Question

Based on their composition and structure, list \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3},\) and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) in order of \((\mathbf{a})\) increasing intermolecular forces, (\mathbf{b} ) increasing viscosity , ( c ) increasing surface tension.

Short answer

(a) Increasing intermolecular forces: Propane (\(CH_3CH_2CH_3\)), Dichloromethane (\(CH_2Cl_2\)), Ethanol (\(CH_3CH_2OH\)). (b) Increasing viscosity: Propane (\(CH_3CH_2CH_3\)), Dichloromethane (\(CH_2Cl_2\)), Ethanol (\(CH_3CH_2OH\)). (c) Increasing surface tension: Propane (\(CH_3CH_2CH_3\)), Dichloromethane (\(CH_2Cl_2\)), Ethanol (\(CH_3CH_2OH\)).

Step-by-step solution

(a) Increasing intermolecular forces

To list the molecules in increasing order of intermolecular forces, we must account for the different types of forces present in each substance. Based on the analysis above, we can see that the order is as follows: 1. Propane (\(CH_3CH_2CH_3\)): London dispersion forces 2. Dichloromethane (\(CH_2Cl_2\)): Dipole-dipole forces 3. Ethanol (\(CH_3CH_2OH\)): Hydrogen bonds

(b) Increasing viscosity

Viscosity is directly related to the strength of intermolecular forces. Substances with stronger intermolecular forces will have higher viscosity since the molecules will be more attracted to each other, making it harder for them to flow past one another. Given this, the order of increasing viscosity is: 1. Propane (\(CH_3CH_2CH_3\)) 2. Dichloromethane (\(CH_2Cl_2\)) 3. Ethanol (\(CH_3CH_2OH\))

(c) Increasing surface tension

Surface tension is also directly related to the strength of intermolecular forces. Stronger intermolecular forces mean that the molecules are more attracted to each other, leading to a higher energy barrier to break the surface. The order of increasing surface tension is: 1. Propane (\(CH_3CH_2CH_3\)) 2. Dichloromethane (\(CH_2Cl_2\)) 3. Ethanol (\(CH_3CH_2OH\))

Key concepts

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction that occurs when a hydrogen atom covalently bonded to a highly electronegative atom, such as oxygen or nitrogen, is attracted to another electronegative atom in a nearby molecule. This bond is stronger than regular dipole-dipole interactions due to the small size of hydrogen and the significant difference in electronegativity, which creates a highly polar bond.

• Occurs in molecules such as water (H₂O) and ethanol (CH₃CH₂OH).
• Significantly influences physical properties like boiling point and solubility.
• Imparts higher surface tension and viscosity when compared to other intermolecular forces.

In the context of our textbook exercise, ethanol exhibits hydrogen bonding, which accounts for its highest intermolecular force, viscosity, and surface tension among the given molecules.

Dipole-Dipole Interactions

Dipole-dipole interactions are attractive forces between the positive end of one polar molecule and the negative end of another. These intermolecular forces arise due to the uneven distribution of electrons within the molecule, which creates a permanent dipole moment.

• Typically found in molecules with distinct partial negative and positive charges.
• The strength of these interactions depends on the polarity of the molecules involved.
• Affects the melting and boiling points of substances.

In dichloromethane (CH₂Cl₂), the chlorine atoms have a greater electronegativity than carbon, leading to dipole-dipole interactions. This results in dichloromethane having higher intermolecular forces than propane, but lower than ethanol due to the absence of hydrogen bonding.

London Dispersion Forces

London dispersion forces, also known as induced dipole-induced dipole interactions, are the weakest among the intermolecular forces. They occur in all molecules, whether polar or nonpolar, due to the spontaneous formation of temporary dipoles as electrons move within an atom or molecule.

• Present in all molecular substances, including noble gases and nonpolar compounds.
• The strength increases with molecular size and the number of electrons.
• Responsible for the existence of liquids and solids among nonpolar compounds.

Propane (CH₃CH₂CH₃), a nonpolar molecule, only exhibits London dispersion forces, making it the weakest in terms of intermolecular forces, viscosity, and surface tension among the molecules discussed in the exercise.