Question

Which pair of molecules gives the species with the lowest viscosity first? CH₃CH₂CH₂Cl and CH₃CH₂CH₂CH₃ CH₃CH₂OH and CCl₃ CBr₄ and CCl₄ CH₃—O—CH₃ and CH₃CH₂OH

A) CH₃CH₂CH₂Cl and CH₃CH₂CH₂CH₃
B) CH₃CH₂OH and CCl₄
C) CBr₄ and CCl₄
D) CH₃—O—CH₃ and CH₃CH₂OH

Answer 1

The pair with the lowest viscosity first is dimethyl ether (CH3—O—CH3) and ethanol (CH3CH2OH), because ethanol has hydrogen bonding increasing its viscosity, while dimethyl ether does not and therefore has a lower viscosity.

Step-by-step explanation:

The question asks which pair of molecules gives the species with the lowest viscosity first. Viscosity is the measure of a fluid's resistance to flow. It can be influenced by intermolecular forces. Substances with stronger intermolecular forces have higher viscosities.

Comparing the pairs given:

• CH₃CH₂CH₂Cl and CH₃CH₂CH₂CH₃: The chlorinated compound will have stronger intermolecular forces due to dipole-dipole interactions, while the unhalogenated counterpart is nonpolar with only London dispersion forces. Hence, CH₃CH₂CH₂CH₃ will have a lower viscosity.
• CH₃CH₂OH and CCl₄: The alcohol has hydrogen bonding, thus higher viscosity than CCl₄ which only has London dispersion forces.
• CBr₄ and CCl₄: Both compounds are nonpolar but CBr₄ has a higher molecular weight, thus higher London dispersion forces and consequently higher viscosity compared to CCl₄.
• CH₃—O—CH₃ (dimethyl ether) and CH₃CH₂OH (ethanol): Ethanol can form hydrogen bonds, giving it a higher viscosity than dimethyl ether, which can't form such bonds.

Considering these factors, the pair with the lowest viscosity first is dimethyl ether (CH₃—O—CH₃) and ethanol (CH₃CH₂OH).