Video Transcript
In this video, we will learn how to
describe polar and nonpolar solvents. We’ll examine how intermolecular
forces between molecules affect solubility and determine the meaning behind the
phrase “like dissolves like.” Before we can examine how polar and
nonpolar solvents interact, let’s first review some key concepts.
Electronegativity is the tendency
of an atom to attract the shared electrons of a bond. Looking at the periodic table,
electronegativity generally increases as you move up a group and from left to right
across a period. The Pauling electronegativity
scale, a unitless scale, which ranges from zero to four, gives us a way to quantify
the electronegativity of atoms. Fluorine has the highest
electronegativity value of 3.98, while elements like potassium have much smaller
electronegativity values. The difference in the
electronegativity values of bonded atoms can be used to determine whether the bond
tends to have ionic, polar, or nonpolar characteristics.
When the electronegativity
difference between two bonded atoms is less than 0.4, the bond tends to be
nonpolar. When the electronegativity
difference between two bonded atoms is greater than 1.8, the bond tends to be
ionic. And when the electronegativity
difference between two bonded atoms is between 0.4 and 1.8, the bond tends to be
polar. In an ionic bond, the
electronegativity difference is so great that the more electronegative atom has
become a negatively charged ion and the less electronegative atom has become a
positively charged ion. Sodium chloride is an example of a
compound that exhibits ionic bonding.
In a polar bond, the electrons of
the bond are unequally shared and are drawn towards the more electronegative
atom. This results in the more
electronegative atom having a partial negative charge represented by the symbol 𝛿−,
while the less electronegative atom gains a partial positive charge represented by
the symbol 𝛿+. A dipole arrow can also be used to
represent the partial negative and partial positive sides of the bond. Hydrochloric acid is an example of
a compound that contains a polar bond.
In a nonpolar bond, the electrons
of the bond are equally shared between the two atoms. So neither atom has a partial
positive or a partial negative charge. A molecule of chlorine contains a
nonpolar bond. When a molecule contains only two
atoms, like the examples shown here, it is fairly easy to determine if the molecule
tends to be polar or nonpolar using electronegativity differences. But when the molecule contains more
than two atoms, this can become quite difficult to determine.
A polar molecule contains polar
bonds and will have a nonuniform electron density. This means that some regions in the
molecule will have a partial negative charge, while other regions in the molecule
have a partial positive charge. Some examples of polar molecules
include water and methanal, commonly called formaldehyde. Nonpolar molecules may contain
polar or nonpolar bonds, but they must have a uniform electron density. In general, if all of the atoms in
the molecule are the same, the molecule only contains carbon and hydrogen atoms, or
the polar bonds are symmetrically distributed, the molecule will be nonpolar.
Now that we can recognize polar and
nonpolar molecules, let’s examine the solubility of these substances in one
another. Solubility is the tendency of one
substance to dissolve in a solvent to form a solution. Let’s consider two containers of
water. To the first container, we’ll add
some vegetable oil. And to the second container, we’ll
add some isopropyl alcohol, commonly called rubbing alcohol. Water and oil do not mix to form a
solution. We can describe water and oil as
immiscible, incapable of being mixed to form a solution. Rubbing alcohol and water do mix to
form a solution. We can describe water and rubbing
alcohol as miscible, capable of being mixed to form a solution. But why do oil and rubbing alcohol
behave differently when mixed with water?
To understand what is occurring, we
need to take a brief look at intermolecular forces. Intermolecular forces are
electrostatic forces that exist between molecules. As we stated earlier in the video,
nonpolar molecules, like a molecule of bromine, do not have regions of partial
charge. Even so, two nonpolar molecules
will exhibit a weak electrostatic attraction towards one another. This electrostatic attraction,
called dispersion force, is due to the random motion of electrons in the molecule
and will be covered in more detail in another video.
Polar molecules do have regions of
partial positive and partial negative charge. When two polar molecules are near
one another, there exists an electrostatic attraction between the partial positive
charge of one molecule and the partial negative charge of another molecule. An even stronger intermolecular
force exists between certain polar molecules that contain a hydrogen atom bonded to
an oxygen, nitrogen, or fluorine atom. There is a very large
electronegativity difference between a hydrogen atom and an oxygen, nitrogen, or
fluorine atom. The large electronegativity
difference results in larger partial negative and partial positive charged regions
in the molecule.
The hydrogen atom of one molecule,
which has a large partial positive charge, experiences a strong electrostatic
attraction to the lone-pair electrons of an oxygen, nitrogen, or fluorine atom of
another molecule. This very strong electrostatic
attraction is called hydrogen bonding and exists between a hydrogen bond donor, a
molecule with a hydrogen atom bonded to an oxygen, nitrogen, or fluorine atom, and a
hydrogen bond acceptor, a molecule that contains an oxygen, nitrogen, or fluorine
atom with lone-pair electrons.
With this understanding of
intermolecular forces, let’s return to the example of oil and rubbing alcohol mixing
with water. We just learned that water
molecules are polar and exhibit hydrogen bonding with other water molecules. Vegetable oil consists of
triglycerides, which we will abbreviate as TG. Triglycerides are compounds that
contain three ester groups with large hydrocarbon chains and are nonpolar. This means that triglycerides
exhibit weak electrostatic forces with one another. A molecule of triglyceride and a
molecule of water do exhibit electrostatic attractions with one another. This electrostatic attraction is
stronger than the attraction between two triglyceride molecules but is weaker than
the attraction between two water molecules.
As water molecules are more
strongly attracted to one another than to the triglycerides, the two substances do
not mix. Like water, the isopropyl alcohol
molecules of rubbing alcohol are polar and can form hydrogen bonds with other
isopropyl alcohol molecules. Water and isopropyl alcohol
molecules can form hydrogen bonds with one another as well. This intermolecular attraction is
similar to the attraction each substance had on its own, and the two liquids mix
together.
In general, solutes and solvents
that exhibit similar intermolecular forces of similar strength are likely to
dissolve in one another. We often express this
generalization with the phrase “like dissolves like,” meaning that nonpolar solutes
tend to dissolve in nonpolar solvents and polar solutes tend to dissolve in polar
solvents. Ionic solutes also tend to dissolve
in polar solvents. A sample of sodium chloride
contains positively charged sodium cations and negatively charged chloride anions
that are electrostatically attracted. When added to water, the negatively
charged chloride anions interact with the partial positively charged regions of the
water molecules. And the positively charged sodium
cations interact with the partial negatively charged regions of the water
molecules. The ions dissociate from one
another and become surrounded by water molecules, thus forming a solution.
It’s important to note that not all
ionic solutes dissolve in polar solvents. Ionic solutes only tend to be
soluble in polar solvents when new stronger electrostatic interactions are
formed. Now that we have examined the
solubility of substances in polar and nonpolar solvents, let’s make a list of some
common solvents that we can choose from.
Some common nonpolar solvents
include hexane, benzene, and carbon tetrachloride. Polar solvents can further be
classified as either protic or aprotic. Protic solvent molecules contain a
hydrogen atom bonded to either an oxygen, nitrogen, or fluorine atom and exhibit
hydrogen bonding. The prefix pro- in protic indicates
that these molecules are proton or hydrogen ion donors. Examples of protic solvents include
water, ethanol, and methanoic acid, commonly called formic acid. The a- in aprotic means “not.” So, aprotic solvents do not contain
a hydrogen atom bonded to an oxygen, nitrogen, or fluorine atom; do not exhibit
hydrogen bonding with other aprotic solvent molecules; and are not proton
donors. Examples of aprotic solvents
include dichloromethane; propanone, commonly called acetone; and
tetrahydrofuran.
It’s often useful to think of
nonpolar and polar solvents as a continuum. Some solvents can be considered
extremely polar or nonpolar, but most solvents fall somewhere in between. Some compounds may even exhibit
both polar and nonpolar characteristics at the same time. Let’s consider, for example,
ethanol and pentanol, two alcohols. While both molecules have a similar
structure, ethanol is miscible with water but pentanol is not. We can see that both molecules
contain an oxygen atom bonded to a hydrogen atom, a group that is polar and can form
hydrogen bonds. We also see that both molecules
contain a hydrocarbon portion that is nonpolar. As pentanol is immiscible with
water, the large nonpolar hydrocarbon chain must affect its solubility more than the
polar OH group.
In general, we find that for small
alcohols and carboxylic acids, the polar portion of the molecule contributes to the
overall characteristics of the solute more than the nonpolar portion. As such, these molecules tend to
dissolve in water. But as the length of the carbon
chain grows, the molecule tends to exhibit more nonpolar characteristics. Thus, large alcohols and carboxylic
acids tend to be immiscible with water. Now that we’ve examined the
solubility of substances in polar and nonpolar solvents, let’s take a look at a
question.
Which of the following statements
describes bromine dissolving in hexane? (A) A nonpolar substance dissolving
in a polar solvent. (B) A polar substance dissolving in
a nonpolar solvent. (C) A polar substance dissolving in
a polar solvent. (D) A nonpolar substance dissolving
in a nonpolar solvent.
First, let’s define the terms polar
and nonpolar. A polar molecule will have a
nonuniform electron density. This means that some portions of
the molecule will have a partial negative charge and some portions of the molecule
will have a partial positive charge. Nonpolar molecules will have a
uniform electron density. This is the case when the electrons
are equally shared in all of the bonds in the molecule or when the polar bonds in
the molecule are symmetrically distributed. Bromine is a diatomic molecule
consisting of two single-bonded bromine atoms. As the atoms are the same, they
will have the same electronegativity. This means that the electrons in
the bond will be equally shared between the two bromine atoms. Thus, the molecule of bromine is
nonpolar.
As bromine is the substance being
dissolved, we can eliminate answer choices (B) and (C). Hexane, chemical formula C6H14,
only contains carbon and hydrogen atoms. The electronegativity of a carbon
atom is approximately equal to the electronegativity of a hydrogen atom. Because the electronegativity
values are so similar, we can consider the electrons in a carbon–hydrogen bond to be
equally shared. As a molecule of hexane only
contains nonpolar carbon–carbon and carbon–hydrogen bonds, hexane will be
nonpolar. Therefore, the statement that
describes bromine dissolving in hexane is answer choice (D), a nonpolar substance
dissolving in a nonpolar solvent.
Now, let’s sum up this video with
the key points. Nonpolar solvents have a uniform
electron density, while polar solvents do not. The nonuniform electron density of
a polar solvent results in the molecule having regions of partial positive and
negative charge. The phrase “like dissolves like”
can be used as a generalization to recognize which solutes tend to dissolve in which
solvents. In general, polar solutes tend to
dissolve in polar solvents and nonpolar solutes tend to dissolve in nonpolar
solvents.
Substances, particularly liquids,
that mix to form a solution are miscible. Substances that do not mix to form
a solution are immiscible. Solvents may be classified as
nonpolar, polar protic, or polar aprotic. Polar protic solvents can form
hydrogen bonds, while polar aprotic solvents cannot. This is because polar protic
solvents contain a hydrogen atom bonded to an oxygen, nitrogen, or fluorine
atom.
