Dipole-Dipole Forces

Overview

Dipole-dipole forces are intermolecular forces that exist between polar molecules where one end has a partial positive charge and the other end has a partial negative charge. These forces result from the attraction between the positive end of one molecule and the negative end of another molecule due to the difference in electronegativity. While these forces are weaker than covalent bonds, they play a significant role in determining the physical properties of substances in different states.

Questions and Answers

1. What are Dipole-Dipole Forces?

   Dipole-dipole forces are intermolecular forces of attraction between polar molecules. These forces arise due to the partial positive and negative charges on different atoms within the molecules.

2. How do Dipole-Dipole Forces compare to London Dispersion Forces?

   Dipole-dipole forces are stronger than London dispersion forces. Dipole-dipole forces occur between polar molecules, while London dispersion forces are present in both polar and non-polar molecules.

3. Give an example of molecules that exhibit Dipole-Dipole Forces.

   An example of molecules that exhibit dipole-dipole forces are HCl (hydrochloric acid) and CHCl₃ (chloroform). In HCl, the chlorine atom is more electronegative, creating a partial negative charge, while hydrogen has a partial positive charge.

4. How do Dipole-Dipole Forces affect the physical properties of substances?

   Greater strength of dipole-dipole forces leads to higher values of thermodynamic parameters like melting points, boiling points, heats of vaporization, and heats of sublimation. These forces contribute to the cohesion between molecules in liquids.

5. Are Dipole-Dipole Forces present in non-polar molecules?

   Dipole-dipole forces are not present in non-polar molecules. These forces specifically occur between polar molecules where there is an electronegativity difference leading to partial charges.

Multiple-Choice Questions

1. What type of intermolecular forces dominate in ammonia ( NH₃ )?

   a) London dispersion forces
   b) Hydrogen bonding
   c) Dipole-induced dipole forces
   d) Dipole-dipole forces
   Answer: b) Hydrogen bonding

2. Which statement about dipole-dipole forces is true?

   a) They are stronger in gases than in liquids
   b) They depend on the distance between molecules
   c) They are not affected by electronegativity differences
   d) They are weaker than London dispersion forces
   Answer: b) They depend on the distance between molecules

3. In which molecule are dipole-dipole attractions observed?

   a) CH₄
   b) H₂
   c) HCl
   d) CO₂
   Answer: c) HCl

4. What increases with the strength of dipole-dipole forces?

   a) Viscosity of liquids
   b) Boiling points of substances
   c) Melting points of substances
   d) Heat of sublimation
   Answer: b) Boiling points of substances

5. Which forces are responsible for the dissolution of an ionic substance in water?

   a) London dispersion forces
   b) Hydrogen bonding
   c) Ion-dipole interactions
   d) Dipole-induced dipole forces
   Answer: c) Ion-dipole interactions