Chemical bonding -HYDROGEN BONDING

 

HYDROGEN BONDING

·     ·       In 1920, Latimer and Rodebush introduced the idea of "hydrogen bond" to explain the nature of association in liquid state of substances like water, hydrogen fluoride, ammonia, formic acid, etc.

     ·       Hydrogen bond can be defined as the attractive force which binds hydrogen atom of one molecule with the electronegative atom (F, O or N) 

     ·       It exerts in polar molecule or molecule having polar group.

     ·       This bond is represented by dotted line (---------) while solid represents the covalent bond.

     ·       The magnitude of the hydrogen bonding depends on the physical state of the compound.

     ·        order of  increasing strength of hydrogen bond in various physical state as

        gas < Liquid< solid  

        Type of hydrogen bonding-

         a)       Intermolecular hydrogen bonding: This type of bonding results between the positive and negative ends of different molecules of the same or different substances. Example-

   a) H^δ+ — F^δ--------H^δ+ — F^δ+--------H^δ+ — F^δ-

   b) H^δ+ — F^δ--------H^δ+ — O —H^δ+-------H^δ+ — F^δ       

        b).    Intramolecular hydrogen bonding: This type of bonding results between hydrogen and an electronegative element both present in the same molecule.

Importance of Hydrogen bond-

   ·      It helpful to explain that H2O exits in liquid state while H2S exists in gaseous state since in H2O, oxygen is more electronegative element than Sulphur in H2S therefore oxygen has ability to form H-bond with other water molecules. so it exists.

   ·      Ortho-nitrophenol has low boiling point than para-nitrophenol since P-nitrophenol form intermolecular hydrogen bond which is stronger than intra molecular hydrogen bond which is formed by ortho-nitrophenol.

     ·      Solubility of compound in water can be explained on the basis of h-bonding. Example alcohol is soluble in water but oil not. Because alcohol forms H-bond with water but oil does not form.

Gas laws - Boyle's law

 

The gaseous state

·      General characteristics are

a.     Shape and volume- gases neither has definite shape nor definite volume

b.     Highly compressible

c.      Gases exert pressure

d.     Have much lower density than the solid and liquids

e.     Diffusion- gases intermix completely in all proportions without any mechanical aid

f.       Liquefaction – can be liquified by cooling and by applying pressure.

This simplicity of gases is due to weak or  negligible force of interaction between gaseous particles

The gas laws

·      From the study of the behaviour of gases, certain generalisation were made. i.e the behaviour of gases are governed by some general laws, these generalisation are called gas laws.

·      These laws are related to measurable properties(pressure, volume, temperature and mass) of gases

These laws are as follows

a.     Boyle’s law(pressure-volume relationship)

b.     Charles’law (Temperature-volume relationship)

c.      Gay lussac’s law(pressure-Temperature relationship)

d.     Avogadro law(volume-Amount relationship)

a.     Boyle’s law (pressure-volume relationship)

·      In 1662 Robert Boyle proposed a relationship between Pressure and temperature. It is called Boyle’s law

·      The Pressure of a fixed amount of a gas are inversely related to its volume at constant temperature and.

                          P α 1/V   ---------(1)

           PV = K₁    ----------(2)      

     

 Let us consider  initial pressure and volume as P_1, V₁    After applying external pressure on gas, the final pressure and volume are as      P_2, V₂

   At constant amount of substance and Temperature

Then, according to Boyle’s law-

 P₁V₁ =K₁     ---------(1)

 P₂V₂ = K₁ ---------(2)

i.e.

    P₁V₁ = P₂V₂ =K₁     --------(3)

Graphical representation of Boyle’s law-

    ·      If graph is plotted between P and V we get curve called rectangular hyperbola)

   The curve clearly shows that when volume is increased, pressure decreases and vice versa.

   Similar curves are obtained at other temperature. Higher curve corresponds to higher temperature.

   Each curve corresponds to a different constant temperature, is known as isotherm (constant temperature curve) 

    ·      If the graph is plotted between P and 1/V, a straight-line graph is obtained passing through origin.

    ·      At high pressure, gasses deviate from Boyle’s law and so at high pressure, a straight-line graph cannot be obtained.

`            relationship between pressure and density

·      If density(d) = m/V    or V = m/d     Here m=mass of gas and V=volume of gas

Then we can give relationship between pressure and density by putting the value of volume (V) in Boyle’s equation as-

                              P m/d = K₁                                     (Boyle’s law  PV= K₁ )

                              So, P α d

Significance of Boyle’s law-

     ·      Gases are compressible      

     ·      Low pressure at high altitude, cause altitude sickness(anoxia) symptoms-uneasiness, sluggish feelings, headache

       Jet and aeroplanes fly at very high altitude with emergency oxygen supply in case of pressure falls