AP Chemistry : Intermolecular Forces

Study concepts, example questions & explanations for AP Chemistry

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Example Questions

Example Question #11 : Intermolecular Forces

What intermolecular forces can be found in a molecule of ethene?

Possible Answers:

Dipole-dipole attraction and ionic bonding

London dispersion forces and dipole-dipole attraction

London dispersion forces, hydrogen bonding, and dipole-dipole attraction

London dispersion forces only

London dispersion forces and hydrogen bonding

Correct answer:

London dispersion forces only

Explanation:

Ethene is an organic molecule composed of two carbon atoms, joined by a double bond, and four hydrogen atoms.

Ethene, like all molecules, exhibits London dispersion forces. This molecule, however, has no net dipole moment, so it will not exhibit dipole-dipole attraction. Also, even though it contains hydrogens, it does not exhibit hydrogen bonding. To exhibit hydrogen bonding, the hydrogen atoms must be attached to more electronegative atoms, namely nitrogen, fluorine, or oxygen. Finally, ionic bonding is only present in ionic compounds, not organic compounds.

Example Question #11 : Intermolecular Forces

Which of the following has the highest boiling point?

Possible Answers:

Correct answer:

Explanation:

Ionic bonds are the strongest type of bonds, followed by covalent bonds, hydrogen bonds, and lastly, van Der waals forces.

Example Question #11 : Intermolecular Forces

Which of the following substances has a higher boiling point?

Dipropyl Ether Untitled_drawing

Propanol Untitled_drawing__1_

Possible Answers:

Propanol

You must know the system pressure in order to determine the answer

Dipropyl ether

You must know the system temperature in order to determine the answer

They have equal boiling points

Correct answer:

Propanol

Explanation:

Although the two molecules seem similar in structure, proponol has a higher boiling point due to the hydrogen bonding allowed by its alcohol group. This creates a strong intermolecular force, and extra energy is subsequently needed to break these bonds, resulting in a higher boiling point.

System pressure and temperature are related to boiling point, but are not necessary when comparing the properties of two molecules.

Example Question #12 : Intermolecular Forces

Which of the following is the strongest intermolecular force?

Possible Answers:

van Der Waals

Ionic bonds

Dipole-dipole forces

Covalent bonds

Ion-dipole forces

Correct answer:

Ion-dipole forces

Explanation:

Ionic and covalent bonds are not intermolecular forces;

Ion-dipole>hydrogen bonds>dipole-dipole>van Der Waals forces

Example Question #12 : Intermolecular Forces

What is the strongest intermolecular force in the following compound?

CH3CH2CH2CH2OH

Possible Answers:

None of these

covalent bond

van Der Waals

Hydrogen bond

ionic bond

Correct answer:

Hydrogen bond

Explanation:

This is butanol. It is an alcohol; OH is the prime example of hydrogen bonding, which is the strongest intermolecular force.

Example Question #11 : Intermolecular Forces

Which of the following intermolecular forces is the strongest?

Possible Answers:

Dipole-dipole forces

Hydrogen bonds

Ionic bonds

Ion-dipole forces

Van der Waals forces

Correct answer:

Ion-dipole forces

Explanation:

Ion-dipole forces are the strongest of the intermolecular forces.

Hydrogen bonding is a specific term for a particularly strong dipole-dipole interaction between a hydrogen atom and a very electronegative atom (oxygen, fluorine, or nitrogen). However, hydrogen bonds are still not as strong as ion-dipole interactions.

In order from strongest to weakest, the intermolecular forces given in the answer choices are: ion-dipole, hydrogen bonding, dipole-dipole, and Van der Waals forces.

Ionic bonding is stronger than any of the given intermolecular forces, but is itself NOT an intermolecular force. Ionic bonds are a permanent chemical connection between two atoms, whereas intermolecular forces as a more transient and temporary attraction between independent molecules.

Example Question #11 : Intermolecular Forces

Formaldehyde  has which of the following types of intermolecular forces?

Possible Answers:

London dispersion forces and dipole-dipole attraction

Dipole-dipole attraction only

Hydrogen bonding and dipole-dipole attraction

London dispersion forces, dipole-dipole attraction, and hydrogen bonding

London dispersion forces only

Correct answer:

London dispersion forces and dipole-dipole attraction

Explanation:

Formaldehyde, like all atoms and molecules, will have very weak London dispersion forces created as electrons shift within the electron cloud. Because it possesses a permanent dipole (based on the polarized carbon-oxygen bond), formaldehyde also exhibits dipole-dipole interactions. It does not, however, exhibit hydrogen bonding, because no hydrogens are attached to oxygen (or other electronegative atoms like nitrogen or fluorine).

Example Question #18 : Intermolecular Forces

Select the option that lists only intermolecular forces.

Possible Answers:

Metallic bonding and covalent bonding

Covalent bonding and ionic bonding

van der Waals forces and metallic bonding

Hydrogen bonding and metallic bonding

Hydrogen bonding and van der Waals forces

Correct answer:

Hydrogen bonding and van der Waals forces

Explanation:

There is a key difference between atomic bonds and intermolecular forces. Metallic bonds, ionic bonds, and covalent bonds are all atomic bonds. This means that they are generally stable and relatively irreversible. An atomic bond will change the identity of a compound by adding an atom to the structure.

Intermolecular forces, in contrast, are more transient and less stable. These attractions are constantly broken and reformed as molecules move around. Hydrogen bonds, dipole-dipole interactions, and van der Waals forces (London dispersion forces) are some common examples of intermolecular forces. Intermolecular forces will never change the identity of the molecule and cannot be used to add atoms to a compound.

Example Question #19 : Intermolecular Forces

Which of these is the strongest intermolecular force?

Possible Answers:

Ionic bonds

Dipole-dipole interactions

Hydrogen bonds

Covalent bonds

van der Waals forces

Correct answer:

Hydrogen bonds

Explanation:

There is a key difference between atomic bonds and intermolecular forces. Ionic bonds and covalent bonds are atomic bonds, meaning they are intramolecular. This means that they are generally stable and relatively irreversible. An atomic bond will change the identity of a compound by adding an atom to the structure.

Intermolecular forces, in contrast, are more transient and less stable. These attractions are constantly broken and reformed as molecules move around. Hydrogen bonds, dipole-dipole interactions, and van der Waals forces (London dispersion forces) are some common examples of intermolecular forces. Intermolecular forces will never change the identity of the molecule and cannot be used to add atoms to a compound.

The strongest intermolecular force is hydrogen bonding, which is a particular subset of dipole-dipole interactions that occur when a hydrogen is in close proximity (bound to) a highly electronegative element (namely oxygen, nitrogen, or fluorine). The hydrogen takes on a partial positive charge and the electronegative atoms takes on a partial negative charge.

Example Question #20 : Intermolecular Forces

Order the following compounds by strength of their intermolecular forces, from weakest to strongest:

I. 

II. 

III. 

IV. 

V. 

Possible Answers:

IV < V < I < II < III

V < I < IV < III < II

I < II < IV < V < III

II < III < I < IV < V

II < III < IV < I < V

Correct answer:

II < III < IV < I < V

Explanation:

This is a question about intermolecular forces, or IMFs. There are four broad categories of IMFs, all of which are represented here. 

Since the question asks us to order the compounds from least strength to greatest, we'll start with the weakest IMF: Van der Waals forces, also called "induced dipoles" or London dispersion forces.

Non-polar molecules like  and  can still exhibit temporary dipoles by induction, when the electrons of one molecule push away the electrons of another. In general, the more electrons that are available to push, the more potential there is for a dipole to occur. Therefore,  is weaker, because it has less electrons available than

The next tier of IMF is permanent dipole interactions (dipole-dipole interactions) that are not hydrogen bonds.  is a polar molecule and experiences dipole interactions, which makes it the next strongest in our list.

Next is hydrogen bonding, an especially powerful form of dipole interactions when hydrogen is bonded to a fluorine, oxygen, or nitrogen atom.  experiences hydrogen bonding because it has hydrogen atoms bonded to oxygen atoms. Because hydrogen bonds are a stronger form of dipole interactions, this puts  next in the list.

The strongest form of intermolecular force is ionic forces, which exist in ionic compounds like . So,  exhibits the strongest IMFs.

Our final order is , or II < III < IV < I < V.

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