Hydrogen Bonding

Understanding Hydrogen Bonding. There are many types of chemical bonds that exist in nature. Remember all chemical bonds whether an ionic, covalent or any bond have the same thing in common that they are all electric in nature. So, all chemical bonds exist because of the existence of charge on different atoms and different molecules. We have studied that ionic bond is formed as a result of transference of electrons while both covalent and coordination bond involves the sharing of electrons. The hydrogen bond is different from them in the sense that it simply represents the dipole interaction between atoms of different electronegativity values.

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What is hydrogen bonding?

Hydrogen bonding occurs when a hydrogen atom is bonded to a highly electronegative atom such as nitrogen, oxygen, or fluorine then this end becomes negatively charged (-δ), while H – end acquires a positive charge (+δ). The negative end (-δ) of one molecule attracts the positive end (+δ) of other molecules. The bond thus formed is known as H-bond. It is represented by dotted line.

The energy of H-bonding is -42 kJ/mol. The actual strength of H- bond depends on the electronegativity difference between the bonded atoms.

What causes hydrogen bonding?

  1. The molecule should contain an atom of high electronegativity such as F, O or N bonded to the hydrogen atom.
  2. The size of the electronegative atom should be small for strong hydrogen bonding.

Types of hydrogen bonding

So we have two categories of bonds.

  • Intra-molecular bonds
  • Inter-molecular bonds

Intramolecular bonds are on average on a one-to-one individual basis stronger than intermolecular bonds

Intra-molecular bonds are the bonds that hold the atoms together within a given molecule and intermolecular bonds are those bonds that hold atoms together on different molecules.

Intramolecular hydrogen bonding

It is formed between two same or different molecule. Intermolecular H-bonding increases the melting point, boiling point., solubility, viscosity, surface tension etc. while Intramolecular H- bonding has opposite effects. This is because intramolecular H-bonding prevents the association of other molecules.

let’s discuss the types of intramolecular bonds.

Nonpolar covalent

A nonpolar covalent bond is a bond in which two atoms equally share their electrons. let’s suppose we examine the bond between two chlorine atoms. The two chlorine are the same exact atoms. They have the same exact electronegativity value and that means they will pull the electrons equally. So, they will have an equal distribution of electrons. It means there will be a net charge of zero on each chlorine. Hence, there will be no electric dipole moment that exists between this two chlorine atoms.Hydrogen Bonding

Covalent bonds can be double bonds, single bonds, or triple bonds. A double bond simply means more electron density between the two atoms. The two atoms will be closer together and that force will be larger and that’s exactly why double bonds are stronger than single bonds and triple bonds are stronger than double bonds or single bonds.

Must Read: Polar and Non-Polar Examples.

Polar covalent

A bond in which two atoms do not share their electrons equally. let’s take an example of a bond exists between hydrogen and fluorine. Here now fluorine is not the same atom as hydrogen. Fluorine is more electronegative than hydrogen and that means it will have a stronger force for those electrons. It will pull the electrons closer to that fluorine and so the fluorine will develop a partial negative charge that hydrogen will develop a partial positive charge. There exists the separation of charge and that will create an electric dipole moment that will point from the hydrogen to fluorine. An electric dipole moment that will create a polar covalent bond.Hydrogen Bonding

So a polar covalent bond is a bond that exists between two different atoms that have two different electronegativity values. Another example is NH3 that the nitrogen is more electronegative than hydrogen and so it will pull the electron density close are developing a partial negative charge it will take away the electrons from hydrogen atoms and will develop a partial positive charge. This is another example of an electric dipole moment in a polar covalent bond.

Intermolecular hydrogen bonding

It is formed within the same molecule.The intermolecular bonds aren’t as strong as the intramolecular bonds are on the one-to-one basis. Usually, we have so many of these intermolecular bonds involved in any given reaction. These intermolecular bonds play a very important role in actually determining what the pathway of a reaction is and what will be the structure of a final molecule. Hydrogen Bonding


Hydrogen Bonding in Water

The atoms that combine to form a water molecule are in a constant tug-of-war over their shared electrons. Oxygen exerts a stronger pull on the shared electrons than hydrogen atoms. So, the electrons get closer to the oxygen atom. Because of this unequal sharing of electrons, the oxygen atom in a water molecule actually has a slight negative charge and each hydrogen atom has a slight positive charge. Even though the water molecule as a whole is neutral because of the unequal sharing of electrons and the resulting positive and negative poles a water molecule is said to be polar. 

The polarity of water molecules causes them to be attractive to each other. Since the positively charged atom involved in this special type of attraction is always a hydrogen atom this kind of bond between molecules is called a hydrogen bond. Each water molecule can hydrogen bond to four other water molecules. A hydrogen bond is weak bond and lasts only a tiny fraction of a second but it takes a lot of energy to overcome the combined attraction of many hydrogen bonds. This explains water’s great capacity to store heat, it’s high boiling point, surface tension, and several other unusual properties

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