Question

What are the strongest attractive forces that must be overcome to (a) melt ice? (b) sublime bromine? (c) boil chloroform \(\left(\mathrm{CHCl}_{3}\right)\) ? (d) vaporize benzene \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)\) ?

Short answer

Question: Identify the strongest attractive force that must be overcome in order to change the physical state of the following substances: (a) Ice (b) Bromine (Br2) (c) Chloroform (CHCl3) (d) Benzene (C6H6). Answer: (a) Hydrogen bonding, (b) London dispersion forces, (c) Dipole-dipole forces, (d) London dispersion forces.

Step-by-step solution

Review types of intermolecular forces

To solve this problem, the student must be familiar with the three main types of intermolecular forces: 1. London dispersion forces (weakest) 2. Dipole-dipole forces (moderate) 3. Hydrogen bonding (strongest) The strength of these forces generally increases from London dispersion forces to hydrogen bonding, although there may be exceptions depending on the specific substance.

Analyze the forces present in each substance

(a) Ice (solid water, H2O): Ice has a structure that allows for hydrogen bonding to occur between the water molecules due to the highly electronegative oxygen atom and polar nature of the H-O bond. Thus, the strongest force that must be overcome when melting ice is hydrogen bonding. (b) Bromine (Br2): Bromine is a diatomic molecule with London dispersion forces as the primary intermolecular force. Therefore, to sublime bromine, the strongest force to be overcome are the London dispersion forces. (c) Chloroform (CHCl3): Chloroform is a polar molecule, with the carbon atom bonded to one hydrogen atom and three chlorine atoms, creating a polar bond due to the electronegativity difference between the central carbon atom and the chlorine atoms. So, the strongest force that must be overcome when boiling chloroform are dipole-dipole forces. (d) Benzene (C6H6): Benzene is a nonpolar molecule with a hexagonal ring structure and delocalized π-electrons. The primary intermolecular force for such nonpolar molecules is London dispersion forces. Thus, the strongest force to be overcome when vaporizing benzene is the London dispersion forces.