Hydrogen Bond
The bond which is formed between an atom of hydrogen and an atom of a negative electronegative element due to the mutually stable electrostatic attraction force of their partial positive and negative charges, is called hydrogen bond.
A covalent bond between a hydrogen and an atom of a strongly electronegative element is a polar covalent bond. For this reason, there is a partial positive charge on the hydrogen atom and a partial negative charge on the atom of the negative electronegative element.
Example: In Hydrogen Fluoride (HF), the bond formed between H and F is a polar covalent bond and due to this there is a partial positive charge on H and a partial negative charge on F.
H+ – F–
That atom of hydrogen which is connected to an atom of a negative electronegative element by covalent bond, can also be attached to any other atom of that negatively electronegative element by means of a stable electrostatic attraction force. This type of bond is called hydrogen bond and it is represented by dotted lines ( – – -).
In this case, the atomic negative electronegativity of Hydrogen forms a bridge between the two atoms of the element. The strength of hydrogen bond depends on the negative electronegativity of the negative electronegativity element.
When the negative power is high, the strength is high. The following is the sequence of occurrence of negative energies of the major elements.
F > O > Cl ~ N > Br > C ~ S ~ I > H
If the atoms of hydrogen and negative electronegativity between which hydrogen bond is formed are part of different molecules of a compound, then this type of bond is called intermolecular hydrogen bonding.
Example: Intermolecular hydrogen bonding occurs in HF and H2O.
Compounds which have intermolecular hydrogen bonding, their molecules are attracted to each other due to hydrogen bonding. Therefore, more energy is required to separate the molecules of these compounds. Therefore, the melting and boiling points of such compounds are often high.
Example: HF and H2O are liquids at room temperature and NH3, HCl and H2S are gases.
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Magnetic Dipole : Magnetic Dipole Moment and Properties
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Electric Potential Energy : Electric Dipole, Potential Gradient
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Coulomb's Law : Electric Field, Potential Difference, Volt
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Gauss Law or Gauss Theorem : Solid Angle and Electric Flux
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Electrolysis : Definition, Principle and Electrolytic Cell
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Adsorption : Adsorption rate, Physisorption and Chemisorption
In other words, the boiling points of HF and H2O are higher than those of NH3, HCl and H2S. The reason for this is that F and O are strongly negatively electronegative elements, so hydrogen bonding is present in HF and H2O.
In HF and H2O there are hydrogen bonds between atoms of different molecules. Therefore, intermolecular hydrogen bonding takes place in HF and H2O. N , Cl and S are not so strongly negatively electronegative elements and hydrogen bonding in NH3, HCl and H2S is not very or very weak.
Therefore, the boiling points of HF and H2O are higher than those of HCl, H2S and NH3. Compounds which have intermolecular hydrogen bonding, when water is added to them, the molecules of the compound form hydrogen bonds with water molecules. Therefore, these types of compounds are usually soluble in water.
If the atoms of hydrogen and the negative electronegative element, between which hydrogen bond is formed, are part of a molecule of a compound, then this type of bonding is called intramolecular hydrogen bonding.
Example: Intramolecular hydrogen bonding occurs in ortho nitrophenol. Similarly, intramolecular hydrogen bonding occurs in selycylaldehyde and celycilic acid.
Compounds that have intramolecular hydrogen bonding usually have low melting and boiling points and are volatile.
Example: Intermolecular in aortho nitrophenol and intramolecular hydrogen bonding in per nitrophenol. The melting point of aortho nitrophenol is 45°C while the melting point of pera nitrophenol is 114°C. Aortho nitrophenol steam is volatile. Whereas pera nitrophenol is not water volatile.