The poorly shielded nucleus of hydrogen in one molecules of hydrogen in one molecule is attracted by the lone pair of electrons of the highly electronegative element of the neighboring molecule. As a result dative relationship is formed between the two molecules, this is why hydrogen bonding is the strongest among the all other vander wall’s forces. The hydrogen bonding in compounds containing fluorine, oxygen or nitrogen is represented by dotted line (------). Hydrogen bonding have influence on the physical properties of molecules like increase in bonding points, solubilities etc.

Effect of hydrogen bonding on some physical properties:

             Hydrogen bonding greatly effects some physical properties of compound. Some example are given below.

 (a)      Ice floats over the surface of water: This is because ice have strong hydrogen bonds. Ice has a tetrahedral structure i-e; each oxygen atom of oxygen have two normal covalent bonds and two hydrogen bonds. The arrangement of water molecules in ice creates a very open structure. Due to larger volume density of ice decreases this is why ice floats over the surface of water.

 (b)      In group VI H2O have high boiling point than H₂S, H₂Se and H₂Te: High boiling point of water than other hydrogen compounds of the same group is due to strong intermolecular forces. In water strong intermolecular forces are due to hydrogen bonding, hence more energy is required to overcome hydrogen bonding. As a result boiling point of water is more than H₂S, Se and H₂Te.

 (c)       Boiling point of NH3 and HF is also higher than other hydrogen compounds of their groups: The increases boiling points of NH₃ and HF as compared to other hydrogen compounds of their groups is also due to hydrogen bonding. It is stronger intermolecular force, hence, enthalpy value of these molecules is higher than other member of their respective groups.

 (d)      The boiling points if alcohols and amines are higher than those of alkanes of comparable molecular mass: .

             MOLECULE                             CH₃CH₂OH                  CH₃CH₂NH₂                CH₃H₈

            MOLECULAR MASS                        46                                  45                         44   

            BOILING POINT  ⁰C                     98 ⁰C                            17⁰C                     -42⁰C

             The large difference in boiling points between alcohol, amine and comparable alkane is due to strong intermolecular forces. Infact alcohol and amine have additional hydrogen bonding which are stronger intermolecular forces, hence more energy is required to break additional hydrogen bonds.

 (e)      2 Nitrophenol is more volatile than 4- nitrophenol.

                               4 – nitrophenol

                     Due to intermolecular  hydrogen bonding i.e; bonding within one molecule, in 2 nitrophenol molecules are mainly held by van du waal’s forces and could easily be separated. On the other hand     4 – nitrophenol have intermolecular hydrogen bonding. As a result molecules are held strongly. Hence, more energy is needed to separate the molecules of 4-nitrophenol from each other, thus, it is less volatile than 2-nitrophenol.

 (f)       Ethanol is solube in water but chloroethane is immiscible.

Hydrogen bonds are already present in water molecules. Ethanol can form hydrogen bonds with water molecules and therefore it get dissolved in water. On the other hand chloroethane can not form hydrogen bonds with water hence, it is slightly soluble.

(g)       Molecular mass of carboxylic acids measured in the vapour phase from solution in organic solvent is twice as large as expected: The reason for his is that carboxylic acid forms hydrogen bonded dimmers in organic solvent.

             Dimerisation: Does not occur when the acid is dissolved in water. This is because water molecules will form intermolecular hydrogen bonds with carboxylic acid molecules.

Dimer of ethanoic acid in vapour phase from organic solvent

(h)      Ammonia is a gas which liquefies easily under pressure: Due to the formation of hydrogen bonds.

 

(i)       Nylon fibres are used in making climbing ropes:  because it has high tensile strength and high elasticity. It is due to strong hydrogen bonds formed between an-NH-hydrogen atom and a C=0 oxygen on a neighboring polymer chain.

 

(j)       Hydrogen bonds: Play a very important put in the structure and properties of biochemical polymers. For example DNA and protein chains often form helical structure. The stability of helical structure primarily depends on the hydrogen bonds, which hold the two parts of the molecules together in a double helix structure.

 

            Overall intermolecular forces are much weaker then forces of attraction found in typical covalent bonds or in ionic bonding. Anyhow, hydrogen bonds are about twice as strong as the other intermolecular forces.

 

DEDUCTION OF THE TYPE OF BONDING PRESENT FROM THE GIVEN INFORMATION:

 

            There are some typical properties for each type of bonding from which we can deduce the type of bonding present in that sample.