viscosities?

Question

which would be more viscous: heptane or 3,3-dimethyl hexane? thanks

Answer 1

You don't have the same number of carbon atoms so it is not easy to tell.

We are talking about Newtonian liquids, so the viscosity is proportional to the strength of the intermolecular forces.
For alkanes the only intermolecular forces present are dispersion forces. These are proportional to the size of the molecule (bigger size stronger forces) and on the surface available for interaction (bigger surface stronger forces).

When the carbon chain is branched you have a smaller surface for interaction than for a non-branched carbon chain with the same TOTAL number of C atoms. So if you were comparing octane with 3,3-dimethyl-hexane or heptane with 3-3 dimethyl-pentane you could easily say that the branched alkane has lower viscosity.

Now that you have a 7-C non-branched and a 8-C branched molecule you can't really say if the effect of more C atoms will dominate, cancel out or lose over the surface effect and have respectively higher, equal or lower viscosity for 3,3-dimethyl-hexane.

Answer 2

Viscosity is a measure of the resistance of a fluid to deform under shear stress. It is commonly perceived as "thickness", or resistance to flow. Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. Thus, water is "thin", having a lower viscosity, while vegetable oil is "thick" having a higher viscosity. All real fluids (except superfluids) have some resistance to shear stress, but a fluid which has no resistance to shear stress is known as an ideal fluid or inviscid fluid.