In Table 11.3, we saw that the viscosity of a series of hydrocarbons increased with molecular weight, doubling from the six-carbon molecule to the ten-carbon molecule.
(a) The eight-carbon hydrocarbon, octane, has an isomer, isooctane. Would you predict that isooctane would have a larger or smaller viscosity than octane? Why?

Verified step by step guidance

1. Understand that viscosity is a measure of a fluid's resistance to flow. In hydrocarbons, viscosity generally increases with molecular weight and the strength of intermolecular forces.
2. Recognize that octane (C8H18) and isooctane (2,2,4-trimethylpentane) are structural isomers, meaning they have the same molecular formula but different structures.
3. Consider the structure of octane, which is a straight-chain alkane, and compare it to isooctane, which is a branched alkane.
4. Recall that branching in hydrocarbons tends to decrease the surface area available for intermolecular interactions, leading to weaker van der Waals forces compared to straight-chain isomers.
5. Predict that isooctane would have a smaller viscosity than octane due to its branched structure, which reduces intermolecular attractions and thus decreases resistance to flow.

Key Concepts

Viscosity

Viscosity is a measure of a fluid's resistance to flow. It is influenced by the size and shape of the molecules in the fluid; larger and more complex molecules typically result in higher viscosity due to increased intermolecular interactions. In hydrocarbons, as molecular weight increases, viscosity tends to increase because larger molecules can entangle more easily, hindering flow.

Isomerism

Isomerism refers to the phenomenon where two or more compounds have the same molecular formula but different structural arrangements or spatial orientations. In the case of octane and isooctane, they are structural isomers, meaning they have different connectivity of atoms. This difference can affect their physical properties, including viscosity, as the arrangement of atoms influences how molecules interact with each other.

Intermolecular Forces

Intermolecular forces are the attractive forces between molecules that influence physical properties such as boiling point, melting point, and viscosity. In hydrocarbons, van der Waals forces (dispersion forces) play a significant role. The strength of these forces can vary based on molecular structure; for example, branched isomers like isooctane generally have weaker intermolecular forces compared to their straight-chain counterparts, potentially leading to lower viscosity.