AP Chemistry : AP Chemistry

Study concepts, example questions & explanations for AP Chemistry

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Example Questions

Example Question #14 : Intermolecular Forces

Which of the following is the strongest intermolecular force?

Possible Answers:

Covalent bonds

Dipole-dipole forces

Ion-dipole forces

Ionic bonds

van Der Waals

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:

van Der Waals

ionic bond

Hydrogen bond

None of these

covalent 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 #15 : Intermolecular Forces

Which of the following intermolecular forces is the strongest?

Possible Answers:

Dipole-dipole forces

Ionic bonds

Van der Waals forces

Hydrogen bonds

Ion-dipole 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 #17 : Intermolecular Forces

Formaldehyde  has which of the following types of intermolecular forces?

Possible Answers:

Dipole-dipole attraction only

Hydrogen bonding and dipole-dipole attraction

London dispersion forces only

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

London dispersion forces and dipole-dipole attraction

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 #15 : Intermolecular Forces

Select the option that lists only intermolecular forces.

Possible Answers:

Covalent bonding and ionic bonding

van der Waals forces and metallic bonding

Hydrogen bonding and metallic bonding

Metallic bonding and covalent 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 #11 : Intermolecular Forces

Which of these is the strongest intermolecular force?

Possible Answers:

Covalent bonds

Hydrogen bonds

van der Waals forces

Dipole-dipole interactions

Ionic bonds

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 #22 : Compounds And Molecules

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

I. 

II. 

III. 

IV. 

V. 

Possible Answers:

II < III < I < IV < V

IV < V < I < II < III

I < II < IV < V < III

II < III < IV < I < V

V < I < IV < III < II

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.

Example Question #21 : Intermolecular Forces

Which of the following compounds will likely have the highest boiling point, and why?

Possible Answers:

, because it is the most massive molecule

, because it is the largest molecule

, because it experiences dipole-dipole intermolecular forces.

, because it experiences hydrogen bonding

, because it is the least massive molecule

Correct answer:

, because it experiences dipole-dipole intermolecular forces.

Explanation:

Stronger intermolecular forces (IMFs) hold molecules together, so the strongest IMFs will produce the highest boiling points because more energy will be required to disrupt those bonds and cause a phase shift. 

The answer is that  should have the highest boiling point, due to dipole-dipole interactions. It is the only molecule on the list that experiences these interactions, which are stronger than the IMFs experienced by the others. While it is true that  is the most massive compound in the list, mass has relatively little to do with boiling point, when compared with IMFs. 

 would have the second highest boiling point because, like , its only IMF is induced dipole moments (or London dispersion forces), but because it is a larger molecule than , it will experience more induced dipoles. It does not actually experience hydrogen bonding since none of its hydrogens are bonded to fluorine, oxygen, or nitrogen.

 is the least massive, but again, that has very little to do with the boiling point compared with IMFs. Since it is only experiencing induced dipoles, and is a smaller molecule than , it will have the weakest IMFs and thus the lowest boiling point.

Example Question #21 : Intermolecular Forces

All molecules have what type of intermolecular force?

Possible Answers:

Ionic

Hydrogen bonding

Dipole-dipole

London dispersion

Covalent

Correct answer:

London dispersion

Explanation:

London dispersion forces are weak attractive forces between molecules or atoms. They involve transient induced polarization of atoms and/or electrons that cause two separate atoms or molecules to be attracted to each other.

Example Question #21 : Intermolecular Forces

The stronger the intermolecular forces, __________.

Possible Answers:

the higher the molar mass

the lower the viscosity

the higher the boiling point

the higher the vapor pressure

the lower the surface tension

Correct answer:

the higher the boiling point

Explanation:

The stronger the intermolecular forces, the higher the: boiling point, surface tension, and viscosity; and the lower the vapor pressure. A bigger molar mass doesn't necessarily mean that there are stronger intermolecular forces (that is determined by the type of intermolecular forces present).

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