Non-Aqueous Solvents (Part - 1) - Acids and Bases, Inorganic Chemistry, CSIR-NET - Class 10 PDF Download

Non-Aqueous Solvents

As we know most of the chemical reactions requires solution phase for them to carry out, hence for this we need different kinds of solvents. In the earlier days of development of chemistry, the reactions have been carried out only in one solvent”water”. Because of different physical and chemical properties of water such as-

Long liquid range(0-100 ℃),High dielectric constant (78.5) ,Specific heat, heat of vapourisation,Dipole moment etc. is used as universal solvent.

Beside these properties, water is non-toxic, non-poisonous, neutral, and colourless and this makes it as an excellent and useful solvent for a large variety of substances.

But nowadays we have many more solvents available which can behave as good solvent like water.

These solvents other than water which are used for carrying out chemical reactions are called non-aqueous solvents. For example:

Benzene, carbon tetrachloride, acetone, ether etc are organic solvents and Liquid ammonia, sulphuric acid, liquid HF, liquid sulphur dioxide etc are inorganic solvents.

Physical Properties Of Solvent

A) Melting & Boiling Point (Liq. Range)​

Water has excellent liquid range( 0 to 100℃) at ordinary temp. And pressure. Acids can also act as solvent at ordinary temp while liq.NH₃ (-77.7 to -33.4℃) and liq. SO₂(-75.5 to -10.2℃) can act as solvent only at low temp.

B) Heat Of Fusion & Heat Of Vapourisation:

helps to know the nature and strength of forces which hold the molecule of the solvent together in solid or liquid state.

A high heat of vapourisation of liquid indicates that the intermolecular forces in it are strong.

∆H_vap/T_{boiling pt.} = Trouton Constant

The ratio of the heat of vapourisation expressed in joules to the boiling point(K) is a constant known as Trouton Constant.

• High value of Trouton constant indicates associated molecules.

*Molar heat of fusion for water (6.0kJ/mol) and ammonia(5.65kj/mol) are almost same .this means the forces which hold molecules together in H₂O and NH₃ are of same magnitude.

C) Dielectric Constant:

in solvent of high dielectric constants ,the ionic substances will dissolve readily .

• With decrease in dielectric constant, the solubility of ionic compounds decreases.
• NH₃ and SO₂ with small value of dielectric const. Values show decreased tendency to dissolve ionic compounds.

D) Dipole Moment:

Greater is the Polarity between bonds in the molecules ,larger is the dipole moment.
And with Increase in dipole moment ,tendency to dissolve ionic substance increases.

E) Viscosity:

it is the measure of fluidity of the solvent.

In solvents of low viscosity the operations such as precipitation , crystallisation ,filtration etc can be easily carried out without any difficulty.

Liquid Ammonia

It is highly used as non-aqueous solvent.

Comparison Of Properties Of Liq.NH₃ & H₂O

• Liquid Range: the melting and boiling points of liq.NH₃ are lower then those of water,its liquid range is -77 to -33℃ and its difficult to handle.
• Hydrogen Bonding: liq.NH₃ also show hydrogen bonding but NH₃ molecules are less strongly associated bcoz N is less electronegative than Oxygen. Therefore freezing point & boiling point of ammonia are less.

Dielectric Constant: dielectric constant of ammonia (22 at -33℃) is lower as compared to water (78.5 at 25℃). Therefore liq. NH₃ is a poor Solvent of ionic Subatances.

However,ammonia has lower viscosity(0.254cp at -33℃) than water (0.959cp at 25℃).this might be expected to permote greater ionic mobilities and thereby compensate to some extent for the effect of dielectric constant.

Self ionisation: the extent of auto ionisation of liq.NH₃ is less than that of water.this is clear from its low specific Conductance and lower ionic product.

K[NH₃]=[NH₄⁺][NH₂^–]= 1.9×10^-23 ,at -50℃

K[H₂O]= [H₃O⁺][OH^–] = 1×10^-14 ,at 25℃

AMMONIA As Better Solvent Than Water

i) Liq. NH₃ is a very poor Conductor of electricity.

ii) Specific heat of ammonia is more than that of water.

iii) Liq.NH₃ has low viscosity which greatly increases the mobility of the ions.

iv) Liq.NH₃ has high critical temp and pressure.

v) it is less associated than water and therefore it is better solvent for organic compounds.

Reactions Of Liq.Ammonia

Acid-Base Reactions

2NH₃ ⇋ NH₄⁺ + NH₂^–

Substance that produces Ammonium ions (NH₄⁺) in liq.NH₃ behave as acid while those produces amide ions(NH₂^–) behave as base.

Example :

NH₄Cl → NH₄⁺ + Cl^– (Acidic Character)

KNH₂ → K⁺ + NH₂^– (basic character)

Protolysis:

some compounds like urea,acetamide ,sulphamide etc which are incapable of donating protons to water can readily donate proton to NH₃ in liquid ammonia .hence these compounds act as acid in liq.NH₃.

Examples:

1. 

2.  

Acid-Base Neutralisation

NH₄Cl(acid) + KNH₂(base) → KCl + 2NH₃

NH₄⁺ + NH₂^– → 2NH₃

Zn(NH₂)₂ shows Amphoteric behaviour in liq.NH₃

Zn(NH₂)₂ (base) +2NH₄Cl (acid) → ZnCl₂ + 4NH₃

Zn(NH₂)₂(acid) + 2KNH₂(base) → K₂Zn(NH₂)₄

Ammono Acid: is a Substance which furnishes NH₄⁺ ion in liq.NH₃ .Ammonium halides ,nitrate or thiocynate are very soluble in liq. NH₃ to give NH₄⁺ ion.

Ammono base: Ammono base is a Substance which in liq.NH₃ solution gives NH₂^– ion .Example ,KNH₂.

Precipitation Reaction

Precipitation reaction involves double decomposition ,due to difference in the Solubilities of various substances in water and liq.NH₃ .

Liq.NH₃ is better in precipitation reaction than water.

Example:

In Aqueous medium, KCl + AgNO₃ → AgCl↓ + KNO₃

In liq.NH₃ ,

i) 2AgCl + Ba(NO₃)₂ → BaCl₂↓ + 2AgNO₃

¡¡) NH₄Cl + LiNO₃ → LiCl↓ + NH₄NO₃

Ammonolysis :

the solvolysis reaction of liq.NH₃ are called Ammonolysis reactions.

1. 

2. 

NaH + NH₃ → NaNH₂ + H₂

HgCl₂ → Hg(NH₂)Cl + HCl

In case of organic halides which undergo slow Ammonolysis at the boiling point of liquid ammonia ,mixture of primary ,secondary, tertiary amines are formed.

RX + 2NH₃ → RNH₂ +NH₄X

RX + RNH₂ + NH₃ → R₂NH + NH₄X

RX + R₂NH + NH₃ → R₃N + NH₄X

Where X=Cl ,Br or I

Complex Formation Reactions:

these rxns are similar to the reactions occurring in the aqueous system.example:

CuCl₂ + 4NH₃ → [Cu(NH₃)₄]Cl₂

AgCl + 2NH₃ → [Ag(NH₃)₂]Cl

Formation Of Ammoniates

Just as hydrates ,ammonia also foem ammoniates with many inorganic salts.

Examples:

BHF₃ + NH₃ → BF₃.NH₃

MgI₂ +6NH₃ → MgI₂.6NH₃

Redox Reaction

In liquid Ammonia , the strong oxidising agent show weaker oxidising character than in water solution.

For example ,HNO₃ does not act as oxidising agent in liq. NH₃ ,On the other hand reducing agents show increased Reactivity.

Example : CuI + Na → Cu + NaI

KMnO₄ + K → K₂MnO₄

•Here liq.NH₃ reduces CuI ,KMnO₄ to Cu & K₂MnO₄ respectively.

Solution Of Alkali Metals In Liquid Ammonia

Properties

• these are blue in colour and colour is independent of the metal dissolved.
• When concentration of metal increased ,the colour changes to bronze and shows metallic appearance.
• these solutions are good conductor and show strong reducing properties.
• These Solutions are paramagnetic and the paramagnetic character decreases with increasing concentration.

Na ⇋ Na⁺ + e^–

Na⁺ + xNH₃ → [Na(NH₃)_x]⁺ ,ammoniated cation

e^– + yNH₃ → [e(NH₃)_y]^– , ammoniated electron

Or

Na + liq.NH₃ → [Na+(NH₃)_x]⁺ + [e(NH₃)_y]^–

• Ammoniated electrons are responsible for blue colour of the solution. the electrical conductivity is also due to the ammoniated cations and ammoniated electrons.
• As Concentration of Solution increased ,ammoniated Cations & electrons associate ,Resulting in a decreased Conductivity.
• Strong Reducing character of these solutions is due to the presence of readily available electrons.

Advantages Of Liq.NH₃

¡) dissolution of alkali metal without chemical reaction is one of the greatest advantage of using liq.NH₃.

¡¡) Alkali metal solutions are strong reducing agent even stronger than hydrogen.

¡¡¡) the tendancy of Solvolysis is less in liq.NH₃ Than water.

iv) Ammonium salt dissolved in liq.NH₃ find application in preparative chemistry. they can be used to precipitate sulphides ,halides, sulphates etc.

Disadvantage Of Liq.NH₃

¡) liquid range of liq.NH₃ is very low (-77℃ to -33℃). Therefore a low temp and high pressure is required.

¡¡) liq.NH₃ has offensive odour & hygroscopic in nature.therefore reactions are to be carried out in sealed tubes.