The Elements of Group 15 (5A, V, VA) The Nitrogen Group (Pnictogens/Pnicogens)

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1 The Elements of Group 15 (5A, V, VA) The Nitrogen Group (nictogens/nicogens) N,, As,, Bi Valence electron configuration: ns 2 np 3 Standard States Nitrogen is the most abundant element in our atmosphere (78%), found as a diatomic gas. N 2 (g) exhibits low reactivity, and is isolated from condensed air. N 2 boils at 77K(-196 o C) and is can thus serve as a refrigerant. 1

2 Standard States hosphorus is a tetrameric solid (white phosphorus) in its standard state ( 4 (s)), although it exists as many allotropes. White phosphorus reacts with oxygen (combusts), so must be stored under water. ormerly used in matches. Hydrides Ammonia (NH 3 ) is produced in higher molar quantities than any other chemical. It is mainly used for fertilizer production (80%), and for explosives, textiles (e.g. rayon), and chemical synthesis. ammonium NH NH 4 N 3 (s) 2 N 2 (g) + 2 (g) + 4 H 2 (g) H < 0 Ammonia is produced industrially by the Haber process: 380 o C, 200 atm, e(s) 3 H 2 (g) + N 2 (g) 2 NH 3 (g) This reaction must be run at high temperatures, catalyzed by iron filings. Arsenic, antimony, and bismuth tend to exist as rhombohedral solids, although (hot) gaseous is a tetramer (like ). Interestingly, it can be done at ambient temperatures, pressures by nitrogenase, as follows (loosely): The Haber rocess Using Le Châtelier to ur Advantage Hydrides hot zone: 400 o C This apparatus helps push the equilibrium to the right by removing the ammonia (NH 3 ) from the system as a liquid. cooled zone: -50 o C b.p.(nh 3 ) = -33 o C b.p.(h 2 ) = -253 o C b.p.(n 2 ) = -196 o C 3 H 2 (g) + N 2 (g) 2 NH 3 (g) ther hydrides of nitrogen. Combustion of N 2 H 4 (hydrazine) is highly exothermic (used as rocket fuel). It is also a good reducing agent. 2

3 Hydrides hosphine (H 3 ) is a highly toxic, volatile gas. Its melting and boiling points are lower than for NH 3 due to lack of H-bonding. hosphines of the form (R 3, R = alkyl or aryl group) are important in coordination chemistry (Lewis bases towards metal cations). Analogous and As compounds are frequently studied in coordination chemistry. Halides Halides are known for all Group 15 elements. They are commonly in the +3 or +5 oxidation state. or nitrogen, the most commonly encountered form is N 3, which is not reactive, and not a Lewis base like other halides. luorines, being very electronegative, render the lone pair on nitrogen inaccessible. ther halides of this form are well known also ( 3, As 3, Bi 3 ) Synthesis: N 2As(s) (g) 2As 3 (l) Halides The pentacoordinate forms are known for, As,, and Bi, but not for N. These are bridged trigonal bipyramidal structures 5 is a tetramer with bridging fluoride groups xides Nitrogen can be found in a variety of different oxidation states. Most oxo-compounds can be found in the gaseous state +1 oxidation state: N 2 (nitrous oxide) A colorless, unreactive gas that is used as an anesthetic and as a propellant Synthesis: NH 4 N 3 N 2 + 2H 2 N 2 is a strong oxidizing agent, but is kinetically slow: N 2 + 2H + + 2e - N 2 + H 2 3

4 xides +2 oxidation state: N (nitric oxide) A neutral, paramagnetic compound that exists as a colorless gas Extra electron exists in π* orbital, and bond order is 2.5. N is generated in the body and serves to control blood pressure and aid in neurotransmission xides +3 oxidation state: N 2 3 (dinitrogen trioxide) N 2 3 N + N 2 (disproportionation) oxidation state: N 2 (nitrogen dioxide), N 2 4 (dinitrogen tetroxide). These interconvert at T> -11 o C. N 2 is a strong oxidizing agent Disproportionates in basic solution 2N 2(aq) + 2H - N - 2 (aq) + N - 3 (aq) + H xides +5 oxidation state: N 2 5 (dinitrogen pentoxide) Colorless, ionic solid but monomeric in gas phase m.p. = 32 o C 4

5 rost Diagram xides hosphorus: commonly 4 10 and is obtained by burning elemental phosphorus in air Hydrolysis yields acids: H 2 H 3 3 (phosphorous acid) H 2 H 3 4 (phosphoric acid) These compounds are useful dehydration reagents in the lab xo-ions Nitrogen-group elements have many oxoanions and oxocations. Again, surveying as a function of their oxidation states: +1 oxidation state: N (hyponitrile) (good reducing agent) +3 oxidation state: N 2- (nitrite) (reducing agent; mild Lewis base) +3 oxidation state: N + (nitrosonium) (oxidizing agent and strong Lewis acid) Nitrous acid converts to N + in a stronger acid HN 2 + H + N + + H 2 +5 oxidation state: N 3- (nitrate ion) (oxidizing agent; very weak base) +5 oxidation state: N 2+ (nitronium) (good oxidizing agent; good Lewis acid) xo-ions +1 oxidation state: H 2 2- (hypophosphite) (good reducing agent) +4 oxidation state: 2 6 (hypophosphate) (Bronsted base) +5 oxidation state: 3-4 (phosphate) and 2 7 (diphosphate) 3-5