^{-1}$) to $HI$ ($299\,kJ\,mol^{-1}$). Acid strength increases from $HF$ (weak) to $HI$ (strong).
• Interhalogen Compounds: Formed between two different halogens (e.g., $AB, AB_3, AB_5, AB_7$). Larger halogen is central. They are strong oxidising agents. Undergo auto-ionization (e.g., $2ICl \rightleftharpoons I^+ + ICl_2^-$). Shapes: Linear ($AX$), T-shaped ($AX_3$), Square pyramidal ($AX_5$), Pentagonal bipyramidal ($AX_7$).

Group 18 (Noble Gases)

• Properties: Monoatomic, colourless, tasteless, non-inflammable. Highly unreactive.
• Xenon Compounds:   - Fluorides: $XeF_2, XeF_4, XeF_6$ prepared by direct reaction at $400^\circ C$.   - Reaction with silica: Forms $XeOF_4$, $XeO_2F_2$, and $XeO_3$.   - Sodium perxenate ($Na_4XeO_6$): Strong oxidising agent; converts $Mn^{2+}$ to permanganate.   - Structures: $XeF_2$ (Linear), $XeF_4$ (Square planar), $XeF_6$ (Distorted octahedron), $XeO_3$ (Pyramidal).
• Uses:   - Helium: Diving mixtures (prevents bends), cryogenics (lowest BP), air balloons.   - Neon: Advertisement signs (brilliant red glow).   - Argon: Filament bulbs (prevents oxidation).   - Krypton/Xenon: Fluorescent and high-speed flash bulbs.   - Radon: Radioactive source for gamma rays; cancer treatment implants.

Questions & Discussion

• Evaluate yourself: Write the products formed in the reaction of nitric acid (both dilute and concentrated) with zinc.