chemistry 2014 7

Inorganic Chemistry

Prof. Frank T. Edelmann

26.04.2015 1

Course on Inorganic Chemistry by Frank T. Edelmann

Chapter 7

Pnictogens (Nitrogen Group)

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Chapter 7 Pnictogens

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Nitrogen

Natural sources:

- elemental: as N2 main component of air (79.5 %)

- in compounds: water (solved nitrate salts 1 % of total N),

salt deposits (Chile, India)

essential part of biosphere (amino acids) -

0.001 % of total N

Manufacture:

- in industry: liquid air rectification (Linde process),

- in laboratory: thermal or catalytic removal of oxygen from air

(product contains noble gases),

pure nitrogen by oxidation of ammonia

NH3 + HNO2 N2 + 2 H2O

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Properties of N2:

- colorless, tasteless and odorless gas

- triple bond between the nitrogen atoms (one s and two p-bonds)

- low solubility in water (3.05 l /100 l H2O)

- essential for life (formation of amino acids, assimilation for most plants as NH4+,

NO2-, NO3

- and urea, special bacteria azotobacter - can convert gaseous N2)

- very low reactivity with almost all elements, only at high temperature, mostly

strongly endothermic or kinetically hindered reactions

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Applications of N2:

- inert purging gas

- cooling agent (in liquid state)

- synthesis of ammonia, hydrazine, hydoxylamine and nitric acid

Nitrogen species:

- N2, N3- (nitride), N3

- (azide)

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Important Nitrogen Hydrogen Compounds

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Industrial Ammonia Synthesis 1. A Synthesis of High Complexity


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Industrial Ammonia Synthesis 2. Synthesis and Purification of Ammonia

equilibrium limited, exothermic reaction with volume decrease

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Process conditions


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Product separation and purification

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Nitrogen Oxides and Acids Overview

Oxides: - endothermic meta-stable compounds (except for N2O4 and N2O5) - are formed during high temperature combustion processes from nitrogen oxidation (instead of N2O) - equilibriums NO/N2O2 and NO2/N2O4 (dimerization primarily at lower temperatures) - N2O, NO/N2O2 and NO2/N2O4 have a high technical importance and environmental relevance as anthropogenic emissions ( ozone and as precursors for acid rain)

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Acids:

- acid strength increases with increasing number of oxygen atoms

- HNO2 (only stable in gas phase or in aqueous solution, salts are stable) and

HNO3 are stable and of high technical importance

- H2N2O2 decomposes at room temperature within days, salts are stable

- HNO and HNO4 are meta-stable at low temperatures and decompose under

normal conditions

Nitrogen Oxides and Acids Overview

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Nitrogen Oxides

Dinitrogen monoxide N2O (NNO)

- Industrial manufacture

careful heating of ammonium nitrate or

of a mixture of NH3 and HNO3

of a mixture of sodium nitrate and ammonium sulfate

(forming NH4NO3) to 200 C (danger of explosions!)

NH4NO3 N2O + 2 H2O + 124.1 kJ/mol

product of biological nitrification and denitrification processes

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Nitrogen Oxides

Dinitrogen monoxide N2O (NNO)

- Properties

colorless gas with weak sweet odor and intoxicating effect,

melting point: - 90.9 C, boiling point: - 88.5 C,

well soluble in water (0.60 l/l H2O) and fats,

oxidation agent, supports combustion processes similar to oxygen

-Applications

narcotic agent,

propellant in ice-cream and whipped cream

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Nitrogen monoxide NO

- formed by nitrogen oxidation at temperatures > 1500 C or by Pt-catalyzed short

contact time combustion of ammonia

Ostwald process: 4 NH3 +5 O2 4 NO + 6 H2O

- highly endothermic compound (DBH = 180.6 kJ/mol),

- molecule contains one unpaired electron = free radical (!)

- colorless, highly toxic gas, low solubility in water

- melting point: - 163.6 C, boiling point: - 151.8 C

- dimerizes especially in the solid and liquid state to N2O2,

- rapid oxidation to NO2 in the presence of air

Nitrogen Oxides

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Nitrogen dioxide NO2 /dinitrogen tetroxide (O2NNO2)

Nitrogen Oxides

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Industrial manufacture of Nitric Acid

OSTWALD Process (3-step process for 50 - 68 % acid) (1) catalytic combustion of NH3 over Pt-Rh alloy gauzes ( short contact times) at 820 950 C and 1 12 bar 4 NH3 + 5 O2 4 NO + 6 H2O + 904 kJ/mol (2) further oxidation of NO to NO2/N2O4 at ca. 150 C 2 NO + O2 2 NO2 + 114 kJ/mol, 2 NO2 N2O4 + 57 kJ/mol (3) absorption in water in the presence of excess air at up to 15 bar: 2 NO2 (=N2O4) + H2O + 0.5 O2 2 HNO3

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Properties and Applications of Nitric Acid

Properties:

- colorless liquid, decomposes slowly (faster at higher temperatures) to nitrous

oxides and water: 2 HNO3 2 NO2 + H2O + 0.5 O2

- melting point: - 41.6 C, boiling point: + 82.6 C

- azeotrope with of 69.2 % HNO3 with water (boiling point: 121.8 C)

- azeotrope = concentrated HNO3

- strong oxidation agent, reacts with Cu, Ag, Hg, S, P and organic substances (not

with Au, Pt, Rh, Ir): 4 H+ + NO3- + 3 e- NO + 2 H2O

- mixture of HNO3 and HCl (1 : 3) oxidises even Au

HNO3 + 3 HCl NOCl + 2 Cl (atomic) + 2 H2O

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Properties:

- mixture of HNO3 and H2SO4 (1 : 9) is used as a nitration agent in organic chemistry

- strong acid (pKs = - 1.44)

- salts (nitrates) have a high solubility in water, low melting points (250 350 C)

and decompose easily upon heating:

alkali and alkaline earth metal nitrates: KNO3 KNO2 + 0.5 O2,

transition metal nitrates: Cu(NO3)2 CuO + 2 NO2 + 0.5 O2,

ammonium nitrate: NH4NO3 N2O + 2 H2O + 124.1 kJ/mol (200-260 C) or

NH4NO3 N2 + 0.5 O2 + 2 H2O + 206.2 kJ/mol (>300 C)


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Applications:

- 75 - 85 % production of ammonium nitrate (NH4NO3) for fertilizers (80 %),

explosives (~ 20 %) and N2O synthesis (from 50 70 % acid)

- 10 % for production of adipic acid (HOOC-(CH2)4-COOH), fiber and plastic

precursor)

- 3 % production of TNT (with highly concentrated acid)

- 3 % nitration of benzene

(aniline precursor, reaction is carried out with highly concentrated acid)

- 2 % alkali and earth alkali nitrates (fertilizers)

- 1 % organic nitro-compounds

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Elemental Phosphorus

Natural sources:

- occurs only in compounds

- 0.1 mass % of the earths crust, 13th most abundant element

- inorganic sources: phosphates

(apatite Ca3(PO4)2 CaX2, X = OH, F, Cl; iron and aluminium phosphates)

- biological importance: participation in metabolism processes as phosphorus acid

esters and phosphates (e.g. ADP/ATP)

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Manufacturing in industry:

- highly endothermic electro-thermal reduction of phosphates with coke and quartz

at 1400 1500 C:

1542 kJ/mol + Ca3(PO4)2 + 3 SiO2 + 5 C 3 CaSiO3 (slag) + 5 CO + P2 (g)

(reduction of P2O5 by C, SiO2 is added to form a slag with Ca)

- condensation and distillation of the resulting phosphorus white phosphorus

(P4)

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Modifications:

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Modifications:

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Phosphorus species in compounds: - occur in all oxidation states from 3 to +5 Applications:

- 90 % for manufacture of P4O10 ( phosphoric acid, phosphates)

- synthesis of P-S and P-Halogen compounds ( organic chemistry)

- safety matches (red phosphorous)

- military purposes

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Phosphorus Oxides

Phosphorus trioxide (P4O6) and phosphorus pentoxide (P4O10)

phosphorus trioxide (P4O6) phosphorus pentoxide (P4O10)

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Phosphorus Oxides

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Phosphorus Oxides

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Phosphoric acid - H3PO4

Manufacture:

(I) Wet processes

Dihydrate process (80 C CaSO4 2 H2O),

hemihydrate process (95 C CaSO4 H2O)

Ca3(PO4)2 (apatite) + H2SO4 3 CaSO4 + 2 H3PO4,

(II) Thermal process

Oxidation of white P4 in excess air and absorption of the formed P4O10

together with water in concentrated H3PO4 (85 %)

P4 + 5 O2 P4O10,

P4O10 + 6 H2O 4 H3PO4

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Properties:

pure acid = colorless, clear, odorless solid, melting point: 42.4 C,

partial condensation (at 200 C complete) 2 H3PO4 H4P2O7 + H2O

concentrated (85 %) acid = high viscous liquid

melting point: 21.1 C, boiling point: 158 C

tribasic medium-strong acid

(pKS1 = 2.16, pKS2 = 7.21, pKS3 = 12.32)

= no oxidizing properties

pH

buffer area

pH

buffer area

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Applications:

- production of phosphates of K, Na and NH4,

- use of the free acid in cleaning agents, in metal treatment and polishing, as an

acidification agent in soft drinks (colas, lemonades),

- organic chemistry (e.g. esterification)

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Salts and Organic Compounds from Phosphorus Acids

Fertilizers - general importance

- Plants need for growing not only light, air, warmth and water, but additionally S, P, N, K, Ca, Mg and Fe

Manufacture of inorganic phosphates and other fertilizer salts - phosphates/phosphites/fertilizer sulphates: neutralization of NaOH, KOH, CaO or NH3 with phosphoric acid (phosphates), phosphorous acid (phosphites) or sulfuric acid (sulfates), precipitation and metathesis reactions -diphosphates and polyphosphates thermal treatment of phosphate mixtures (condensation - e.g. 2 Na2HPO4 Na2H2P2O7 + H2O), reaction time and temperature (250 - 900 C) control polymerization degree

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Salts mainly used for fertilization

-phosphates: superphosphates, obtained by treatment of Ca3(PO4)2 (low solubility)

with 50 % H2SO4 = mixture of Ca(H2PO4)2 (high solubility) and CaSO4

superphosphate 16 22 % P2O5,

double superphosphate 35 % P2O5,

triple superphosphate > 46 % P2O5

ammonium phosphates (made by neutralisation and

thermal condensation), used as solids and

in H2O solution (high solubility)

ammonium nitrate/ammonium sulfate (made by neutralisation)

- urea (made by CO2 + NH3 NH2COONH4 OC(NH2)2 + H2O)

- potassium chloride (mining), sulfate and nitrate (KCl + H2SO4/HNO3 or nitrates)

Salts and Organic Compounds from Phosphoric Acids

chemistry 2014 7

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