The p-Block Elements: Class XI, Unit 7

Unit 7: The p-Block Elements

• Highlights the chemical diversity of p-block elements, focusing on their ability to react with s,d, and f-block elements as well as themselves.

Overview of p-Block Elements

• Learnt in Class XI, they are situated in groups 13-18 of the periodic table.
• General valence shell electronic configuration is ns²np¹⁻⁶, excluding helium which is 1s².
• Properties influenced by: atomic sizes, ionization enthalpy, electronegativity, and electron gain enthalpy.
• Absence of d-orbitals in the second period vs. presence of d/f orbitals in heavier elements significantly affects properties.
• All three types of elements (metals, metalloids, and non-metals) are present, adding to chemical diversity.

Group 15 Elements

• Consist of nitrogen, phosphorus, arsenic, antimony, and bismuth.
• There is a shift from non-metallic to metallic characteristics down the family.
• Nitrogen and phosphorus are non-metals, arsenic and antimony are metalloids, and bismuth is a typical metal.

Occurrence of Nitrogen and Phosphorus

• Molecular nitrogen accounts for 78% of the atmosphere's volume.
• In the Earth's crust, nitrogen exists as sodium and potassium nitrate.
• It is found in proteins in plants and animals.
• Phosphorus can be found in minerals of the apatite family like fluorapatite.

Properties of Group 15 Elements

• Valence shell electronic configuration is ns²np³.
• Filled s-orbital and half-filled p-orbitals contribute extra stability.
• The size of ionic and covalent radii increases down the group.
• There is a notable increase in covalent radius from nitrogen to phosphorus.
• However, a much smaller increase is seen from arsenic to bismuth because of filled d and/or f orbitals in heavier elements.
• Ionization enthalpy generally decreases down the group because of increasing atomic size.
• Group 15 elements' ionization enthalpy exceeds that of group 14 elements in corresponding periods as well as successive ionization enthalpies increase.
• Electronegativity decreases down the group, but differences aren't significant among heavier elements.
• All are polyatomic.
• Dinitrogen is a diatomic gas, the rest are solids.
• Metallic characteristics increase down the group.
• Boiling points increase from top to bottom, but melting points increase up to arsenic and then decrease to bismuth.
• Excluding nitrogen, all exhibit allotropy.

Chemical Properties of Group 15

• Common oxidation states: -3, +3, and +5.
• The trend to show a -3 oxidation state declines, stability of +5 oxidation states lowers, while +3 rises as you go down the group.
• While nitrogen displays +1, +2, and +4 oxidation states from reacting with oxygen, phosphorus shows +1 and +4 in oxoacids.
• Nitrogen in +1 to +4 oxidation states disproportionates in acidic solutions, e.g. 3HNO₂ → HNO₃ + H₂O + 2NO.
• In phosphorus, virtually all intermediate oxidation states disproportionate into +5 and -3 in both alkaline and acidic environments.
• Arsenic, antimony, and bismuth exhibit increased stability in the +3 oxidation state with respect to disproportionation.
• Nitrogen is restricted to a maximum covalency of 4 because only four (one s and three p) orbitals are available for bonding
• Heavier elements have vacant d orbitals for bonding, expanding their covalency.

Anomalous Properties of Nitrogen

• Differs from the rest of the group due to its small size, high electronegativity/ionization enthalpy, and non-availability of d-orbitals.
• It can form pπ-pπ multiple bonds with itself and elements like C and O.
• Does not form pπ-pπ bonds because their atomic orbitals are large and have ineffective overlapping.
• Exists as diatomic molecule with a triple bond.
• Single N-N bond is weaker as there is high interelectronic repulsion of non-bonding electrons owed to the short bond length.

Reactivity of Group 15 Elements towards Hydrogen

• Form hydrides of type EH₃.
• Hydride stability decreases from NH₃ to BiH₃.
• The reducing character of the hydrides increases.
• Basicity decreases in the order NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃.

Reactivity of Group 15 Elements towards Oxygen

• Form two types of oxides: E₂O₃ and E₂O₅.
• Higher oxidation state oxides are more acidic.
• For nitrogen and phosphorus (E₂O₃), oxides are purely acidic, arsenic and antimony oxides are amphoteric, bismuth oxides are basic.

Reactivity of Group 15 Elements towards Halogens

• Form two series of halides: EX₃ and EX₅.
• Nitrogen cannot form pentahalides becauses of non-availability of d Orbitals
• Pentahalides are more covalent.
• All trihalides are stable with the exception of nitrogen.
• Trihalides are covalent in nature.

Reactivity of Group 15 Elements towards Metals

• Metals react to form binary compounds with a -3 oxidation state.
• Examples Ca₃N₂, Ca₃P₂, Na₃As₂, Zn₃Sb₂, and Mg₃Bi₂.

Key example fact

• Nitrogen with n=2, has "s" and "p" orbitals only, has "s" and "p" orbitals only, It does not have "d" orbitals to expand its covalence beyond four.

Key difference

• Unlike NH₃, PH₃ molecules are not associated through hydrogen bonding, that is why the boiling point of PH₃ is lower than NH₃.

Dinitrogen Preparation

• Commercially produced through air liquefaction and fractional distillation.
• Liquid nitrogen distills first (b.p. 77.2 K).
• Lab prep: treating aqueous ammonium chloride with sodium nitrite, small amounts of NO and HNO₃, which are removed using aqueous sulphuric acid with potassium dichromate
• NH₄Cl(aq) + NaNO₂(aq) → N₂(g) + 2H₂O(l) + NaCl (aq)*.
• Thermal decomposition of ammonium dichromate (NH₄)₂Cr₂O₇ → N₂ + 4H₂O + Cr₂O₃
• Very pure nitrogen made through thermal decomposition of sodium or barium azide.
• Ba(N₃)₂ → Ba + 3N₂*

Properties of Dinitrogen

• Colorless, odorless, tasteless, non-toxic gas, made of ¹⁴N and ¹⁵N isotopes.
• Low freezing and boiling points, with limited water solubility (23.2 cm³ per liter).
• Room temperature inertness from high bond enthalpy of N≡N bond.
• Higher temperature increases reactivity, directly combines with metals into ionic nitrides and w/ non-metals: covalent nitrides.

Reactions of Dinitrogen

6Li + N₂ → 2Li₃N 3Mg + N₂ → Mg₃N₂

Uses of Dinitrogen

• Major use is the production of ammonia and other nitrogen-based industrial chemicals such as calcium cyanamide.
• Used in inert atmospheres (e.g. iron and steel) and liquid dinitrogen (refrigerant) to preserve biological materials, food and in cryosurgery.

Ammonia Preparation

• Small amounts in air and soil from organic matter decay, e.g., NH₂CONH₂ + 2H₂O → (NH₄)₂CO₃ ⇌ 2NH₃ + H₂O + CO₂
• In labs, through ammonium salts with caustic soda or calcium hydroxide, like:
• 2NH₄Cl + Ca(OH)₂ → 2NH₃ + 2H₂O + CaCl₂*
• (NH₄)₂SO₄ + 2NaOH → 2NH₃ + 2H₂O + Na₂SO₄*
• Large-scale ammonia manufacture involves Haber's process:
• N₂(g) + 3H₂(g) ⇌ 2NH₃(g); ∆H⁰ = -46.1 kJ mol⁻¹*
• Optimum conditions are a pressure of 200 × 10⁵ Pa, temp of 700 K, and iron oxide as a catalyst.

Physical and Chemical Properties of Ammonia

• Properties include colourless gas with a pungent odour and freezing/boiling points of 198.4/239.7 K.
• Hydrogen-bonded in solid and liquid states, accounting for high melting and boiling points.
• Is trigonal pyramidal molecule w/ nitrogen at the apex, and three bond pairs with one lone pair of electrons.
• NH₃(g) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq) is also weakly basic.
• Can give ammonium salts w/ acids, including NH₄Cl, (NH₄)₂SO₄
• Causes hydroxides/hydrated oxides of metals from solutions through ppt.
• Acts as Lewis base becauses of donation of the electron pair and formation of metal-ion linkages, applies in detection of metal ions:
• Cu²⁺(aq) + 4NH₃(aq) ⇌ [Cu(NH₃)₄]²⁺(aq)*
• Ag⁺(aq) + Cl⁻(aq) → AgCl(s) followed by AgCl(s) + 2NH₃(aq) → [Ag(NH₃)₂]Cl(aq)*

Uses of Ammonia

• Create nitrogenous fertilisers i.e. NH₄NO₃ urea, NH₄ phosphate etc.
• Used make compound like nitric acid Acts like refregerants.

Example Question

• What makes NH₃ a Lewis base? Ans: lone-pair of electrons for nitrogen atom, used donation,

Nitrogen Oxides

• Nitrogen exhibits a spectrum of oxides, presented in Table form.

Examples: Write w/ description w/ Uses/Formula:

Formula N₂O is:

• Dinitrogen oxide or Nitrogen(I) oxide, or Nitrous Oxide, is colourless gas, is neutral, created through NH₄NO₃ w/ heat.

Formula NO:

• Nitrogen monoxide or Nitrogen(II) oxide is colourless gas, neutral, created through 2NaNO₂ + 2FeSO₄ + 3H₂SO₄ → Fe₂(SO₄)₃ + 2NaHSO₄ + 2H₂O + 2NO

Formula N₂O₃ is:

• Dinitrogen trioxide, is unstable w/ acidic/blue color is form as 2NO + N₂O₄

Formula NO₂ is:

• Nitrogen dioxide has brown color is and acidic and created w/ 2Pb(NO₃)₂ w/O²

Formula N₂O₄ is:

• Dinitrogen tetroxide is colorless, liquid/solic w/ heat +2(NO₂) (Nitrogen dioxide)

Formula N₂O₅ is:

• Dinitrogen pentoxide colourless solid/ is acidic formula is: 4HNO₃ or P₄O₁₀→ 4HPO₃ + 2N₂O₅
• Nitric acid HNO₃ is acidic nitrogen forms oxoacids and is planar and color less

Note

• Main thing to not is with each increase to a new Nitrogen Oxide w/ formula, this has a +4 on Oxidation.

Key points.

• Key resonating main structures along with bond paramters for each oxide are contained in table 7.4
• Dimerisation in nitrogen is why the compound reacts.

Nitric Acid (HNO₃)

• Nitrogen gives oxoacids including H₂N₂O₂, HNO₂.

Description of Production

• Labs made throrugh KNO₃ or NaNO₃ tht combines w/ H₂SO to glass retort -> NaNO₃ + H₂SO₄→ N₂HSO₄ or HNO₃ +H₂O.
• Massive-production mostly happens via the process of *Ostwald".
• Ammonia is oxidized w/ Pt/Rh catalyst when put w/ atmosphereic oxygen:4NH₃ (g) + 5O₂ Pt/Rh gauge catalyst
• *500 K, 9 bar→ 4NO(g) or 6H₂O(g) + NO₂(g) *Pt/Rh gauge catalyst
• *Nitric oxide formed and combos w/ dioxygen w/ g→:2H₂O-> + 2NO₂
• Nitrogen dioxide goes to give +NO₂ (g) + H₂O(1) → 2HNO₃ (aq) + NO(g)
• Note NO created gets recycling/ to 6.9-8% concentration. Dehydration yields 98"%.

Key traits

It is color less solution (p + 231.4 K boils w/ 355.6 K, which are Lab standards +69# HNo has a density of 1.5

White/Brown phosphorus key aspects

• Is found in both while/red and black formations
• Transulcent is waxy and while: toxic and insoluble yet disulfied. Bakes well inside NaOH inside unreactive w/ inert air or when P4 O 3+ NaHO32

Red phosphorus Key Facts

• Heating white @ 573K give this. No ordour/Toxic and less reactive.

Key Formulae regarding Nitrogen Oxides:

• Pt/Rh gauge catalyst->:I2 + 10HNO3 w. 2HIO3 of 10 NO2 of 4H2O with 07.42 wHich acts lieka Lewis Basy and it produces with +S07. *Heat/Coll w.
• Z₂: C+83=83/58
• heat O-atom + S->*