THE HYDROGEN BOND (hydrogen bonding)

THE HYDROGEN BOND (hydrogen bonding)

 

Let’s start with the topic THE HYDROGEN BOND (hydrogen bonding)…..when a covalent bond is formed between hydrogen and a more electronegative atom, the more electronegative atom tries to pull the shared pair of electrons towards itself. Due to this, it will acquire a partial negative charge on it and there will be the development of a partial positive charge on the hydrogen.

Covalent character in ionic bonds

When this positive charge hydrogen comes in contact with a negatively charged atom of some other molecule, the two molecules can be linked together through a weak force of attraction. This weak force of attraction is known as the hydrogen bond or hydrogen bonding. It is represented by a dotted line (——-).

For example- In the molecule of hydrogen fluoride (HF), F is more electronegative than hydrogen and because of this, it will pull the shared pair of electrons. Now, due to polarity hydrogen acquires a partially positively charged and fluorine acquires a partially negatively charge on it.

Now, when this polar molecule comes in contact with another polar molecule of HF, the positive hydrogen of one molecule is attracted by negatively charged fluorine atom of another molecule, thus the formation of hydrogen bond takes place between two hydrogen fluoride.

Likewise, several molecules of hydrogen fluoride come in contact, and several hydrogen and fluorine are bound together and the formation of hydrogen bonds takes place. So we may define the hydrogen bond as follow.

“The weak attractive force which binds the partially positively charged hydrogen atom of one molecule, with the partially negatively charged atom of some other molecule of similar or different type (intermolecular) or with some other negative centre of the same molecule (intramolecular); is referred to as hydrogen bond or hydrogen bonding.”

  • The hydrogen bond is much weaker as compared to a covalent bond.
  • The bond energy of hydrogen bond lies between 3.5 to 40 kJ/mol.
  • The bond energy of the covalent bond usually of the order of 400 kJ/mol.

Essential Condition for Hydrogen Bonding: THE HYDROGEN BOND(hydrogen bonding):-

As it is clear that there is a dipole-dipole interaction in the formation of the hydrogen bond, Therefore following conditions favour the formation of hydrogen bonding.

i- High electronegativity of the atom bonded to hydrogen:-

hydrogen atom should be bonded to a highly electronegative atom such as F, O or N. Higher the electronegativity of the atom bonded to hydrogen, greater is the strength of hydrogen bond. Since the electronegativities of F, O and N follow the order

F > O > N, the strength of hydrogen bond decreases in the order.

H—-F > H—-O > H—-N

ii- Small size of electronegative atom: ( NH3 shows hydrogen bonding while HCl does not. give a reason.)

For an effective hydrogen bonding, it is necessary that the size of the electronegative atom is small. If the size of the electronegative atom is large, its attraction on the bonded pair will be less. consequently, less polarity will be developed in the molecule and the resulting hydrogen bond will be weaker. For example N and Cl having the same value of electronegativity (3.0), yet NH3 shows hydrogen bonding while HCl does not due to a larger size of Cl atom.

 

 

Type of hydrogen bonding and example:

Hydrogen bonding is of following types.

1- Intermolecular hydrogen bonding-

When hydrogen bonding exists between two molecules of the same or different substances it is known as intermolecular hydrogen bonding. Some examples of the compounds containing this type of hydrogen bonding are as follows-

i- Hydrogen fluoride-

In the solid-state HF consists of a long zig-zag chain of HF molecules linked together through hydrogen bonds as shown in the given figure. These chains persist to a large extent in the liquid and gaseous phase also. But in the liquid and gaseous state the, the chain is short.

 

bonding in hydrogen fluoride

 

ii- Water-

In H2O molecule, the central oxygen atom is covalently linked to two H atom. Each H can form a hydrogen bond. In addition to these two hydrogen bonds, the central oxygen atom also forms two hydrogen bonds with the neighbouring molecules. Therefore, 4 hydrogen bond are formed by a single molecule of water as shown in the figure.

 

intermolecular hydrogen bonding

 

iii- Ammonia-

In NH3 molecule, the more electronegative nitrogen atom is covalently linked to three hydrogen atoms. In the liquid state, several molecules of ammonia gas get associated through hydrogen bonds as shown in the figure.

 

 

intermolecular hydrogen bonding

 

iv- Hydrogen bonding between alcohol and water-(intermolecular hydrogen bonding between different molecule)

As mentioned above intramolecular hydrogen bonding is also possible between the molecule of different substances. For example, when ethyl alcohol is dissolved in water the molecules of the two substances get associated through intermolecular hydrogen bonding as shown in the figure below.

 

intermolecular hydrogen bonding

 

2- Intramolecular hydrogen bonding-

When hydrogen bonding exists within the same molecule, it is called as intramolecular hydrogen bonding. In case of this type of hydrogen bonding, a hydrogen bond is formed between a hydrogen atom covalently attached to some electronegative atom and some other electronegative atom present close to it in the same molecule. It results in the cyclization of the molecule.

For example– o-nitrophenol and salicylaldehyde possess intramolecular hydrogen bonding as shown in the figure below.

 

intramolecular hydrogen bonding

 

Effect of hydrogen bonding on the properties of the substances-

Several physical properties of substances are affected by hydrogen bond. Some of the physical properties are as follows:

i- Molecular state:-

Due to intermolecular hydrogen bonding, two or more molecules of a substance may get associated resulting in the formation of clusters of molecules as we have seen in case of HF and water.

Two molecules of acetic Acid (CH₃COOH) can associate together through intramolecular hydrogen bonding and form a dimer as shown below.

 

 

ii- Melting and boiling point-

The compounds containing intermolecular hydrogen bonding forces abnormally higher melting and boiling points. This is because a large amount of energy is required to break intermolecular hydrogen bonds and to set the molecules free.

The effect of hydrogen bonding on melting and boiling points of substances can be understood by comparing the melting and boiling point of the hydrides of the elements of group 14 15 16 and 17.

Q- The melting and boiling points of water (H2O) are much higher than those of  H₂S, H2Se and H₂Te. explain.

Ans- This is because, in water, the H2O molecules are clustered due to intermolecular hydrogen bonding, while hydrogen bonding is practically absent in other hydrides due to low electronegativities of elements S, Se and Te.

 

 

iii- Physical state-

Intermolecular hydrogen bonding also changes the physical state of the substance.

For example- Oxygen and sulphur are from the same family. They both form hydrides i.e., – H2O and H₂S having almost similar structures, yet H2O is a liquid whereas H₂S is a gas. (reasoning question).

Explanation:- 

Due to hydrogen bonding, the molecules of water get associated with one another and therefore water exists in the liquid state. On the other hand, the hydrogen bonding is almost negligible in H₂S due to low electronegativity of sulphur. Hence in H₂S, molecules are not associated and remain free. This is why H₂S exists in the gaseous state.

iv- Solubility in water- 

Such compounds which form hydrogen bonding with water are soluble in water.

For example- Alcohols are covalent compound but dissolve in water because alcohols form hydrogen bonding when mixing with water as shown below in the picture.

 

solubility of alcohol in water

 

Many other covalent compounds like carboxylic acids, sugar, glucose, urea, honey, etc., are also soluble in water due to their ability to form hydrogen bonding with water.

Thanks for reading THE HYDROGEN BOND (hydrogen bonding).

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