Physical properties
-at rtp: colourless, odourless, diatomic, neutral gas
-high mp & bp (due to strong N≡N triple bond) lack of reactivity of N2
nitrogen: very inert (more reactive at high temp)
inertness due to short bond length, 0.110 nm & high bond energy of triple bond, +944kJ/mol > large amt of energy needed to form N atoms to react
N≡N(g) > 2N(g), ΔH= +944kJ/mol
Ammonia, NH3 Industrial manufacture of ammonia by Haber Process
-nitrogen & hydrogen gas reacted at high pressure, 350atm, and temp, 500°C, w/ finely divided iron catalyst; N2(g) + 3H2(g) 2NH3(g)
-w/ these conditions, abt 30% conversion to ammonia, gases cooled under pressure, ammonia removed as liquid (bp-33°C), unused gases recycled
Chemical kinetics & chemical equilibrium in Haber Process N2(g) + 3H2(g) 2NH3(g), ΔH+ = -46kJ/mol
According to Chatelier's Principle;
-increase in pressure > increase yield (higher pressure: decrease in total # of moles of NH3 formed, more gaseous collisions, faster prod rate of NH3) (increasing pressure: increases prod cost > compromise bet high yield & cost / low yield & cost)
-decrease in temp > equilibrium shift to right (since exothermic) > more NH3 produced
(increasing pressure: increases prod cost, lower temp: slower reaction rate > compromise bet high yield in longer time / low yield in short time)
industrial conditions: 30% conversion to NH3
-temp: 450°C - 500°C
-pressure: 350atm
-catalyst: finely divided Fe w/ Al2O3 (Al2O3: a promoter)
Physical properties of ammonia
-colourless, pungent, alkaline gas
-less dense than air
-very soluble in water [NH3(g) + H2O(l) NH4+(aq) + OH-(aq)]
Chemical properties of ammonia
-weak base: ammonia + water ammonium ion + hydroxide ion (low conc)
NH3(g) + aq > NH3(aq), NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)
-ammonia can ppt insoluble metal hydroxides for analysis
-ammonia is a complex agent due to its lone pair of electrons, NH3 acts as a ligand when lone pair donated to empty d-orbital of central metal ion (usually transition metals) > complex ions [Zn(NH3)42+- tetrammine zinc(II) ion, Cu(NH3)42+- tetrammine copper(II) ion]
Zn2+(aq) + OH-(aq) > Zn(OH)2(s) (white ppt)
Zn(OH)2(s) + 4NH3(aq) > 2OH-(aq) + Zn(NH3)42+(aq) (colouless sol) Cu2+(aq) + OH-(aq) > Cu(OH)2(s) (light blue ppt)
Cu(OH)2(s) + 4NH3(aq) > 2OH-(aq) + Cu(NH3)42+(aq) (deep blue sol)
AgCl(s) + 2NH3(aq) > Ag(NH3)2+(aq) + Cl-(aq)
Industrial importance of ammonia
-solvent in laundry work; removes acids left by evaporation of sweat
-manufacture of HNO3(aq)
4NH3(g) + 5O2(g) >(Pt/Rb catalyst, 900°C)> 4NO(g) + 6H2O(l)
2NO(g) + O2(g) > 2NO2(g)
4NO2(g) + O2(g) + 2H2O(l) > 4HNO3(l)
HNO3 used in manufacture of org nitrocpds (TNT, azodye, fertilizers)
-manufacture of nitrogenous fertilizers (NH4SO4, NH4NO3) (80% of NH3 used for fertilizers)
Nitrates
nitrate(III) ion, NO2- (nitrite ion)
charge delocalised over the 2 N-O bonds which are equivalent
nitrate(V) ion, NO3-
tests
nitrite, NO2-
nitrate, NO3-
+ dil H2SO4
brown fumes evolved (NO2)
no observable reaction
+ NaOH (aq), + Al foil, warm
ammonia produced
ammonia produced
Environmental problems
-nitrate fertilizers can dissolve easily > all N available to plants, but also some washed / leached into rivers & streams > eutrophication & health problems
eutrophication: large amt of nitrate in water encourages algae growth which quickly die and decompose > oxygen in water used up > aquatic plants & animals suffocate & die
excessive nitrates in drinking water > kidney damage, illness in young children
Oxides of nitrogen
nitrogen monoxide, NO (nitric oxide); colourless gas that turns brown in air (NO2 formed); formed in combustion engines, thunderstorms, in soil (by denitrifying bact)
nitrogen dioxide, NO2; brown gas; formed by oxidation of NO in air
dinitrogen oxide, N2O; colourless gas ('laughing gas'); formed by denitrifying bact in soil oxidation no of N in cpds
+5: NO3-, nitrate(V) ion
+4: NO2, nitrogen dioxide
+3: NO2-, nitrate(III) ion
+2: NO, nitorgen monoxide
+1: N2O, dinitrogen oxide
0: N2, nitrogen gas
-3: NH3, ammonia, N3-, nitirde ion
Environmental problems nitrogen oxides, NOx, from combustion of fossil fuels (due to v.high temp > break triple bond); N2(g) + O2(g) > 2NO(g) -Acid rain: NOx dissolves in water > nitrous acid (HNO2) + nitric acid, nitric acid + nitrous acid + sulphurous acid (from SO2) + sulphuric acid (from SO3) + water > acid rain
2NO2(g) + H2O(l) > HNO2(aq) + HNO2(aq)
SO2(g) + H2O(l) > H2SO3(aq)
SO3(g) + H2O(l) > H2SO4(aq)
-NOx combine w/ hydrocarbons, from car exhaust > photochemical smog (consists of oxidising subs: ozone, NO2, PAN- peroxyacetylnitrate) which irritates respiratory system (NO2 in smog > brown haze characteristics)
[PAN: CH3C(=O)OONO2 -irritates mucous membrane]
Catalytic role of NO in oxidising SO2
NO catalyses formation of SO3 from SO2
2NO(g) + O2(g) > 2NO2(g)
SO2(g) + NO2(g) > SO3(g) + NO(g)
SO3(g) + H2O(l) > H2SO4(aq)
NO in Ozone depletion
CFC & NO can deplete ozone
NO formed -during thunderstorms: N2(g) + O2(g) > 2NO(g), -by oxidation of N2O from denitrifying bact: N2O(g) + O(g) > 2NO(g) [O- in stratosphere]
NO(g) + O3(g) > NO2(g) + O2(g)
NO2(g) + O > NO(g) + O2(g)
like CFC's, NO reproduced > further ozone destruction
(small amt of NO formed in nature not great sig in ozone depletion)
Catalytic removal of nitrogen oxides in car exhaust gases
car exhaust using unleaded petrol: nitrogen, CO2, water vapour, unreacted hydrocarbon (CxHy), NO, NOx
harmful products > catalytic converter consisting of transition metal catalysts- palladium, platinum, rhodium > harmless subs
Pd, Pt: oxidation of CO > CO2, & hydrocarbons
Rh: reduction of NO > N2
2CO(g) + O2(g) >(Pd & Pt)> 2CO2(g)
CxHy(g) + (x + y/4)O2(g) >(Pd & Pt)> xCO2(g) + (y/2)H2O(g)
2NO(g) + 2CO(g) >(Rh)> N2(g) + 2CO2(g)
