Video Transcript
Which of the following alkanes has
the greatest viscosity? (A) Octane, (B) pentane, (C)
methane, (D) propane, (E) ethane.
First of all, alkanes are saturated
hydrocarbons that contain only single bonds between carbon atoms. Alkanes have the general formula
C𝑛H2𝑛+2, where 𝑛 represents the number of carbon atoms. So, if we know the number of carbon
atoms of each alkane in the answer choices, we can write its chemical formula.
The names of straight-chain alkanes
contain two parts: a stem and a suffix. The stem represents the number of
carbon atoms in the molecule, and the suffix tells the type of hydrocarbon. The suffix A-N-E is used for all
alkanes. Let’s make use of the following
table to help us determine the number of carbon atoms in each alkane.
Octane has eight carbon atoms,
pentane has five carbon atoms, methane has one carbon atom, propane has three carbon
atoms, and ethane has two carbon atoms. Now, let’s use the general formula
to determine the chemical formula of each alkane. For octane, the number of hydrogen
atoms will be two times eight plus two. Therefore, the chemical formula of
octane is C8H18. Using the general formula, the
remaining alkanes have the chemical formulas shown.
Viscosity is a quantity that
describes a fluid’s resistance to flow. The stronger the dispersion forces
between alkane molecules, the greater the viscosity of the alkane. Dispersion forces are weak
electrostatic attractions formed between molecules, which become stronger the longer
the carbon chain gets. For example, methane contains only
one carbon atom and has such weak dispersion forces that it is a gas at room
temperature.
The carbon chain of ethane only
contains two carbon atoms. Therefore, the dispersion forces
are quite weak, and ethane is also a gas. Propane, with its three-carbon
chain, also has weak dispersion forces and is a gas at room temperature.
Because methane, propane, and
ethane are all gases, their viscosities are quite low. However, octane and pentane are
liquids at room temperature and have higher viscosities. The dispersion forces between
pentane molecules are stronger than those between methane, propane, and ethane
molecules, because pentane has a longer carbon chain. Because octane has a longer carbon
chain than pentane, the dispersion forces are even stronger, and octane has a higher
viscosity.
In conclusion, the alkane with the
greatest viscosity is octane, or answer choice (A).
