Question

Which is more viscous and why?

4. (After Mon/Tues class) Both molecules shown below are octane isomers. Which one has the highest boiling point? Which one is more viscous? Which one would tend to bead more on a surface? Explain each part. C& Hig Co His H H H H H H H H CH3 CH3 marc polariza CH3-C-CH2-CH H-C-CC-CC-C-C-CH a) CH3 CH3 b) H H H H H

Answer 1

2,2,4-trimethylpentane (option a) is a branched chain isomer while the n-octane (option b) is a linear chain isomer.

Linear chain isomer always have higher boiling point than the branched chain because of higher surface area. Surface area is directly proportional to the intermolecular forces. Linear chain isomer means more surface area, stronger is the intermolecular forces and hence, to overcome these strong intermolecular forces, higher temperature (boiling point) will be needed.

# n-octane has higher boiling point.

Viscosity is the resistance to fluid motion. Viscosity depends upon the interactions between the molecules. More interaction means more viscosity.

Viscosity increases with increase in intermolecular forces and the increased ability to which molecules become entangled.

Since, linear chain isomer have higher degree of intermolecular forces and its molecule can easily be entangled (high packing fraction), it will be more viscous.

# n-octane is more viscous.

The molecule having lower surface area will bead more on a surface. Since, branched chain isomers have lower surface area, hence it will bead more.

# branched chain isomer will bead up more.