In each of the following pairs, which one has the lower viscosity?
a. #"HCOOH"# or #"C"_8"H"_18#
b. #"C"_2"H"_5"Cl"# or #"CH"_3"OH"#
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a. #"C"_8"H"_18"# and b. #"C"_2"H"_5"Cl"# have the lower viscosities.
The viscosity of a liquid is determined by its intermolecular forces.
The stronger the intermolecular forces, the more difficult it is for the molecules to slide past each other.
a. #"HCOOH"# or #"C"_8"H"_18#
#"C"_8"H"_18# has only weak London dispersion forces, while #"HCOOH"# has strong intermolecular hydrogen bonds.
So we predict that #"C"_8"H"_18# has a lower viscosity than #"HCOOH"#.
The prediction is confirmed by the actual viscosities.
The viscosity of #"C"_8"H"_18# is 0.542 mPa·s, while the viscosity of #"CH"_3"-OH"# is 1.78 mPa·s.
b. #"C"_2"H"_5"Cl"# or #"CH"_3"-OH"#
#"C"_2"H"_5"Cl" # or #"CH"_3"-OH"#
#"C"_2"H"_5"Cl"# has weak dipole-dipole forces from the #"C-Cl"# bonds, but #"CH"_3"-OH"# has strong hydrogen bonds.
Confirmation: The viscosity of #"C"_2"H"_5"Cl"# is 0.279 mPa·s, while the viscosity of #"CH"_3"-OH"# is 0.593 mPa·s.
