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HYDROGEN BONDING: Definition, effects and examples

hydrogen bonding in water

1.0 Definition of Hydrogen Bonding

The attractive force between hydrogen atoms bonded to strongly electronegative atom and lone pair electrons of another strongly electronegative atom is termed as hydrogen bonding.

Hydrogen bonds are not real chemical bonds in formal sense. These are weaker than covalent bonds. However, hydrogen bonds are stronger than dipole- dipole interactions, which are stronger then London dispersion forces.

hydrogen bonding simple example

2.0 Explanation of Hydrogen Bonding

Hydrogen bonds are not real chemical bonds in formal sense. These are weaker than covalent bonds. However, hydrogen bonds are stronger than dipole- dipole interactions, which are stronger then London dispersion forces.

Hydrogen bonding occurs among polar covalent molecules containing H and one of the small sized strongly electronegative elements such as N O or F. Common examples of molecules exhibiting hydrogen bonding are NH3, H2O and HF etc. The boiling point and heat of vaporization of H2O are higher than those of H2S because H2O molecules attract each other through hydrogen bonding whereas H2S molecules attract each other though dipole- dipole interactions which is a weather attractive forces than hydrogen bonding.

2.1 Hydrogen bonding between donors and acceptors           

Hydrogen bonding is not limited to the above mentioned electronegative atoms. The three chlorine atoms in chloroform can also form H- bonding with other molecules. These atoms deprive the carbon atoms of its electrons and the partial positively charged hydrogens can form strong hydrogen bonds with oxygen atom of other molecules such as those of acetone.

hydrogen bonding donar and acceptor

The hydrogen bonding present in the molecules of ammonia or hydrochloric acid can be described as follows:

hydrogen bonding in hydro chloride

3.0 Effect on Compound properties by hydrogen bonding

3.1  Hydrogen bonding effects Boiling point (B.P)

Example hydrides of group IVA are lowest

The B.P of hydrides of group IVA are the lowest as compared to groups VA-VIIA. The reason is that these elements are least electronegative. Methane (CH4) has lowest boiling point because of the lack of hydrogen bonding as the electronegativity difference between carbon and hydrogen is so small that partial charges are not sufficient to establish hydrogen bonding between the molecules of methane. In other words the polarizability of methane molecule is the least. Thus, molecules of methane behave independently of one another. Thus, hydrides of IVA have lower B.P than those of 5-7th groups.

Ammonia and hydrofluoric acid have high B.P than the hydrides of other elements of group VA-VIIA respectively

Water is hydride of oxygen which is the 1st member of the 6thA group. Similarly NH3 and HF are hydrides of nitrogen and fluorine which occupy 1st position in 5th A and 7thA groups of the periodic table. Consequently the electronegativties of N, O and F are maximum in their respective groups. This causes the appearances of strong hydrogen bonding amongst the hydrides of oxygen (6th group), nitrogen (5th Group) and fluorine (7th Group) than the remaining elements of their respective groups associated with lower electronegativities and polarizabilities.

3.2 Hydrogen bonding can change physical state

H2O, H2S and H2Se are hydrides of the 6th group.  Water is hydride of oxygen and H2S and H2Se are hydrides of sulpher and selenium. Oxygen occupies top position in the 6th group and is the most electronegative element in the group which creates powerful hydrogen bonding as compared to hydrides of sulpher and selenium. Strong hydrogen bonding in water is sufficient to make it a liquid because physical state depends on intermolecular attractions.

3.3 Effect Hydrogen bonding in Cleansing

The cleansing action of detergents and soaps is related to their molecular structure. They contain two ends, one being polar (water loving) and the other non polar (water hating). the detergent molecules orient themselves with their ends attached to groups of water molecules. Thus, the non polar oil, grease or dirt is dissolved by the cluster of non-polar ends of detergents. In this way oil droplets containing dirt get dispersed in water which are subsequently washed away.

hydrogen bonding role in cleasing

3.4 Effect of hydrogen bonding on strength of acids

The exceptionally low acidic strength of HF molecule as compared to HCl, HBr and HI is due to its strong hydrogen bonding because the partial positively charged hydrogen is entrapped between two highly electronegative F atoms. So hydrogen bonding is the electrostatic force of attraction between partial positive hydrogen atom and a highly electronegative atom like F, O and N belonging to another molecule. The stronger hydrogen bonding between H and F of two bonded HF molecules hardly allows hydrogen to be a proton as compared in HCl or HBr.

hydrogen bonding in acids

4.0 EXAMPLES of Hydrogen bonding  

4. HYDROGEN BONDING IN PROTEINS         

Hydrogen bonding exists in the living systems. Proteins are the important parts of living organisms. Hair, silk and muscles are the important modifications of proteins. Proteins consist of long chains of amino acids. Thus, long chains are coiled about one another into a spiral. This spiral is called helix and hydrogen bonding plays a major role in proteins coiling. Each turn of the helix contain 27 amino acids.

4.2 HYDROGEN BONDING IN DNA 

Deoxyribonucleic acid (DNA) has two spiral chains. These are coiled about each other on a common axis. In this way a double helix is formed. This is 18-20 Ao in diameter. They are linked by hydrogen bonding.

4.3 HYDROGEN BONDING IN PAINTS AND DYES

The paints and dyes adhere to the surfaces because of hydrogen bonding. Similar type of hydrogen bonding exists in glue and other sticky substances.

4.4 HYDROGEN BONDING IN CLOTHING FIBERS  

Hydrogen bonding is responsible for the rigidity and the tensile strength of clothing fibers like cotton and silk.

4.5 HYDROGEN BONDING IN FOOD MATERIALS    

The food materials like carbohydrates include glucose, fructose and sucrose. These compounds contain hydroxyl (OH) groups which are responsible for hydrogen bonding in these substances.

4.6 HYDROGEN BONDING IN HONEY   

Hydrogen bonding in responsible for the stickiness of honey. Both water and sugar molecules in honey are hydrogen bonded substances.

5.0 Hydrogen bonding in Water (Liquid and Ice )

The molecules of water have tetrahedral structure as two lone pairs of electrons on oxygen atom occupy two corners of the tetrahedron. In the liquid state water molecules are extensively associated with each other and these association break and deform as in liquid state water molecules are mobile. When the temperature of water is decreased, ice is formed. Hydrogen bonding is responsible for arrangement of water molecules in ice where each water molecule is surrounded tetrahederally by four others and held with each other through hydrogen bonding. This arrangement is repeated in thee dimension and thus hexagonal empty spaces appear in the crystal lattice of ice. That’s why ice occupies 9% more space as compared to liquid water and consequently density of ice decreases. This is the reason why ice floats on water surface. The layer of ice insulates the water underneath for further heat loss. Fish and plants survive under this blanket of ice for months. Hence, it can be said that the life pattern of plants and animals would have been totally different in the absence of hydrogen bonding.

hydrogen bonding in water

Boiling point of water is higher than that of HF

Fluorine is more electronegative than oxygen so hydrogen bonding in HF is stronger than in water and as a result boiling point of HF should be higher than that of water. However, BP of HF is lower than water because on the average fluorine of HF can make only one hydrogen bond with neighboring molecule while water can from two hydrogen bonds per molecule as it has two hydrogen atoms and two lone pairs on oxygen atom.

  • NH3 on the average can from only one hydrogen bond per molecule as it has only one lone pair. The number of lone pairs is less than the number of hydrogen atoms in NH3 so the number of hydrogen bonds will be limited by lone pairs and extra hydrogens will be left unbonded.
  • The boiling point of HCl is slightly higher than that of HBr because chlorine is more electronegative than bromine and results in stronger hydrogen bonding which makes its B.P higher.

Both water and ethyl alcohol (C2H5OH) can from hydrogen bonds so ethyl alcohol can dissolve in water and vice versa. Similarly carboxylic acids are also soluble in water provided the chain length of carboxylic acids is size (containing less no. of carbon atoms).

Hydrocarbons (methane CH4, Ethane C2H6) are not soluble in water because they are non-polar compounds and hence unable to from hydrogen bonding with water.

 Related Links

More about liquids and solids 

Hydrogen Bonds 

About Syed Noman ud din

Syed Noman ud din is an Electrical Engineer and working in Industry from last 3 years. He writes technical articles for electrical and electronic engineers. He has also published several research publications in renowned international journals.

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