Hydrogen Bonding

Hydrogen Bonding is simply the formation of Hydrogen bonds. Hydrogen bonding is a type of chemical bonding that possess an electrostatic force of attraction between a hydrogen atom and an atom containing a lone pair of electrons in a chemical substance. The hydrogen must be attached to an electronegative atom for a hydrogen bond to form. It should be noted that the hydrogen bond is not a covalent bond however, the atoms present in it are covalent.

Hydrogen Bonding is possible in cases when the molecule contains a highly electronegative atom linked to the hydrogen atom. The hydrogen bonding in water is a vigorous bond between the nearest water molecule containing one Hydrogen atom between two oxygen atoms. Hydrogen bonding is major of two types of intramolecular and intermolecular hydrogen bonding, on the basis of the atoms involved in it. Hydrogen bonding is maximum in solid-state compounds.

What is Hydrogen Bonding?

Hydrogen bonding is the formation of hydrogen bonds. Hydrogen bonds are the type of attractive intermolecular forces caused by the dipole-dipole interaction between a hydrogen atom bonded to a strongly electronegative atom of the same or another nearby electronegative atom. 

Hydrogen is covalently bound to the more electronegative oxygen atom in water molecules (H₂O). As a result of dipole-dipole interactions between the hydrogen atom of one water molecule and the oxygen atom of another H₂O molecule, hydrogen bonding occurs in water molecules.

The bond pair of electrons in the O-H bond is quite close to the oxygen nucleus in this case (due to the large difference in the electronegativities of oxygen and hydrogen). As a result, the hydrogen atom generates a partial positive charge (+) whereas the oxygen atom develops a partial negative charge (-). 

The electrostatic interaction between the hydrogen atom of one water molecule (with + charge) and the oxygen atom of another water molecule (with – charge) can now cause hydrogen bonding. As a result, hydrogen bonds are a unique type of intermolecular attractive force that only occurs when hydrogen atoms are bound to a highly electronegative atom. In comparison to typical dipole-dipole and dispersion forces, hydrogen bonds are primarily strong. They are, however, weaker than actual covalent or ionic connections.

Important Conditions for Hydrogen Bonding

• The atom that is bonded to the hydrogen atom has a high electronegativity, making the bond suitably polar. A strongly electronegative atom connected to the hydrogen atom must be present in the molecule. The degree of polarisation of the molecule increases with increasing electronegativity.
• The electronegative atom attached to the Hydrogen atom should be smaller in size. Greater the electrostatic attraction, the smaller the size. So, the smaller size should attract the bonding electron pair effectively.

Effects of Hydrogen Bonding on Elements

1. Dissociation: HF dissociates in water and sends off the difluoride ion instead of the fluoride ion. In HF, this is due to hydrogen bonding. HCl, HBr, and HI molecules do not form hydrogen bonds. This explains why chemicals like KHCl₂, KHBr₂, and KHI₂ don’t exist.
2. Association: Because of hydrogen bonding, carboxylic acid molecules exist as dimers. Such compounds have molecular weights that are twice as large as those calculated from their simple formula.

Examples of Hydrogen Bonding

Water

An excellent example of Hydrogen Bonding is Water. A highly electronegative oxygen atom is connected to a hydrogen atom in the water molecule. The shared pair of electrons are attracted to the oxygen atoms more, and this end of the molecule becomes negative, while the hydrogen atoms become positive.

Hydrogen Fluoride 

A stronger-than-average hydrogen bond is created by hydrofluoric acid and is known as a symmetric hydrogen bond. Formic acid can also make this type of bond.

Ammonia 

Between the hydrogen in one molecule and the nitrogen in another, hydrogen bonds are formed. Since each nitrogen has a single electron pair, the bond that develops in the case of ammonia is relatively weak. Methylamine also has this form of hydrogen bonding with nitrogen.

Alcohol and Carboxylic Acid 

A type of chemical molecule with a -OH group is alcohol. In most cases, hydrogen bonding is easily generated if any molecule containing the hydrogen atom is immediately coupled to either oxygen or nitrogen.

Strength of Hydrogen Bond: The hydrogen bond is a relatively weak one. Hydrogen bonds have a strength that is halfway between weak van der Waals forces and strong covalent bonds. The attraction of the shared pair of electrons, and hence the atom’s electronegativity, determines the hydrogen bond’s dissociation energy.

Properties of Hydrogen Bonding

1. Volatility – The boiling point of compounds incorporating hydrogen bonding between distinct molecules is greater, hence they are less volatile.
2. Solubility – Because of the hydrogen bonding that can occur between water and the alcohol molecule, lower alcohols are soluble in water.
3. The lower density of ice than water – In the case of solid ice, hydrogen bonding causes water molecules to form a cage-like structure. In fact, each water molecule is tetrahedrally connected to four other water molecules. In the solid state, the molecules are not as tightly packed as they are in the liquid state. This case-like structure collapses as ice melts, bringing the molecules closer together. As a result, the volume of water reduces while the density increases for the same quantity of water. As a result, at 273 K, ice has a lower density than water. Ice floats because of this.
4. Viscosity and surface tension – Hydrogen bonding is found in compounds that have an associated molecule. As a result, their flow becomes more complicated. They have high surface tension and higher viscosity.

Why do hydrogen-bonded compounds have high melting and boiling points?

The melting and boiling temperatures of hydrogen-bonded compounds are unusually high. The elevated melting and boiling points of hydrogen-bonded compounds are attributable to the additional energy required to break these bonds.

• At room temperature, H₂O is a liquid, whereas H₂S, H₂Se, and H₂Te are all gases. Hydrogen bonding produces links in the water molecules, resulting in water having a higher boiling point than the other chemicals.
• Because there is hydrogen bonding in NH₃, but not in PH₃, ammonia has a higher boiling point than PH₃.
• The presence of hydrogen bonding accounts for hydrogen fluoride’s particularly high boiling point among the halogen acids.
• Because ethanol contains hydrogen bonds, it has a higher boiling point than diethyl ether.

Types of Hydrogen Bonding

There are two types of H bonds, which are labeled as follows:

Intermolecular Hydrogen Bonding  

Intermolecular hydrogen bonding occurs when hydrogen bonds are formed between molecules of the same or distinct substances. Hydrogen bonding in water, alcohol, and ammonia, for example.

Intramolecular Hydrogen Bonding

Intramolecular hydrogen bonding refers to hydrogen bonding that occurs within a single molecule. It occurs in compounds with two groups, one of which has a hydrogen atom linked to an electronegative atom and the other of which has a highly electronegative atom linked to a less electronegative atom of the other group. The link is created between the more electronegative atoms of one group and the hydrogen atoms of the other group.

FAQs on Hydrogen Bonding

Question 1: State Octet Rule.

Answer:

Atoms are most stable when their valence shells are filled with eight electrons, according to the octet rule. It is based on the observation that the atoms of the major group elements have a proclivity for chemical bonding in such a way that each atom in the resulting molecule has eight electrons in the valence shell. Only the core group elements are subject to the octet rule.

Question 2: What are the factors that affect the formation of Ionic Bond?

Answer: 

Factors affecting the formation of Ionic Bond are:- 

• Ionization Enthalpy
• Electron Gain Enthalpy
• Lattice Energy

Question 3: List any three characteristics of Ionic Compounds.

Answer:

Characteristics of Ionic Compounds are as follows:-

1. The melting and boiling points of ionic compounds are usually quite high. This is due to the strong electrostatic forces that hold ions together in ionic compounds.
2. Ionic compounds are frequently found in solid form.
3. Ionic compound solutions are excellent electrical conductors. In their molten condition, they are also good conductors of electricity.

Question 4: What is Hydrogen Bonding?

Answer:

When a hydrogen atom is coupled to a highly electronegative atom, the shared pair of electrons are attracted more by this atom, and the molecules’ negative end becomes slightly negative while the positive end becomes slightly positive.The negative end of one molecule attracts the positive end of the other, resulting in the formation of a weak bond. This connection is referred to as Hydrogen Bonding.

Question 5: What are the conditions required for Hydrogen Bonding?

Answer:

Conditions required for Hydrogen Bonding are:

• A highly electronegative atom must be coupled to the hydrogen atom in the molecule. The polarisation of a molecule is proportional to its electronegativity.
• The electronegative atom should be modest in size. The greater the electrostatic attraction, the smaller the size.