Intermolecular forces - Home of Knowledge

Intermolecular forces

Intermolecular forces: Relatively weak type of attractive forces which are present in between the molecules are called intermolecular forces.

Comparison of chemical bond forces and intermolecular forces present in HCl;- The intermolecular forces are very weak as compare to chemical bond. In HCl the bond in between the hydrogen is very stronger as compared to intermolecular forces between HCl molecules. It is easier to break the intermolecular forces in between HCl molecules as compared to chemical bond between hydrogen and chlorine.

17 KJ energy is required to break the intermolecular forces where as 430 KJ energy is required to break the chemical bond. In the following diagram, the complete lines in between hydrogen and chlorine shows chemical bond while the dotted lines shows intermolecular forces.


DIPOLE-DIPOLE interactions or vander walls forces: The net attractive forces between oppositely charged ends of two adjacent molecules are called dipole dipole interactions.

Explanation: These forces are elictrical in nature and may be either temporary or permanant which are formed by unequal sharing of electrons between two different electronegative type of bonded atoms. the more electronegative attract the shared pair of electrons more toward itself and become slightly negatively charged and the other slightly positively charged. So there exist a net force of attraction between these two opposite charges which is called dipole dipole interactions.


Hydrogen Bonding: The forces of attraction between hydrogen atom of one molecule and high electronegative atom in another molecule is called hydrogen bonding. OR The force of attraction between hydrogen atom of one molecule and nitrogen, oxygen and flourine of other molecule is called hydrogen bonding.

Explanation: Hydrogen bonding is a special type of intermolecular forces in permanant polar molecules. Hydrogen bond is only formed between hydrogen and a small high electronegative atom with lone pair of electrons, such as nitrogen, oxygen and flourine. The covalent bond in between hydrogen atom and high electronegative small atoms like nitrogen , oxygen and flourine becomes polar and it results in the formation of partial positive charge on hydrogen and partial negative charge on other atom. In this way opposite charges attract each other and results in the formation of hydrogen bond. Hydrogen bond is represented by dotted lines between the molecules. Example: H2O, HF, NH3, C2H5OH ,etc.


Hydrogen bonding affects the physical properties of the compounds: Due to this boiling points of the compounds are affected greatly. For example, boiling point of water (100 °C) is higher than that of alcohol (78 °C) because of more and stronger hydrogen bonding in water.The important phenomenon of floating of ice over water is because of hydrogen bonding. The density of ice at 0 °C (0.917 gcm-3) is less than that of liquid water at 0°C (1.00 gcm-3). In the liquid state water molecules move randomly. However, when water freezes, the molecules arrange themselves in an ordered form, that gives them open structure. This process expands the molecules, that results in ice being less dense as compared to water.

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