p-Block Elements: Groups 15-18 Overview

Objectives of the p-Block Elements Unit

• Comprehend general trends within groups 15, 16, 17, and 18
• Learn to prepare, identify characteristics, and use dinitrogen, phosphorus, and related compounds
• Describe preparation methods, properties, and uses of dioxygen and ozone, including simple oxides' chemistry
• Detail the allotropic forms of sulfur, its compounds' chemistry, and the configuration of its oxoacids
• Describe the means of preparing chlorine and hydrochloric acid, in addition to their properties and uses
• Delve into the chemistry of interhalogens and the configurations of halogen oxoacids
• List the applications of noble gases
• Identify the importance of the elements to your life

Group 15 Elements Overview

• Encompasses nitrogen, phosphorus, arsenic, antimony, bismuth, and moscovium
• As one descends the group, properties shift from non-metallic to metallic, passing through metalloid characteristics
• Nitrogen and phosphorus are nonmetals
• Arsenic and antimony are metalloids
• Bismuth and Moscovium are metals

Occurrence of Group 15 Elements

• Molecular nitrogen accounts for 78% of the atmosphere’s volume
• Sodium nitrate (Chile saltpetre) and potassium nitrate (Indian saltpetre) constitute the Earth’s crust
• Proteins found in plants and animals contain it
• Phosphorus is present in minerals of the apatite family, like fluorapatite Ca9(PO4)6. CaF2, major constituent of phosphate rocks
• Phosphorus is vital in animal and plant matter, present in bones and living cells
• Milk and eggs contain phosphoproteins
• Arsenic, antimony, and bismuth exist largely as sulfide minerals
• Moscovium (Mc) is a lab-created radioactive element with an atomic number of 115, an atomic mass of 289, and an electron configuration of [Rn] 5f146d107s27p3
• Moscovium's chemistry remains largely unknown due to its short half-life and scarcity

Electronic Configuration of Group 15

• The valence shell electronic configuration is ns2np3
• The s orbital is full and the p orbitals are half-filled, which contributes to extra stability

Atomic and Ionic Radii of Group 15

• As size increases down the group, both covalent and ionic radii also climbs
• Radii increases significantly from N to P
• The increase in covalent radius is smaller from As to Bi because of filled d and/or f orbitals

Ionization Enthalpy of Group 15

• A gradual reduction in ionization enthalpy occurs down the group with increasing atomic size
• The ionization enthalpy surpasses that of Group 14 due to the extra stable half-filled p orbitals and smaller size
• With successive ionization, ionization enthalpies increase

Electronegativity of Group 15

• Electronegativity typically decreases down the group due to increasing atomic size
• The difference is negligible between heavier elements

Physical Properties of Group 15

• All Group 15 elements are polyatomic
• Dinitrogen is a diatomic gas, while other elements are solids
• Metallic character boosts down the group
• Nitrogen and phosphorus are nonmetals; arsenic and antimony are metalloids; bismuth is a metal
• Boiling points increase from top to bottom, but melting points increase up to arsenic, then decrease to bismuth
• All elements except nitrogen exhibit allotropy

Chemical Properties of Group 15

• Elements display oxidation states of -3, +3, and +5
• The -3 oxidation state becomes less stable descending the group due to increasing size and metallic properties
• Bismuth hardly forms any-3 oxidation state compounds
• Stability of the +5 oxidation state also decreases down the group
• BiF5 is the only well-characterized Bi(V) compound
• The +3 state gets stability (inert pair effect) descending the group
• Nitrogen also displays +1, +2, and +4 oxidation states when reacting with oxygen
• Nitrogen doesn't form +5 oxidation state compounds with halogens because of the absence of d-orbitals
• Some oxoacids of phosphorus exhibit +1 and +4 oxidation states
• All nitrogen oxidation states, from +1 to +4, tend to disproportionate in an acidic solution
• Similarly, phosphorus tends to disproportionate into +5 and –3 states in alkali and acid; arsenic, antimony, and bismuth become more stable, w.r.t disproportionation
• Nitrogen is restricted to a maximum covalency of 4 because it has only four orbitals (one s and three p) for bonding, while heavier elements can expand their covalence using vacant d orbitals
• The capacity of heavier elements to extend their covalency is manifested as in PF6–

Anomalous Properties of Nitrogen

• Unique qualities due to its small size, high electronegativity and ionization enthalpy, and the absence of d orbitals
• Capability to form pπ - pπ multiple bonds with itself and other small, highly electronegative elements
• Heavier elements within Group 15 do not form pπ - pπ bonds because their atomic orbitals are too large for effective overlapping
• Nitrogen exists as a diatomic molecule with a triple bond and a high bond enthalpy of 941.4 kJ mol-1
• Phosphorus, arsenic, and antimony primarily form single bonds (P-P, As-As, Sb-Sb), whereas single bonds of bismuth have metallic characteristics

Reactivity of Group 15 towards Hydrogen

• Group 15 elements form hydrides EH3, the properties include
• Stability diminishes from NH3 to BiH3, as observed via bond dissociation enthalpy
• Reducing character of hydrides boost
• While ammonia serves as a mild reducing agent, bismuth hydride represents the strongest
• Basicity diminishes from NH3 to BiH3, as high electronegativity and small size cause ammonia (NH3) to exhibit hydrogen bonding in both solid and liquid phases, leading to greater melting and boiling points compared to phosphine

Reactivity of Group 15 towards Oxygen

• Group 15 elements form two types of oxides: E2O3 and E2O5
• Higher oxidation state oxides tend to be more acidic than lower ones, acidity descends the series
• Trioxides of N and P are acidic, trioxides of As and Sb are amphoteric, trioxides of Bi are basic

Reactivity of Group 15 towards Halogens

• Halides generated in two main series: EX3 and EX5
• Only trihalides of nitrogen exist, as nitrogen has no d orbitals to form pentahalides
• Pentahalides tend to be more covalent

Reactivity of Group 15 towards Metals

• Reacts with metal to form binary compounds that exhibits the -3 oxidation state including, calcium nitride, calcium phosphide, sodium arsenide, zinc antimonide, magnesium bismuthide

Dinitrogen Preparation

• Generated industrially by liquefaction and fractional distillation of air
• In laboratories, prepare dinitrogen by treating an aqueous solution of ammonium chloride with sodium nitrite: NH4Cl(aq) + NaNO2(aq) → N2(g) + 2H2O(l) + NaCl (aq)
• Remove NO and HNO3 impurities by passing the gas through aqueous sulfuric acid with potassium dichromate or by way of thermal ammonium dichromate breakdown: (NH4)2Cr2O7 —Heat—> N2 + 4H2O + Cr2O3
• Can be obtained by the thermal breakdown of sodium or barium azide: Ba(N3)2 → Ba + 3N2

Properties of Dinitrogen

• Dinitrogen is non-toxic and consists of two stable isotopes, 14N and 15N
• It is colorless, odorless, and tasteless and has very low water solubility (23.2 cm3 per liter at S.T.P.) and low freezing and boiling points
• Dinitrogen is inert at room temperature due to high bond enthalpy, making it increases fast, rising in temperature. Combining with metals to predominantly form ionic nitrides, and with non-metals to generate covalent nitrides
• It combines with hydrogen at about 773 K in the presence of a catalyst (Haber’s Process) to form ammonia: N2(g) + 3H2(g) ↔ 2NH3(g); ΔH⦵ = –46.1 kJ mol-1
• Combines with dioxygen at very high temperature (at about 2000 K) to form nitric oxide, NO: N2+ O2(g) → 2NO(g)

Dinitrogen Uses

• The main use of dinitrogen is in the manufacture of ammonia and other industrial chemicals containing nitrogen (e.g., calcium cyanamide)
• Helpful where an inactive atmosphere is required (e.g., in iron and steel industry, inert diluent for reactive chemicals)
• Liquid dinitrogen is used as a refrigerant to preserve biological materials, food items and in cryosurgery

Ammonia Preparation

• Ammonia is present in small quantities in air and soil where it is formed by the decay of nitrogenous organic matter e.g., urea
• NH2CONH2 + 2H2O → (NH4)2CO3= 2NH3 + H2O + CO2
• On a small scale ammonia is obtained from ammonium salts which decompose when treated with caustic soda or calcium hydroxide
• 2NH4Cl + Ca(OH)2 → 2NH3 + 2H2O + CaCl2; (NH4)2 SO4 + 2NaOH → 2NH3 + 2H2O + Na2SO4
• On a large scale, ammonia is manufactured by Haber's process
• In accordance with Le Chatelier's principle, high pressure would favour the formation of ammonia
• The optimum conditions for the production of ammonia are a pressure of ~200 atm, a temperature of ~700 K, and the use of a catalyst

Properties of Ammonia

• Colorless gas having a pungent smell, and its freezing and boiling points are 198.4 and 239.7 K
• It is related through hydrogen bonds, and it has three bond pairs with nitrogen atom at its apex
• It is weakly basic and precipitates the hydroxides of many metal from their salt solutions: ZnSO4 (aq) + 2NH4OH(aq) → Zn(OH)2 (s) + (NH4)2SO4 (aq); FeCl3 (aq) + NH4OH (aq) → Fe2O3.xH2O(s) + NH4Cl (aq)
• Its presence of a lone pair of electrons on the nitrogen atom makes it a Lewis base

Uses of Amonia

• Ammonia is used to produce various nitrogenous fertilizers like ammonium nitrate and liquid ammonia is used as a refrigerant

Oxides of Nitrogen

• Nitrogen forms a number of oxides in different oxidation states, including dinitrogen oxide (N2O), nitrogen monoxide (NO), dinitrogen trioxide (N2O3), nitrogen dioxide (NO2), dinitrogen tetroxide (N2O4), and dinitrogen pentoxide (N2O5)
• Dinitrogen oxide (N2O) is prepared by heating ammonium nitrate and is a colorless, neutral gas
• Nitrogen monoxide (NO) is prepared by combining 2NaNO2 + 2FeSO4 + 3H2SO4, and is also neutral and colorless

Nitric Acid

• Nitrogen has oxoacids: hyponitrous acid, nitrous acid, nitric acid
• Nitric acid is the most important oxoacid, prepared in a laboratory by heating KNO3 or NaNO3 and concentrated H2SO4
• Concentrated Nitric acid is a oxidizing agent, attacking all metals
• Cu + 4HNO3(conc.) → Cu(NO3)2 + 2NO2 + 2H2O

Phosphorus Allotropic forms

• Phosphorus exists in allotropic, including white, red, and black
• White Phosphorus is a translucent, poisonous, insoluble in water but soluble in carbon dissolvable: P4 + 3NaOH + 3H2O → PH3 + 3NaH2PO2
• Red Phosphorus is iron grey color, it is insoluble in water unlike white

Phosphine

• PH3, prepare by calcium phosphide with water CA3P2 + 6H2O → 3CA(OH)2
• It has a rotten fish smell
• P4 + 3NaOH + 3H2O → PH3 + 3NaH2PO2

Phosphorus halidies

• Obtained by passing dry chlorine over heat phosphorus P4 + 6CL2 → 4PCL3
• PCl5 + H2O → POCl3 + 2HCl
• React with organic compounds containing alcohol group

Oxoacids of Phosphorus

• Phosphorus forms multiple oxoacids with formulas, production techniques, and structures all differing
• Hypophosphorous Acid Formula: H3PO2
• Orthophosphorus Acid: H3PO3 orthophosph, can't isolate in pure set
• Pyrophorus Acid H4P2O5 Can't isolate that, pure state don't exist