Characteristics of ammonia and catalytic oxidation
What properties are typical for ammonia?
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:
- 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).
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);
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.
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.
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).
- 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).
- 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.
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);
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).
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.