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Hydrogen Bonding-Definition, Factors Affecting the Strength, Types, Effect on Various Physical Properties, Practice problems, FAQs

Hydrogen Bonding-Definition, Factors Affecting the Strength, Types, Effect on Various Physical Properties, Practice problems, FAQs

You may have noticed water has unusual properties like high freezing point, high boiling point, high heat of vaporisation and high heat of fusion, compared to other chalcogen hydrides. 

For example,

But why does this abnormality occurs?

This is due to the presence of significant hydrogen bonding between water molecules.

Why do we study the concept of
Many of water's crucial, life-sustaining characteristics are provided by hydrogen bonding, this also serve to fix the structures of proteins and DNA, the cellular building block. Hydrogen bonding can be found in both inorganic and organic compounds, such as water and DNA and proteins. We should look out for such an important concept!

 

Table of contents

  • What is -bonding?
  • Conditions for hydrogen bonding
  • Factors affecting the strength of H-bond
  • Types of Hydrogen Bonding
  • Effect of hydrogen bonding on physical properties of compounds.
  • Practice problems
  • Frequently asked questions-FAQs 

What is H- bonding?

Hydrogen bonding is a special class of attractive intermolecular forces that exists due to the dipole-dipole interaction between a covalently bonded hydrogen atom and another highly electronegative atom (like , ).


Conditions for hydrogen bonding

  • Presence of a highly electronegative atom which is covalently attached to the hydrogen atom.
     Example: has 4.0 electronegativity whereas H has 2.1 electronegativity value. The electronegativity difference is sufficient to form -bond

     
  • Smaller electronegative  donor atom with lone pair  are required for effective overlapping. 

Factors affecting the strength of -bond

  • Higher the electronegativity of the covalently bonded atom to , greater the 𝛅+ charge on

 atom and thus the strength of bond increases. Hence this is the major factor to explain -bond.

  • Higher the coulombic interaction of the lone pair of the E.N. atom, stronger the -bond.
  • If size increases for the bonding atoms, then H-bonding will not take place effectively.

     

Related video: https://youtu.be/NdE9Oms4kZM (51:33 -  56:55)

Types of Hydrogen Bonding

  • Intermolecular -bonding

Hydrogen bonding, existing between more than one molecule of the same or different

compounds is known as intermolecular hydrogen bonding. 

Eg: Hydrogen bonding in water, alcohol, ammonia.

  • Intramolecular -bonding              

The hydrogen bonding  present within the molecule is known as intramolecular hydrogen bonding.


For example, 
Intramolecular hydrogen bonding exists in ortho nitrophenol

The hydrogen bond between the oxygen atom of the group

 and the hydrogen atom of the group will result in the formation of a ring.

Effect of hydrogen bonding on various physical properties of compounds

  • Solubility
    The formation of hydrogen bonds increases the solubility of the compound in the solvent.

Examples:

Intermolecular hydrogen bonding and its effect on  solubility:

Ethanol (is soluble in water due to the presence of intermolecular hydrogen bonding with the water molecules. Intermolecular hydrogen bonding increases the solubility.



Intramolecular hydrogen bonding and its effect on solubility

In the case of intramolecular hydrogen bonding, chelation takes place that leads to the non-availability of hydrogen atoms to interact with other atoms of the solvent and hence, solubility decreases.

  • Viscosity
    Viscosity is defined as the degree up to which a fluid can resist the flow under an applied force. The substances that contain hydrogen bonding exists as associated molecules. So, their flow becomes comparatively difficult or slow and they have higher viscosity.
    Example: Syrup jar analogy to understand viscosity. 



Due to increased interparticle attraction, honey is an extremely viscous fluid.

  • Boiling point
    Increase in the strength of intermolecular forces will lead to increase in the boiling point. For substances with intermolecular hydrogen bonding, more energy will be required to convert the liquid into its vapour state. 

 

Intramolecular hydrogen bonding decreases the boiling point. This is because the hydrogen present in the molecule, which is covalently bonded to a highly electronegative atom, has already formed a hydrogen bond with another electronegative atom within the molecule. Therefore, they cannot form hydrogen bonds with the electronegative atoms of other molecules. Therefore, there is less force of attraction between the molecules. So, less energy is required to convert them into gas and hence, the boiling point decreases.

  

  • Physical state

The magnitude of Hydrogen bonding depends on the physical state of the compound

  • Acidic and basic strength
    Hydrogen bond in a compound generally decreases its acidic strength as the removal of involved in hydrogen bonding is really difficult.

Related video: https://youtu.be/NdE9Oms4kZM 

Practice problems

Q1  is highly soluble in acetone but insoluble in water. Why?

Solution: This is due to the presence of hydrogen bonding between and acetone. Carbon in ethyne is sp hybridised, which is highly electronegative. As a result, there is a development of partial negative and positive charges on carbon and hydrogen atoms, respectively. The hydrogen atom with a partial positive charge can form a hydrogen bond with the oxygen atom present in acetone. This is the reason why is highly soluble in acetone but insoluble in water.

Q2 Which of the following molecules has higher acidic strength?

Answer: A

Molecule A is an alcohol and the hydrogen of the group is involved in hydrogen bonding with the surrounding molecules and thus, cannot be removed easily. Hence, it is less acidic. The hydrogen atom in group of molecule B will not undergo hydrogen bonding as there is no considerable electronegativity difference. Hence, can be easily released that increases the acidity of molecule B. Thus, B will have higher acidity than A.

Q3 The boiling point of water is exceptionally high it is because

A. There is a covalent bond between and
B. Water molecule is linear in structure
C. Water molecules associate due to hydrogen bonding
D. There is an ionic bond between and
Answer: C

The boiling point of water is exceptionally high because water molecules are associated with each other due to hydrogen bonding

Q4 The molecule which contains hydrogen bond is

Answer: A

Due to high electronegativity difference between and , shows the

Frequently asked questions-FAQs


1. and both have 3.0 electronegativity. However, hydrogen bonding is more commonly seen in . why?

Answer: It is due to the smaller size of the atom than that of the atom. But, in some cases also forms bond. For example Chloral hydrate

2.What is a dipole?
Solution: A dipole is an electrically neutral molecule. It carries both a positive and negative charge, with  a little space separating the two charges.

3. The boiling point increases with the increase in molecular mass however the boiling point comparison of hydrides of Group 15 is observed as > > > Why?

Solution: The anomaly in the actual orders of boiling points is because of the presence of intermolecular hydrogen bonding in , which increases its boiling point. Thus, the correct order of boiling point is given as follows:> > >

4. Why is the density of ice lower than that of water?

Solution: The density of ice is lower than that of water because of the open cage-like structure in ice due to the presence of hydrogen bonding.
Related topics

Covalent bond VSEPR Theory
dipole moment Water

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