College Chemistry : Polarity and Dipoles

Study concepts, example questions & explanations for College Chemistry

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Example Questions

Example Question #1 : Polarity And Dipoles

Which of the following molecules is polar?

Possible Answers:

Correct answer:

Explanation:

In order for a molecule to be polar, it must have a net dipole moment. To determine whether there is a dipole moment, you must first figure out the molecular geometry. There are several molecular geometries whose symmetries prevent a dipole moment—even when a charge is present. They include the follwoing: linear, trigonal planar, tetrahedral, octahedral, and trigonal ​bipyramid.  has a molecular geometry that is trigonal pyramidal and has a dipole due to the negativity of nitrogen's lone pair of electrons.

Example Question #1 : Polarity And Dipoles

Which of the following has dipole-dipole forces?

 

Possible Answers:

Correct answer:

Explanation:

Recall that dipole-dipole forces only form in polar molecules.

 is nonpolar because the difference in electronegativities between carbon and hydrogen is very slight, not enough to make it polar.

 is nonpolar even though the individual  bonds are polar. The geometry of the molecule is tetrahedral, which means the polarities of the bonds will cancel each other out.

 is nonpolar because the difference in electronegativities between carbon and hydrogen is not large enough to make it polar.

Example Question #3 : Polarity And Dipoles

Which of the following would you expect to have the highest boiling point?

Possible Answers:

Correct answer:

Explanation:

 has the highest boiling point because it is the only polar molecule. Recall that polar molecules will have dipole-dipole forces, in addition to London Dispersion forces. The added dipole-dipole forces experienced by  will increase its boiling point.

Example Question #4 : Polarity And Dipoles

Which of the following substances has polar bonds, but is nonpolar overall?

Possible Answers:

Correct answer:

Explanation:

A polar bond occurs when two atoms of vastly different electronegativities bond to create a compound.

 consists of hydrogen and oxygen. Oxygen is very electronegative, while hydrogen is not. This means that when oxygen and hydrogen are bonded, oxygen pulls the electrons that they share toward itself. This gives oxygen a partial negative charge and hydrogen a partial positive charge, making the overall water molecule polar.

 consists of hydrogen, an atom which is not very electronegative, and chlorine, an atom which is highly electronegative. The same principle applies here; chlorine pulls the electrons that they share toward itself, giving chlorine a partial negative charge and hydrogen a partial positive charge. Because this compound consists only of one polar bond, the entire species is polar.

 consists of sodium and fluorine. Sodium is not a very electronegative species, while fluorine is extremely electronegative. As a result, fluorine pulls the electrons that they share toward itself, giving sodium a partial positive charge and fluorine a partial negative charge. This compound also consists only of one polar bond, so the entire species is polar.

 consists of two carbons, which are each double bonded to the same oxygen. Carbon is less electronegative than oxygen, making the individual  bonds polar. However, because these bonds are situated symmetrically, the polarities cancel each other out, and the species as a whole becomes nonpolar.

Example Question #2 : Polarity And Dipoles

Which of these substances is polar?

Possible Answers:

Correct answer:

Explanation:

Polar bonds develop when a bond is formed between two atoms of elements with significantly different electronegativities. The more electronegative element will pull the shared electrons toward itself, so that the electrons are not evenly distributed throughout the molecule.

In , carbon is the central atom, with a single bond to each hydrogen atom. Because the central carbon is connected to four of the same atom, and has no lone pairs of electrons left, this molecule is nonpolar overall.

The same principle applies to . Chlorine is much more electronegative than carbon, so the individual  bonds are polar. However, the central carbon is connected to four of the same atom with no lone pairs of electrons left, and the orientation of this molecule is tetrahedral. This causes the polarities of each of the individual polar bonds to cancel out, rendering the molecule as a whole nonpolar.

 consists of two oxygens, each double-bonded to the same carbon. Oxygen is much more electronegative than carbon, so each  bond is polar. However, the central carbon is connected to two of the same atom with no lone pairs of electrons left. This causes the polarities of each of the individual polar bonds to cancel out, leaving the molecule as a whole nonpolar.

 consists of a central carbon single-bonded to a hydrogen and triple-bonded to a nitrogen. This is a linear molecule, and the nitrogen also has a lone pair of electrons. Of these three elements, nitrogen is the most electronegative. Carbon is more electronegative than hydrogen, and nitrogen is more electronegative than carbon. This leads to nitrogen being the most electronegative atom, so the electrons are distributed more towards the nitrogen than any other atom. This causes the molecule as a whole to be polar.

Example Question #6 : Polarity And Dipoles

Which of the following compounds would be expected to dissolve in a polar solvent?

Possible Answers:

Correct answer:

Explanation:

In this question, we're asked to identify a compound that will be most likely to dissolve in a polar solvent. To do this, we'll need to look at each compound and determine whether it is polar or not.

Remember that like dissolves like. Thus, since we have a polar solvent, we're going to be looking for an answer choice that gives a polar compound.

First let's look at . This is not a polar compound because it has no dipole moment. Even though each individual bond is polar, they all cancel each other out due to their orientation with respect to the central carbon atom.

Next, let's look at . Once again, this compound is not polar because it has no dipole moment due to the orientation of its atoms.

Looking at the next compound, , we also see that it is not polar because it has no dipole moment.

The  compound is certainly not polar, because it is just a hydrocarbon, where all of the bonds are non-polar.

Finally, we arrive at , ethanol. This compound would be expected to dissolve in a polar solvent because it has a hydroxyl group. This functional group is very polar, and it also allows ethanol to form hydrogen bonds with other compounds that are capable of participating in hydrogen bonds.

Example Question #7 : Polarity And Dipoles

A collection of  molecules would experience which of the following intermolecular forces?

Possible Answers:

Hydrogen bonding

Ion-ion interaction

Dipole-dipole interaction

Dispersion/van der Waals forces

Correct answer:

Dispersion/van der Waals forces

Explanation:

Boron has three valence electrons. When it forms three single covalent bonds with bromine atoms, each of these electrons is shared with a more electronegative bromium atom, resulting in a stable octet configuration for Boron. Since the central Boron atom has no additional lone pair, these three covalent bonds will be in a trigonal planar configuration (for maximum distance between electrons). Thus,   is a symmetrical molecule, and all of the dipoles along the B-Br bonds will cancel out. Thus,   is nonpolar, and will undergo dispersion or van der Waals intermolecular forces.

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