Hydrides of group 15 elements, Chemistry tutorial

Introduction to Hydrides:

All the elements of group 15 form trihydrides of the type EH3. Moreover to these, nitrogen as well forms N2H4 and N3H, and phosphorus gives PaH^. Bond energy of the M-H bond reduces from NH3 to BiH3 due to increase in the size of the element on descending the group. As a result, the stability of the hydrides reduces from NH3 to BiH3; bismuth hydride decomposes quite fast at room temperature. The variation in boiling point of trihydrides is one of the strongest pieces of proof for hydrogen bond formation by nitrogen. Properties of such trihydrides are summarized in the table given below:

Table: Properties of Group 15 hydrides, EH3

825_Properties of Group 15 hydrides.jpg

The central atom in the trihydrides is sp3 hybridized. As one of the positions in the tetrahedron is occupied via a lone pair, the structure of such hydrides is pyramidal, (figure shown below). The tetrahedron is distorted due to the repulsion between the lone pair of electrons and the bond pairs. By the decrease in the electronegativity of the central atom, the bond pairs of electrons go further away from the central atom. This yields in the decrease in repulsion of the bonding pairs in the vicinity of the central atom resultant in decrease in the bond angles that become close to 90° (table shown above). E-H bond then comprises of almost pure p- orbitals and the lone pair is nearly in a pure s-orbital. As we go down the group, lone pair resides mostly in the s-orbital from where it is harder to be removed. This is the cause why ammonia acts as a better donor than other hydrides of the group. Therefore, NH3 readily forms ammonium salts having H+. Phosphonium salts are made up only with H+ under anhydrous conditions however other hydrides, AsH3, SbH3 and BiH3 don't form these salts.

1529_Structure of Ammonia.jpg

Fig: Structure of Ammonia

Ammonia:

Ammonia is the significant industrial chemical of all the nitrogen compounds; it is generated in the largest quantities. Ammonia can be obtained in the lab by heating ammonium salts by an alkali:

2NH4Cl + Ca(OH)2 → CaCl2 + 2NH3 + 2H2O

Ammonia is prepared industrially via Haber method. Nitrogen from air and hydrogen from synthesis gas, are reacted altogether at high pressure of around 250 atmosphere and at a temperature of 800 K in the presence of a finely divided catalyst.

N2 + 3H2 ↔ 2NH3, ΔHo = - 92.0 kJ mol-1

As Le Chatelier's principle would state, high pressure will favor the forward reaction that carry on with the reduction in volume. As the forward reaction is exothermic, high temperatures would favor the back reaction resultant in dissociation of ammonia. Though, to let the reaction to carry on at a reasonable rate, the reaction is taken out at 800 K in the presence of a finely divided iron catalyst having traces of oxides of Mo, K and Al. The above temperature and pressure is optimum to give around 15% yield of NH3. The unreacted gases are recycled.

Ammonia encompasses a strong characteristic pungent smell. This is a colourless gas at room temperature and can be simply liquefied by either increasing the pressure or reducing the temperature. It is highly soluble in water; it is more soluble than any other gas due to hydrogen bonding with water. The ammonia solution in water is usually termed as ammonium hydroxide. The name ammonium hydroxide is, though, misleading and the solution should better be termed as aqueous ammonia. The solution comprises NH3 (aq), NH4 (aq) and OH- (aq) ions:

NH3 (aq) + H2O ↔ NH4 (aq) + OH- (aq)

The equilibrium constant 'K' for the above reaction is just 1.81 x 10-5. This value is fairly low, recommending that the aqueous solution is a weak alkali. Ammonia solution, in the presence of NH4Cl, is employed as a buffer solution in the pH range of 10. This mixture is as well employed for precipitation of metal hydroxides selectively.

NH3 molecule consists of a pyramidal structure, having a lone pair of electrons at the apex. The ammonium ion made on reaction with H+ consists of a tetrahedral structure. Ammonia functions as a donor molecule towards Lewis acids and most of the complex ions having ammonia as a ligand are recognized, example: [Cu(NH3)4]2+, [Ag(NH3)2]+, [Ni(NH3)6]2+ and so on. Ammonia is associated to the metal ions via its lone pair.

Liquid Ammonia as a Non-aqueous Solvent:

Liquid ammonia functions as a good solvent for numerous substances and numerous kinds of reactions. In this behavior, it resembles water as the solvent. Both are self-ionizing, the difference being in the lower degree of ionization of ammonia:

2NH3 ↔ NH4+ + NH2-

2H2O ↔ H3O+ + OH-

As we are familiar that all such substances, which on dissolving in water produce hydronium   ion, are acids and all such that produce hydroxide ion are bases, example, HCl, HNO3 are acids in water and NaOH, Ca(OH)2 are bases. Likewise, all such substances which dissolve in liquid ammonia to provide NH4 ions are acids and those which give amide ions, NH2, are bases. Therefore, NH4Cl, NH4NO3 are acids and NaNH2 is a base in liquid ammonia. Acid-base neutralisation reaction in liquid ammonia, therefore, can be a reaction giving a salt and the solvent. Compare the corresponding reaction in water, example:

NH4Cl → NH4+ + Cl-

NaNH2 → Na+ + NH2

NH4Cl + NaNH2 + (NH3) → NaCl + 2NH3

HCl + NaOH + (H2O) → NaCl + H2O

Likewise,

2NH4Cl + PbNH + (NH3) → PbCl2 + 3NH3

              Lead imide

3NH4Cl + AlN + (NH3) → AlCl3 + 4NH3

       Aluminium nitride

Therefore, we observe that imides such as PbNH and nitrides like AlN, as well function as bases in liquid ammonia. Liquid ammonia is the fundamental solvent as it can simply accept a proton. Thus, those acids that are weak in water will be highly acidic in liquid ammonia. Therefore, acetic acid is a weak acid in water (pka = 5) however will function as the strong acid in liquid ammonia:

CH3COOH + H2O ↔ CH3COO- + H3O+

CH3COOH + NH3 ↔ CH3COO- + 4NH+

Reactions related by complex formation are as well comparable in water and liquid ammonia.

ZnCl2 + 2NaOH + (water) → Zn(OH)2 + (excess NaOH) → [Zn(OH)4]2-

                                              Precipitate                           Soluble complex

ZnCl2 + 2NaNH2 + (Liquid NH3) → Zn(NH2)2 + (excess NaNh2) → [zn(NH2)4]2-

                                                       Precipitate                Soluble complex

Hydrazine, N2H4:

Hydrazine is made up by the action of sodium hypochlorite on ammonia in the presence of the small amount of gelatin that helps to suppress the side reactions:

NH3 + NaOCl → NH2Cl + NaOH

NH3 + NH2Cl + NaOH → NH2NH2 + NaCl + H2

N2H4 + 2NH2Cl → N2 + 2NH4Cl (side reaction)

3NH2Cl + 2NH3 → N2 + 3NH4Cl (side reaction)

Anhydrous hydrazine might be acquired by the distillation over NaOH or via precipitating N2H6SO4, which is then treated by liquid NH3 to precipitate (NH4)2SO4:

N2H4(aq) + H2SO4(aq) → N2H6SO4 + (2NH3) → N2H4 + (NH4)2SO4

Hydrazine is a colourless, fuming liquid. It makes a dihydrate, N2H4-2H2O and two series of salts, example - N2H5Cl and N2H6Cl2. Hydrazine burns in air providing nitrogen and water. As this reaction is highly exothermic (ΔHo = - 622 kJ), hydrazine is employed as a rocket fuel all along with liquid air or oxygen as the oxidant.

2124_Structure of Hydrazine.jpg

Fig: Structure of Hydrazine

Hydrazine consists of a structure identical to that of hydrogen peroxide having two lone pairs of electrons and can act as the coordinating ligand making complexes having metal ions such as Co2+, Ni2+ and so on. The bond energy of N-N bond in hydrazine is extremely small due to the repulsion of the nonbonding electrons that weaken the N-N bond. Thus hydrazine is not stable; it decomposes to N2, NH3 and H2 at 500 K.

Hydrazoic Acid, HN3 and Azides:

Hydrazoic acid is as well termed as hydrogen azide. It is a colourless, extremely explosive liquid. Whenever sodamide is reacted by N2O at 450 K, sodium azide is made up which on treatment with sulphuric acid provides hydrazoic acid:

2NaNH2 + N2O → NaN3 + NaOH + NH3

2NaN3 + H2SO4 → 2HN3 + Na2SO4

This is a weak acid and dissociates only slightly in water. By using electropositive metals it forms azides. Pb(N3)2 is covalent and explosive in nature and it is employed as a detonator. NaN3 is ionic and non-explosive. For the azide ion, N3, three resonance structures can be drawn:

1700_Resonance structures of azide ion.jpg

Fig: Resonance structures of azide ion

For covalent azides and hydrazoic acid, this is largely covalent; three resonance structures might as well be drawn. However, according to the Pauling's adjacent charge rule, one structure is excluded, since in it, two adjacent atoms encompass the similar charge. Therefore the increased stability of the ionic azides might be due to the larger number of resonance forms.

Phosphine, PH3:

Phosphine, PH3, is the most stable hydride of phosphorus. This is intermediate thermal stability between ammonia and arsine. Phosphine can simply be made up by any of the given methods:

Hydrolysis of metal phosphide like AlP or Ca3P2:

Ca3P2 + 6H2O → 2PH3 + 3Ca(OH)2

Pyrolysis of phosphorus acid at 480 - 485 K:

4H3PO3 → PH3 + 3H3PO4

Alkaline hydrolysis of Phosphonium iodide:

PH4I + KOH → PH3 + KI + H2O

Alkaline hydrolysis of the white phosphorus (industrial method):

P4 + 3KOH + 3H2O → PH3 + 3KH2PO2

Phosphine is a colourless, very poisonous gas having faint garlic odor. As the P-H bond is not polar adequate to form P-H-----P or P-H---- O bonds, dissimilar ammonia, phosphine is not related in the liquid state and is much less soluble in water. In contrary to the fundamental nature of solutions of ammonia in water, aqueous solutions of phosphine are neutral that is due to the much weaker tendency of PH, to protonate in water. Though, it does react by HI to form the Phosphonium iodide:

PH3 + HI → PH4I

Pure phosphine ignites in air at around 435 K, however whenever contaminated with traces of P2H4 it is spontaneously inflammable:

PH3 + 2O2 → H3PO4

Arsine, Stibine and Bismuthine:

AsH3, SbH3 and BiH3 are exceptionally poisonous, thermally unstable, colourless gases whose physical properties are compared by those of NH3 and PH3. As stated earlier, the thermal stability and fundamental character of these hydrides reduce from NH3 to BiH3.

AsH3 and SbH3 can be made by acid hydrolysis of arsenides and antimonides of electropositive elements such as Na, Mg, Zn and so on:

Mg3As2 + 6HCI → 2AsH3 + 3MgCl2

Zn3Sb2 + 6HCl → 2SbH3 + 3ZnCl2

Bismuthine is very unstable and is best made up by the disproportionation of methylbismuthine at 230 K:

3MeBiH2 → 2BiH3 + BiMe3

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