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?

Video transcript

Welcome back everyone in this example, we have neo plantain which is a nice summer of plantain predict which compound would have a lower viscosity. Recall that viscosity describes a resistance to flow and that a higher viscosity, it's associated with stronger London dispersion forces. So looking at the structures of Neo Plantain and plantain, we would see that they consist of carbon and hydrogen only and because they have just carbon and hydrogen, we should recall that these two structures are considered hydrocarbons, meaning that these two structures are going to be considered non polar since we have symmetry throughout these structures and therefore they will have no dipole moments. So that's why the molecules are non polar. And if these molecules are non polar then they will only have London dispersion forces. So we can't compare into molecular force to determine which compound has lower viscosity. However, if we think of the structure and how the structures are displayed, we can consider how strong these London dispersions these London dispersion forces within these molecules will be compared to one another. And so we should recall that when we have branched molecules they will have less surface area and therefore a less efficient area of interaction, which overall results in a weaker inter molecular force. And the same can be true for a molecule with a smaller molecular weight it will have a smaller area of interaction and therefore a weaker inter molecular force. So we can see that plantain is considered a straight chain. However, when we look at our neo plantain, we have branches of methyl group Ch three groups. So we'll say that neo plantain is branched and because neo panting Bc is branched, it will have less surface area, less efficient areas of interaction and therefore a weaker London dispersion force. So because it has weaker London dispersion force compared to our plantain, which is straight chained and non branch, we would say that therefore neo plantain has lower viscosity, and so our final answer is going to be neo plantain. I hope everything I reviewed was clear. If you have any questions, leave them down below and I will see everyone in the next practice video.