Video Transcript
Which of the following molecules is a nonpolar solvent? (A) Acetic acid, (B) ammonia, (C) ethanol, (D) water, (E) benzene.
In this question, we are given the names of five different compounds, and we must determine which of them is a nonpolar solvent. First of all, a solvent is the major component of a solution in which a solute dissolves. Solvents can be classified as polar or nonpolar. The tendency of a solute to dissolve in a solvent and form a solution is a property called solubility. One of the factors that determines if a solute will dissolve in a polar or nonpolar solvent is how electrons are distributed in the molecules.
A polar molecule must contain at least one polar bond. A covalent bond is polar if the atoms in the bond have a sizable difference in electronegativity values. Polar covalent bonds have a difference in electronegativity between about 0.4 and 1.8. Electronegativity measures the tendency of an atom to attract a bonding pair of electrons in a chemical bond. In a polar bond, the atom with the greater electronegativity value withdraws most of the electron density from the atom with the smaller electronegativity value, but having polar bonds does not guarantee that a molecule will be polar. So we also have to examine the chemical structure of the molecule.
In polar molecules, the polar bonds are not arranged in a symmetrical way. This means that different parts of the molecule will have different electric charge, creating a permanent electric dipole moment. Nonpolar molecules may or may not contain polar bonds, but if they do contain polar bonds, the bonds will be organized symmetrically. All parts of the nonpolar molecules will have about the same electric charge. Therefore, nonpolar molecules do not have dipole moments.
Our job in solving this problem is to determine if any polar bonds are present in the molecules and then take a look at the molecular structure to determine if the molecules are polar or nonpolar. Let’s begin with answer choice (A). Acetic acid is a carboxylic acid. It contains the carboxyl functional group, or COOH. Let’s sketch out a structural formula for acetic acid. The carboxyl functional group contains one carbon atom, two oxygen atoms, and one hydrogen atom.
It’s not necessary to use the exact electronegativity values to solve this problem if we can remember that electronegativity increases from left to right in a period and from bottom to top in a group on the periodic table. Therefore, oxygen has a greater electronegativity value than both carbon and hydrogen. In the carbon-to-oxygen double bond and the carbon-to-oxygen single bond, the difference in electronegativity values is 0.8, which means these bonds are polar. As for the oxygen-to-hydrogen single bond, the electronegativity difference is 1.2, which means that this bond is also polar. In all of these bonds, oxygen has the greater electronegativity value, which means that the oxygen atoms will attract most of the electron density. Therefore, in the carboxyl group, the oxygen atoms will have a partial negative charge and the carbon and hydrogen atoms will have a partial positive charge.
The rest of the acetic acid molecule contains only carbon-to-hydrogen single bonds. The difference in electronegativity between a carbon atom and a hydrogen atom is very small, about 0.4. This means that the three carbon-to-hydrogen single bonds are nonpolar bonds, and none of the atoms in these bonds will carry partial charges. We can see from the molecular structure that the electrical charge of the carboxyl group will be different from the rest of the molecule. Therefore, acetic acid molecules are polar, and acetic acid is a polar solvent.
Next, for answer choice (B), let’s sketch the displayed formula of ammonia. The central atom of the molecule is nitrogen, and there are three covalent bonds with hydrogen atoms. The difference in electronegativity between a nitrogen atom and a hydrogen atom is 0.8, which means that these three bonds are polar. In each of the bonds, nitrogen more strongly attracts the electron density, leaving nitrogen with a partial negative charge and the hydrogen atoms with partial positive charges. Because these bonds are not arranged in a symmetrical way, the bond dipoles reinforce each other instead of canceling out. This gives the ammonia molecule an electric dipole moment. Therefore, ammonia is composed of polar molecules and is a polar solvent.
Now, let’s look at answer choice (C). Ethanol is an alcohol, which means that ethanol molecules contain the hydroxyl group, or OH. An ethanol molecule contains two polar bonds, an oxygen-to-hydrogen single bond and a carbon-to-oxygen single bond. The electronegativity value of oxygen is much greater than the other atoms. In each bond, the oxygen atom attracts the electron density more strongly. Therefore, both bond dipoles point toward the oxygen atom. Due to the presence of two lone pairs of electrons on the oxygen atom, this part of the molecule has a bent, nonsymmetrical shape. This means that the ethanol molecule has a dipole moment and is a polar molecule. Therefore, ethanol is a polar solvent.
By now, we’re probably pretty familiar with the molecular structure of water, which is composed of two very polar oxygen-to-hydrogen single bonds. Oxygen has a greater electronegativity than hydrogen. So, both bond dipoles point toward the oxygen atom. Because the water molecule has a bent, nonsymmetrical shape, these bond dipoles reinforce each other. Therefore, water is composed of polar molecules and is classified as a polar solvent.
As for answer choice (E), benzene has a chemical formula of C6H6. The covalent bonds between the carbon and hydrogen atoms in benzene are nonpolar bonds. This is because the difference in electronegativity between carbon and hydrogen is only 0.4. Because benzene only contains nonpolar bonds, it is composed of nonpolar molecules and is therefore a nonpolar solvent.
Which of the following molecules is a nonpolar solvent? The correct answer is benzene or answer choice (E).
