Comparison Of Liquid Nh3 And Water

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Comparison of Liquid NH3 and Water

1.    Hydrogen Bonding:

 Like water, liquid NH3 is an associated solvent through extensive hydrogen bonding. However, due to the lower electronegative of nitrogen as compared to oxygen, ammonia molecules are less strongly associated than H-O-H molecules in water. The O-H...O hydrogen bond is stronger than the N-H..N bond. Also hydrogen bonding in liquid NH3 cannot form cross-linked network since each NH3 molecule has only 1 lone pair of electrons compared with 2 for each H2O molecule. It is because of weaker hydrogen bonding in liquid NH3 that it has the lower mp , bp, density, viscosity, dielectric constant and electric conductivity.

2.    Liquid range:

Due to weaker hydrogen bonding in liquid NH3 (than in liquid H2O), the freezing point and boiling point of liquid. NH3 are lower than those of water. The liquid range in ammonia is only between (-77.7 to -33.4 0C, i.e., ≈ 44 0C) as compared to that of water which ranges between 0 to 1000C. The small liquid range creates a few handling problems. Its high heat of vaporization makes it way to handle in simple vacuum flasks without rapid bonding of the ammonia.

3.    Dielectric constant:

 The dielectric constant of ammonia (22 at – 33.40C) is much lower as compared to that of water (81 at 180C), suggesting that ammonia is a poor solvent for ionic substances. Generally, ammonium salts of large uninegative ions such as nitrates (NO3-), thiocyantes (SCN-) and iodides (I-) are soluble in liquid ammonia.

4.    Viscosity:

 Liquid ammonia has lower viscosity (0.254 at – 33.40C) than that of water (0.959 cP at 250C). This suggests that liquid ammonia provides less resistance towards the mobilities of ions. Therefore, ions of soluble salts have greater ionic mobility in liquid NH3 than in water. This compensates for its having lower dielectric constant value.

5.    Self-ionization:

The extent of self-ionization in ammonia is much les than in water.

2NH <==>  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.  

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