Non Aqueous Solvents
Reactions in Liquid Ammonia (NH3)
Chemical Reactions which occur in liquid ammonia can be classified as follows-
1. Acid-Base Reactions
2. Precipitation Reactions
3. Ammonolysis Reactions
4. Ammoniation Reactions
5. Redox Reactions
6. Complex formation Reaction
1. Acid-Base Reactions
Auto ionisation of ammonia generates ammonium ion and amide ion.
2NH3(liquid) ⇌ NH4+ + NH2−
Ammonium ions in liquid ammonia will act as acids whereas amide ions will act as base. Such acids are known as Ammono Acids and bases are known as Ammono Base. When these two are mixed with each other then the resultant reaction is known as Neutralization Reaction.
NH4Cl + KNH2 ⇌ KCl + 2NH3
2. Precipitation Reaction
Precipitation reaction depends upon the solubility of reaction product in given solvent. So nature of precipitation reaction also depends upon the nature of solvent. Because of the differences of solubilities of various substances in liquid ammonia and water, a number of reactions which do not take place in water, can be carried out in Liquid Ammonia.
2AgCl + Ba(NO3)2 → BaCl2↓ + 2AgNO3
The above reaction proceeds in forward direction due to insolubility of barium chloride in liquid ammonia.
3. Ammonolysis Reactions
Liquid ammonia ammonolyses some salts to Ammono basic salts.
TiCl4 + 8NH3 → Ti(NH2)4 + 4NH4Cl
4. Ammoniation Reactions
When a solute is dissolved in liquid ammonia and the resultant product contains a number of molecules of liquid ammonia, then such reaction is known as ammoniation reaction. It is anexample of solvation reaction in which solvent is ammonia.
CuSO4 + 4NH3 → CuSO4.4NH3
5. Redox Reaction
Redox Reactions can be classified in three categories-
a. When alkali metals act as reducing agent in liquid ammonia-
Dilute solutions of alkali metal in liquid ammonia have a deep blue colour. These are good conductor of electricity, paramagnetic in nature and show strong reducing property.
2Na + Se → Na2Se
b. Where liquid ammonia acts as reducing agent-
In some reactions, liquid ammonia itself act as reducing agent.
4NH3 + 5O2 ---Pt---> 4NO + 6H2O
c. Where liquid ammonia acts as solvent-
There are number of reactions in which Liquid Ammonia acts only as solvent. Potassium permanganate which act as strong oxidising agent in water, behaves as weak oxidising agents in Liquid Ammonia.
6KMnO4 + 6KNH2 → 6K2MnO4 + 4NH3 + N2
6. Complex formation reaction
A number of Complex formation reactions also take place in liquid ammonia. These reactions are similar to the reactions occurring in aqueous system.
Zn(NO3)2 + 2KNH2 → Zn(NH2)2 + 2KNO3
Zn(NH2)2 + 2KNH2 → K2Zn(NH2)4(Soluble Amide Complex)
Chemical Reactions which occur in liquid ammonia can be classified as follows-
1. Acid-Base Reactions
2. Precipitation Reactions
3. Ammonolysis Reactions
4. Ammoniation Reactions
5. Redox Reactions
6. Complex formation Reaction
1. Acid-Base Reactions
Auto ionisation of ammonia generates ammonium ion and amide ion.
2NH3(liquid) ⇌ NH4+ + NH2−
Ammonium ions in liquid ammonia will act as acids whereas amide ions will act as base. Such acids are known as Ammono Acids and bases are known as Ammono Base. When these two are mixed with each other then the resultant reaction is known as Neutralization Reaction.
NH4Cl + KNH2 ⇌ KCl + 2NH3
2. Precipitation Reaction
Precipitation reaction depends upon the solubility of reaction product in given solvent. So nature of precipitation reaction also depends upon the nature of solvent. Because of the differences of solubilities of various substances in liquid ammonia and water, a number of reactions which do not take place in water, can be carried out in Liquid Ammonia.
2AgCl + Ba(NO3)2 → BaCl2↓ + 2AgNO3
The above reaction proceeds in forward direction due to insolubility of barium chloride in liquid ammonia.
3. Ammonolysis Reactions
Liquid ammonia ammonolyses some salts to Ammono basic salts.
TiCl4 + 8NH3 → Ti(NH2)4 + 4NH4Cl
4. Ammoniation Reactions
When a solute is dissolved in liquid ammonia and the resultant product contains a number of molecules of liquid ammonia, then such reaction is known as ammoniation reaction. It is anexample of solvation reaction in which solvent is ammonia.
CuSO4 + 4NH3 → CuSO4.4NH3
5. Redox Reaction
Redox Reactions can be classified in three categories-
a. When alkali metals act as reducing agent in liquid ammonia-
Dilute solutions of alkali metal in liquid ammonia have a deep blue colour. These are good conductor of electricity, paramagnetic in nature and show strong reducing property.
2Na + Se → Na2Se
b. Where liquid ammonia acts as reducing agent-
In some reactions, liquid ammonia itself act as reducing agent.
4NH3 + 5O2 ---Pt---> 4NO + 6H2O
c. Where liquid ammonia acts as solvent-
There are number of reactions in which Liquid Ammonia acts only as solvent. Potassium permanganate which act as strong oxidising agent in water, behaves as weak oxidising agents in Liquid Ammonia.
6KMnO4 + 6KNH2 → 6K2MnO4 + 4NH3 + N2
6. Complex formation reaction
A number of Complex formation reactions also take place in liquid ammonia. These reactions are similar to the reactions occurring in aqueous system.
Zn(NO3)2 + 2KNH2 → Zn(NH2)2 + 2KNO3
Zn(NH2)2 + 2KNH2 → K2Zn(NH2)4(Soluble Amide Complex)
Reactions in Liquid Sulphur Dioxide (SO2)
Chemical reactions which occur in Liquid Sulphur Dioxide can be classified as follows
1. Acid-Base Reactions
2. Solvolytic Reactions
3. Precipitation Reactions
4. Solvation Reactions
5. Complex Formation Reactions
6. Redox Reactions
Chemical reactions which occur in Liquid Sulphur Dioxide can be classified as follows
1. Acid-Base Reactions
2. Solvolytic Reactions
3. Precipitation Reactions
4. Solvation Reactions
5. Complex Formation Reactions
6. Redox Reactions
Comparison of Liquid Ammonia and Water as Solvent
Hydrogen Bonding
Both water and liquid ammonia are an associated solvent through hydrogen bonding. Due to the lower electronegative of nitrogen as compared to oxygen, ammonia molecules are less strongly associated than water molecules in water. The hydrogen bond in waater is stronger than that of in ammonia. Hydrogen bond in liquid ammonia can not form cross-linked network as each amminia molecule has only one lone pair of electrons compared with two for each water molecule.
Liquid Range
Due to weaker hydrogen bonding in liquid ammonia the freezing point and boiling point are lower than those of water. The liquid range in ammonia is only between -77.7 to -33.4oC as compared to that of water which ranges between 0 to 100oC. Due to small liquid range, it has few handling problems.
Dielectric Constant
The dielectric constant of ammonia (22 at – 33.4oC) is much lower as compared to that of water (81 at 18oC). Due to this, ammonia is a poor solvent for ionic substances.
Viscosity
Liquid ammonia has lower viscosity (0.254 at – 33.4oC) than that of water (0.959 at 25oC). Due to this, liquid ammonia causes less resistance towards the mobilities of ions. Therefore, ions of soluble salts have greater ionic mobility in liquid ammonia than in water.
Self-ionization
The extent of self-ionization in ammonia is much less than that of in water.
2NH3 ⇌ NH4+ + NH2- K = 1.0 X 10-33 (-400C)
2H2O ⇌ H3O+ + OH- K = 1.0 X 10-14 (250C)
It shows that ammonia conducts current to a very limited extent and is a better solvent for covalent substances.
Hydrogen Bonding
Both water and liquid ammonia are an associated solvent through hydrogen bonding. Due to the lower electronegative of nitrogen as compared to oxygen, ammonia molecules are less strongly associated than water molecules in water. The hydrogen bond in waater is stronger than that of in ammonia. Hydrogen bond in liquid ammonia can not form cross-linked network as each amminia molecule has only one lone pair of electrons compared with two for each water molecule.
Liquid Range
Due to weaker hydrogen bonding in liquid ammonia the freezing point and boiling point are lower than those of water. The liquid range in ammonia is only between -77.7 to -33.4oC as compared to that of water which ranges between 0 to 100oC. Due to small liquid range, it has few handling problems.
Dielectric Constant
The dielectric constant of ammonia (22 at – 33.4oC) is much lower as compared to that of water (81 at 18oC). Due to this, ammonia is a poor solvent for ionic substances.
Viscosity
Liquid ammonia has lower viscosity (0.254 at – 33.4oC) than that of water (0.959 at 25oC). Due to this, liquid ammonia causes less resistance towards the mobilities of ions. Therefore, ions of soluble salts have greater ionic mobility in liquid ammonia than in water.
Self-ionization
The extent of self-ionization in ammonia is much less than that of in water.
2NH3 ⇌ NH4+ + NH2- K = 1.0 X 10-33 (-400C)
2H2O ⇌ H3O+ + OH- K = 1.0 X 10-14 (250C)
It shows that ammonia conducts current to a very limited extent and is a better solvent for covalent substances.
Q. What is the color of the solution when Sodium is dissolved in liquid ammonia
When sodium is dissolved in liquid ammonia, a solution of deep blue colour is obtained. The blue colour of the solution is due to the ammoniated electron which absorbs energy in the visible region of light and thus imparts blue colour to the solution.
Na + (x + y)NH3 → [Na(NH3)x]+ + [e(NH3)y]−
When sodium is dissolved in liquid ammonia, a solution of deep blue colour is obtained. The blue colour of the solution is due to the ammoniated electron which absorbs energy in the visible region of light and thus imparts blue colour to the solution.
Na + (x + y)NH3 → [Na(NH3)x]+ + [e(NH3)y]−
Q. The blue color of the sodium solution in liquid ammonia is discharged by addition of ammonium chloride. Explain.
When sodium is dissolved in liquid ammonia, a solution of deep blue colour is obtained. The blue colour of the solution is due to the ammoniated electron which absorbs energy in the visible region of light and thus imparts blue colour to the solution.
Na + (x + y)NH3 → [Na(NH3)x]+ + [e(NH3)y]−
The color is quickly discharged by adding ammonium chloride because the ammonium ion are rapidly reduced to ammonia and free hydrogen.
NH4+ + [e(NH3)y]− → NH3 + 1/2H2↑
Hence, the blue color of the sodium solution in liquid ammonia is discharged by addition of ammonium chloride
When sodium is dissolved in liquid ammonia, a solution of deep blue colour is obtained. The blue colour of the solution is due to the ammoniated electron which absorbs energy in the visible region of light and thus imparts blue colour to the solution.
Na + (x + y)NH3 → [Na(NH3)x]+ + [e(NH3)y]−
The color is quickly discharged by adding ammonium chloride because the ammonium ion are rapidly reduced to ammonia and free hydrogen.
NH4+ + [e(NH3)y]− → NH3 + 1/2H2↑
Hence, the blue color of the sodium solution in liquid ammonia is discharged by addition of ammonium chloride
Q. The paramagnetic behaviour of alkali metal solution in liquid ammonia decreases with increases of concentration, why ?
Dilute solution of alkali metals are paramagnetic due to the presence of unpaired electrons but with increasing metal concentrations, the solvated electrons undergo pairing.
M(s) + (x + y)NH3 ⇌ [M(NH3)x]+ + [e(NH3)y]−
2[e(NH3)y]− ⇌ [e 2(NH3)y]−2
Hence, paramagnetic behaviour decreases and furthur, ionizaion decreases on increasing concentration.
Dilute solution of alkali metals are paramagnetic due to the presence of unpaired electrons but with increasing metal concentrations, the solvated electrons undergo pairing.
M(s) + (x + y)NH3 ⇌ [M(NH3)x]+ + [e(NH3)y]−
2[e(NH3)y]− ⇌ [e 2(NH3)y]−2
Hence, paramagnetic behaviour decreases and furthur, ionizaion decreases on increasing concentration.
Q. Which of the following statements is not true about the dilute solutions of alkali metals in liquid ammonia ?
a. They are deep blue coloured solutions
b. They are highly conductive in nature
c. They are diamagnetic in natured
d. Ammoniated cation and solvated electrons are formed in the solution
a. They are deep blue coloured solutions
b. They are highly conductive in nature
c. They are diamagnetic in natured
d. Ammoniated cation and solvated electrons are formed in the solution
