Characteristics of ammonia and catalytic oxidation

What properties are typical for ammonia?

Fluorescein in ammonia solution [Wikimedia]

One of the most im­por­tant com­pounds of ni­tro­gen is am­mo­nia. By its phys­i­cal prop­er­ties, it is a col­or­less gas with a harsh, suf­fo­cat­ing smell (this is the smell of the aque­ous so­lu­tion of am­mo­ni­um hy­drox­ide NH₃·H₂O). The gas dis­solves well in wa­ter. In an aque­ous so­lu­tion, am­mo­ni­um is a weak base. NH₃ is a good re­duc­er, as in the am­mo­ni­um mol­e­cule, ni­tro­gen has the low­est pos­si­ble ox­i­da­tion state for ni­tro­gen, -3.

Many char­ac­ter­is­tics of am­mo­nia are de­ter­mined by the lone pair of elec­trons in the ni­tro­gen atom – at­tach­ment re­ac­tions with am­mo­nia take place be­cause of its pres­ence (this lone pair is lo­cat­ed on the free or­bital of the pro­ton H⁺).

How to ob­tain am­mo­nia

There are two main prac­ti­cal meth­ods for ob­tain­ing am­mo­nia – one of them in the lab­o­ra­to­ry, the oth­er in in­dus­try:

Liquid ammonia [Wikimedia]
  • ob­tain­ing am­mo­nia in in­dus­try – in­ter­ac­tion of molec­u­lar ni­tro­gen and hy­dro­gen:

N₂ + 2H₂ = 2NH₃ (re­versible re­ac­tion).

This method of ob­tain­ing am­mo­nia is called the Haber re­ac­tion. For molec­u­lar ni­tro­gen and hy­dro­gen to re­act, they must be heat­ed to 500 ᵒC or 932 ᵒF, pres­sure of 25-30 MPa must be cre­at­ed, and por­ous iron must be present as a cat­a­lyst.

  • ob­tain­ing in the lab­o­ra­to­ry – re­ac­tion be­tween am­mo­ni­um chlo­ride and cal­ci­um hy­drox­ide:

Ca(OH)₂ + 2N­H₄­Cl = Ca­Cl₂ + 2N­H₄OH (as NH₄OH is a very weak com­pound, it im­me­di­ate­ly breaks down into gaseous am­mo­nia and wa­ter: NH₄OH = NH₃ + H₂O).

NH₄Cl smoke [Wikimedia]

As am­mo­nia is a gas that is lighter than air, it should be gath­ered in a test tube, keep­ing it up­side down. M(air) = 29 g/mol, M(am­mo­nia) = 17 g/mol, ac­cord­ing­ly, air is heav­ier, and the am­mo­nia ris­es up­wards.

In ob­tain­ing am­mo­nia in the lab­o­ra­to­ry, it is not manda­to­ry to use am­mo­ni­um chlo­ride. Am­mo­ni­um sul­fate (NH₄)₂SO₄ and any oth­er equiv­a­lent salt will also be suit­able. The gas ob­tained is dried by pass­ing it through a mix­ture of sodi­um hy­drox­ide with lime.

Am­mo­nia can also be ob­tained in the in­ter­ac­tion of cyanides with wa­ter with boil­ing:

NaCN + 2H₂O = HCOONa + NH₃ (the cyanide so­lu­tion must be con­cen­trat­ed).

*Click here for an ex­cit­ing ex­per­i­ment with am­mo­nia.

The for­ma­tion of am­mo­nia is pos­si­ble in the ir­re­versible hy­drol­y­sis of amides, imides and ni­trides re­spec­tive­ly (a base and am­mo­nia are formed):

  • NaN­H₂ + H₂O = NaOH + NH₃ (hy­drol­y­sis of sodi­um amide);
Sodium hydroxide [Wikimedia]
  • CaNH + 2H₂O = Ca(OH)₂ + NH₃ (hy­drol­y­sis of cal­ci­um imide);

  • Zn₃N₂ + 6H₂O = 3Zn(OH)₂ + 2NH₃ (hy­drol­y­sis of zinc ni­tride).

Am­mo­nia has many chem­i­cal prop­er­ties – for ex­am­ple, it is a good re­duc­er and lig­and (mol­e­cule in a com­plex com­pound bond­ed with a com­plex for­mer – the cen­tral atom of the com­plex, for ex­am­ple Cu(NH₃)₄₂ – cop­per (II) hy­drox­ide tetraamine.

Qual­i­ta­tive re­ac­tions to am­mo­nia

The qual­i­ta­tive re­ac­tion to am­mo­nia is plac­ing fab­ric or cot­ton wool soaked in con­cen­trat­ed hy­drochlo­ric acid in a flask with NH₃: white smoke will be re­leased be­cause am­mo­ni­um chlo­ride NH₄­Cl forms:

NH₃ + HCl = NH₄­Cl.

NH₄Cl [Wikimedia]

As am­mo­nia is a weak base, it has a weak al­ka­line re­ac­tion of the medi­um. This can be no­ticed by the change in the col­or of in­di­ca­tors:

  • lit­mus turns blue;

  • phe­nolph­thalein turns crim­son;

  • methyl or­ange turns yel­low.

Am­mo­nia and am­mo­ni­ac com­pounds can also be de­tect­ed by their char­ac­ter­is­tic smell.

Phenolphthalein in alkaline solution [Wikimedia]

Chem­i­cal prop­er­ties of am­mo­nia: re­ac­tions with­out a change in ox­i­da­tion states

Typ­i­cal chem­i­cal re­ac­tions with am­mo­nia with­out a change in the ox­i­da­tion state of ni­tro­gen:

  • re­ac­tion with wa­ter:

NH₃ + H₂O = NH₄OH = NH₄⁺ + OH⁻ (the re­ac­tion is re­versible, as am­mo­ni­um hy­drox­ide NH₄OH is an un­sta­ble com­pound);

  • re­ac­tion with acids with the for­ma­tion of nor­mal or acid salts:

NH₃ + HCl = NH₄­Cl (the nor­mal salt am­mo­ni­um chlo­ride is formed);

NH₃ + H₂­SO₄ = NH₄H­SO₄ (in the re­ac­tion of am­mo­nia with cold con­cen­trat­ed sul­fu­ric acid, the acid salt am­mo­ni­um hy­dro­sul­fate forms);

2NH₃ + H₂­SO₄ = (NH₄)₂SO₄ (in the re­ac­tion be­tween am­mo­nia and di­lut­ed hot sul­fu­ric acid, the nor­mal salt am­mo­ni­um sul­fate forms).

Ammonium sulfate [Wikimedia]
  • re­ac­tion with salts of heavy met­als with for­ma­tion of com­plex­es:

2NH₃ + AgCl = [Ag(NH₃)₂]Cl (the com­plex com­pounds sil­ver (I) chlo­ride di­amine forms);

  • re­ac­tion with haloalka­nes:

NH₃ + CH₃­Cl = [CH₃N­H₃]Cl (methy­lam­mo­ni­um hy­drochlo­ride forms – this is the sub­sti­tut­ed am­mo­nia ion NH₄⁺);

  • re­ac­tion with al­ka­line met­als:

2NH₃ + 2K = 2KN­H₂ + H₂ (potas­si­um amide KNH₂ forms; ni­tro­gen does not change its ox­i­da­tion state, al­though the re­ac­tion is an ox­i­diza­tion-re­duc­tion one).

At­tach­ment re­ac­tions take place in the ma­jor­i­ty of cas­es with­out changes in ox­i­da­tion states (all the above re­ac­tions apart from the last are clas­si­fied as this type of re­ac­tion).


Ox­i­da­tion of am­mo­nia: re­ac­tions with a change in ox­i­da­tion states

Re­ac­tions of the ox­i­da­tion of am­mo­nia take place with a change in the ox­i­da­tion state of ni­tro­gen. As am­mo­nia is a good re­duc­er, it can be used to re­duce heavy met­als from their ox­ides, for ex­am­ple:

  • Re­duc­tion of met­als:

2NH₃ + 3CuO = 3Cu + N₂ + 3H₂O (when cop­per (II) ox­ide is heat­ed in the pres­ence of am­mo­nia, metal­lic cop­per of a red­dish col­or is re­duced).

Copper (II) oxide [Wikimedia]
  • Ox­i­da­tion of am­mo­nia in the pres­ence of strong ox­i­diz­ers (for ex­am­ple halo­gens) takes place ac­cord­ing to the equa­tion:

2NH₃ + 3Cl₂ = N₂ + 6HCl (heat­ing is re­quired to car­ry out this ox­i­da­tion-re­duc­tion re­ac­tion).

  • When potas­si­um per­man­ganate has an im­pact on am­mo­nia in an al­ka­line medi­um, the for­ma­tion of molec­u­lar ni­tro­gen, potas­si­um man­ganate K₂M­nO₄ and wa­ter is ob­served:

2NH₃ + 6KM­nO₄+ 6KOH = 6K₂M­nO₄+ N₂ + 6H₂O;

  • At in­tense heat­ing (up to 1200 ᵒC or 2192 ᵒF) am­mo­nia can break down into sim­ple sub­stances:

2NH₃ = N₂ + 3H₂.

  • At 1000 ᵒC or 1832 ᵒF, am­mo­nia re­acts with meth­ane CH₄:

2CH₄ + 2NH₃ + 3O₂ = 2HCN + 6H₂O (hy­dro­cyan­ic acid and wa­ter form).

  • By the ox­i­da­tion of am­mo­nia with sodi­um hypochlo­rite, hy­drazine N₂H₄ can be ob­tained:

2NH₃ + NaO­Cl = N₂H₄ + NaCl + H₂O.

N₂H₄ hydrate [Wikimedia]

Com­bus­tion of am­mo­nia and its cat­alyt­ic ox­i­da­tion by oxy­gen

The ox­i­da­tion of am­mo­nia by oxy­gen has cer­tain spe­cial fea­tures. There are two dif­fer­ent types of ox­i­da­tion:

  • cat­alyt­ic (with a cat­a­lyst);

  • rapid (com­bus­tion).

In com­bus­tion, an ox­i­da­tion-re­duc­tion re­ac­tion takes place, the prod­ucts of which are molec­u­lar ni­tro­gen and wa­ter:

4NH₃ + 2O₂ = 2N₂ + 6H₂O (the flame is yel­low-green, com­bus­tion starts at 651 ᵒC or 103.8 ᵒF, as this is the self-ig­ni­tion tem­per­a­ture of am­mo­nia).

Cat­alyt­ic ox­i­da­tion of oxy­gen also takes place with heat­ing (around 800 ᵒC or 1472 ᵒF), but one of the prod­ucts of re­ac­tion is dif­fer­ent:

4NH₃ + 5O₂ = 4NO + 6H₂O (in the pres­ence of plat­inum (or iron, man­ganese, chromi­um or cobalt ox­ides) as a cat­a­lyst, the ox­i­da­tion prod­ucts are ni­tro­gen (II) ox­ide and wa­ter).

MgO [Wikimedia]

Up to 100 mil­lion tons of am­mo­nia is man­u­fac­tured world­wide ev­ery year: this sub­stance is one of the most pop­u­lar and wide­ly used in in­dus­try. It is used for the man­u­fac­ture of am­mo­nia so­lu­tions, the pu­rifi­ca­tion and dye­ing of nat­u­ral fab­rics, the man­u­fac­ture of ni­tric acid, and in syn­thet­ic fab­rics, and the salts am­mo­nia, car­bamide and urotropine. It is an in­ex­pen­sive cool­ing agent. Am­mo­nia is harm­ful to the skin and mu­cous mem­branes, as it caus­es se­vere ir­ri­ta­tion.