JEE Main & Advanced Chemistry Chemical Bonding and Molecular Structure Hydrogen bonding

Hydrogen bonding

Category : JEE Main & Advanced

In 1920, Latimer and Rodebush introduced the idea of "hydrogen bond".         

For the formation of H-bonding the molecule should contain an atom of high electronegativity such as F, O or N bonded to hydrogen atom and the size of the electronegative atom should be quite small.         

Types of hydrogen bonding           

(1) Intermolecular hydrogen bond : Intermolecular hydrogen bond is formed between two different molecules of the same or different substances.           

(i) Hydrogen bond between the molecules of hydrogen fluoride.           

(ii) Hydrogen bond in alcohol and water molecules           

(2) Intramolecular hydrogen bond (Chelation)         

Intramolecular hydrogen bond is formed between the hydrogen atom and the highly electronegative atom (F, O or N) present in the same molecule. Intramolecular hydrogen bond results in the cyclisation of the molecules and prevents their association. Consequently, the effect of intramolecular hydrogen bond on the physical properties is negligible.           

For example : Intramolecular hydrogen bonds are present in molecules such as o-nitrophenol, o-nitrobenzoic acid, etc.                                                                                                                     

The extent of both intramolecular and intermolecular hydrogen bonding depends on temperature.           

Effects of hydrogen bonding           

Hydrogen bond helps in explaining the abnormal physical properties in several cases. Some of the properties affected by H-bond are given below,         

(1) Dissociation : In aqueous solution, hydrogen fluoride dissociates and gives the difluoride ion \[(HF_{2}^{-})\] instead of fluoride ion \[({{F}^{-}})\]. This is due to H-bonding in HF. This explains the existence of\[KH{{F}_{2}}\]. H-bond formed is usually longer than the covalent bond present in the molecule (e.g. in \[{{H}_{2}}O,\,\,O-H\] bond = 0.99 Å but H-bond = 1.77 Å).         

(2) Association : The molecules of carboxylic acids exist as dimers because of the hydrogen bonding. The molecular masses of such compounds are found to be double than those calculated from their simple formulae. For example, molecular mass of acetic acid is found to be 120.         

(3) High melting and boiling point : The compounds having hydrogen bonding show abnormally high melting and boiling points.           

The high melting points and boiling points of the compounds \[({{H}_{2}}O,\ HF\] and \[N{{H}_{3}})\] containing hydrogen bonds is due to the fact that some extra energy is needed to break these bonds.         

(4) Solubility : The compound which can form hydrogen bonds with the covalent molecules are soluble in such solvents. For example, lower alcohols are soluble in water because of the hydrogen bonding which can take place between water and alcohol molecules as shown below,

\[\begin{align}   & \overset{\delta +}{\mathop{H}}\,-\overset{\delta -}{\mathop{O}}\,................\overset{\delta +}{\mathop{H}}\,-\overset{\delta -}{\mathop{O}}\,...............\overset{\delta +}{\mathop{H}}\,-\overset{\delta -}{\mathop{O}}\, \\  & \,\,\,\,\,\,\overset{\,}{\mathop{\,}}\,\,{{C}_{2}}{{H}_{5}}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,H\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{{C}_{2}}{{H}_{5}} \\ \end{align}\]           

The intermolecular hydrogen bonding increases solubility of the compound in water while, the intramolecular hydrogen bonding decreases.

(5) As the compounds involving hydrogen bonding between different molecules (intermolecular hydrogen bonding) have higher boiling points, so they are less volatile.         

(6) The substances which contain hydrogen bonding have higher viscosity and high surface tension.

(7) Explanation of lower density of ice than water and maximum density of water at 277K : In case of solid ice, the hydrogen bonding gives rise to a cage like structure of water molecules as shown in following figure. As a matter of fact, each water molecule is linked tetrahedrally to four other water molecules. Due to this structure ice has lower density than water at \[273K.\] That is why ice floats on water. On heating, the hydrogen bonds start collapsing, obviously the molecules are not so closely packed as they are in the liquid state and thus the molecules start coming together resulting in the decrease of volume and hence increase of density. This goes on upto 277K. After 277 K, the increase in volume due to expansion of the liquid water becomes much more than the decrease in volume due to breaking of H-bonds. Thus, after \[277K\], there is net increase of volume on heating which means decrease in density. Hence density of water is maximum \[277K\].

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