CBSE Class 11-science Answered

Polar molecules, such as water molecules, have a weak, partial negative charge at one region of the molecule (the oxygen atom in water) and a partial positive charge elsewhere (the hydrogen atoms in water).

Thus when water molecules are close together, their positive and negative regions are attracted to the oppositely-charged regions of nearby molecules. The force of attraction, shown here as a dotted line, is called a hydrogen bond. Each water molecule is hydrogen bonded to four others.

The hydrogen bonds that form between water molecules account for some of the essential — and unique — properties of water.

• The attraction created by hydrogen bonds keeps water liquid over a wider range of temperature than is found for any other molecule its size.
• The energy required to break multiple hydrogen bonds causes water to have a high heat of vaporization; that is, a large amount of energy is needed to convert liquid water, where the molecules are attracted through their hydrogen bonds, to water vapor, where they are not.

Two outcomes of this:

• The evaporation of sweat, used by many mammals to cool themselves, achieves this by the large amount of heat needed to break the hydrogen bonds between water molecules.
• Moderating temperature shifts in the ecosystem (which is why the climate is more moderate near large bodies of water like the ocean)

The hydrogen bond has only 5% or so of the strength of a covalent bond. However, when many hydrogen bonds can form between two molecules (or parts of the same molecule), the resulting union can be sufficiently strong as to be quite stable.

Multiple hydrogen bonds

• hold the two strands of the DNA double helix together
• hold polypeptides together in such secondary structures as the alpha helix and the beta conformation;
• help enzymes bind to their substrate;
• help antibodies bind to their antigen
• help transcription factors bind to each other;
• help transcription factors bind to DNA
• The electronegative atom must have one or more unshared electron pairs as in the case of oxygen and nitrogen, and has a negative partial charge. The hydrogen, which has a partial positive charge tries to find another atom of oxygen or nitrogen with excess electrons to share and is attracted to the partial negative charge. This forms the basis for the hydrogen bond.

  In other words - The hydrogen on one molecule attached to O or N that is attracted to an O or N of a different molecule.

  Hydrogen bonding is usually stronger than normal dipole forces between molecules. Of course hydrogen bonding is not nearly as strong as normal covalent bonds within a molecule - it is only about 1/10 as strong. This is still strong enough to have many important ramifications on the properties of water.

  A hydrogen atom attached to a relatively electronegative atom is a hydrogen bond donor. This electronegative atom is usually fluorine, oxygen, or nitrogen. An electronegative atom such as fluorine, oxygen, or nitrogen is a hydrogen bond acceptor, regardless of whether it is bonded to a hydrogen atom or not. An example of a hydrogen bond donor is ethanol, which has a hydrogen bonded to oxygen; an example of a hydrogen bond acceptor which does not have a hydrogen atom bonded to it is the oxygen atom on diethyl ether.

  Carbon can also participate in hydrogen bonding, especially when the carbon atom is bound to several electronegative atoms, as is the case in chloroform, CHCl₃. The electronegative atom attracts the electron cloud from around the hydrogen nucleus and, by decentralizing the cloud, leaves the atom with a positive partial charge. Because of the small size of hydrogen relative to other atoms and molecules, the resulting charge, though only partial, nevertheless represents a large charge density. A hydrogen bond results when this strong positive charge density attracts a lone pair of electrons on another heteroatom, which becomes the hydrogen-bond acceptor.

  The hydrogen bond is often described as an electrostatic dipole-dipole interaction. However, it also has some features of covalent bonding: it is directional, strong, produces interatomic distances shorter than sum of van der Waals radii, and usually involves a limited number of interaction partners, which can be interpreted as a kind of valence. These covalent features are more significant when acceptors bind hydrogens from more electronegative donors.

  The partially covalent nature of a hydrogen bond raises the questions: "To which molecule or atom does the hydrogen nucleus belong?" and "Which should be labeled 'donor' and which 'acceptor'?" Usually, this is easy to determine simply based on interatomic distances in the X-H...Y system: X-H distance is typically ~1.1 Å, whereas H...Y distance is ~ 1.6 to 2.0 Å. Liquids that display hydrogen bonding are called associated liquids.

  Hydrogen bonds can vary in strength from very weak (1-2 kJ mol⁻¹) to extremely strong (>155 kJ mol⁻¹), as in the ion HF₂⁻.Typical values include:

  • F—H^...:F (155 kJ/mol or 40 kcal/mol)
  • O—H^...:N (29 kJ/mol or 6.9 kcal/mol)
  • O—H^...:O (21 kJ/mol or 5.0 kcal/mol)
  • N—H^...:N (13 kJ/mol or 3.1 kcal/mol)
  • N—H^...:O (8 kJ/mol or 1.9 kcal/mol)
  • HO—H^...:OH₃⁺ (18 kJ/mol^[4] or 4.3 kcal/mol)