Define intermolecular forces .

Weak electrostatic forces that exist between atoms or molecules, responsible for physical properties such as boiling point, melting point, density, viscosity and solubility.

True or False? Intermolecular forces are generally stronger than intramolecular forces such as covalent bonds.

False. Intermolecular forces are generally weaker than intramolecular forces (ionic bonds, covalent bonds, metallic bonds). They act between molecules rather than within them.

Define London dispersion forces .

Weak intermolecular forces produced by instantaneous dipoles arising from the temporary uneven distribution of electrons. They exist in all atoms and molecules and are the only intermolecular force present in nonpolar molecules and noble gases.

London dispersion forces arise because electrons are in constant motion, creating an .......... that induces a dipole in a neighbouring molecule.

London dispersion forces arise because electrons are in constant motion, creating an instantaneous dipole that induces a dipole in a neighbouring molecule.

Why do London dispersion forces increase in strength as molecular size increases?

Larger molecules have more electrons, which increases polarizability — the ease with which the electron cloud can be distorted. Greater polarizability produces stronger instantaneous dipoles and therefore stronger London dispersion forces.

True or False? A more branched molecule has stronger London dispersion forces than a less branched molecule of the same molecular mass.

False. Greater branching makes a molecule more compact and less polarizable. This reduces the strength of London dispersion forces, which is why more branched isomers have lower boiling points.

Why do the boiling points of noble gases increase down Group 18?

Each successive noble gas has more electrons, increasing polarizability . This leads to stronger London dispersion forces between atoms, requiring more energy to overcome them and raising the boiling point.

Define dipole-induced dipole interaction .

A weak intermolecular attraction between a polar molecule and a nonpolar molecule, in which the permanent dipole of the polar molecule distorts the electron distribution of the nonpolar molecule, creating a temporary induced dipole .

How does iodine dissolve partially in water via a dipole-induced dipole interaction?

The partial negative charge on the oxygen atom of a polar water molecule distorts the electron cloud of the nonpolar iodine molecule, inducing a temporary induced dipole . The resulting weak attraction between the induced dipole and the water molecule allows limited dissolution.

The strength of a dipole-induced dipole interaction depends on the magnitude of the dipole moment in the polar molecule and the .......... of the nonpolar molecule.

The strength of a dipole-induced dipole interaction depends on the magnitude of the dipole moment in the polar molecule and the polarizability of the nonpolar molecule.

True or False? Dipole-induced dipole interactions can occur between two nonpolar molecules.

False. A dipole-induced dipole interaction requires one polar molecule to provide the permanent dipole that distorts the electron cloud of the nonpolar molecule. Two nonpolar molecules interact only via London dispersion forces.

Why is iodine only slightly soluble in water rather than freely soluble?

The dipole-induced dipole interaction between water and I 2 is weak. Because the induced dipole in I 2 is temporary and relatively small, the attractive force is insufficient to fully overcome iodine's tendency to associate with other I 2 molecules.

Define dipole-dipole interactions .

Intermolecular forces between polar molecules that arise from the electrostatic attraction between the permanent partial positive end of one molecule and the permanent partial negative end of a neighbouring molecule.

How are polar molecules arranged in the solid state compared to the liquid state?

In the solid state, polar molecules are arranged in an ordered way with the positive end of each molecule aligned with the negative end of its neighbour. In the liquid state, molecules move more freely, so the arrangement is disordered with both attractive and repulsive orientations present.

Dipole-dipole forces exist only in .......... molecules, whereas London dispersion forces are present in all covalent molecules.

Dipole-dipole forces exist only in polar molecules, whereas London dispersion forces are present in all covalent molecules.

True or False? Acetonitrile (CH 3 CN, 41 amu) has a higher boiling point than propane (C 3 H 8 , 44 amu) primarily because it has greater London dispersion forces.

False. The two molecules have similar molecular masses, so their London dispersion forces are comparable. Acetonitrile's higher boiling point is due to the additional dipole-dipole interactions that arise from its permanent dipole moment.

What two factors determine the strength of a dipole-dipole interaction?

The magnitude of the molecular dipole moment and the proximity of the interacting molecules. Larger dipoles and closer molecular distances both produce stronger dipole-dipole interactions.

True or False? Repulsive dipole-dipole interactions can occur when two polar molecules are oriented with their negative ends facing each other.

True. When the same partial charges (both δ− or both δ+) face each other, the interaction is repulsive. In liquids, molecules adopt random orientations so both attractive and repulsive interactions occur simultaneously.

Define ion-dipole interaction .

An electrostatic force between an ion ( cation or anion ) and the oppositely charged end of a polar molecule's permanent dipole .

How do ion-dipole interactions explain why ionic compounds dissolve in polar solvents such as water?

Water molecules orient so that their δ− oxygen surrounds cations and their δ+ hydrogens surround anions . These electrostatic attractions between the ions and the molecular dipoles are strong enough to pull the ions apart from the crystal lattice , dissolving the compound.

When NaCl dissolves in water, the Na + ion is attracted to the .......... end of the water dipole.

When NaCl dissolves in water, the Na + ion is attracted to the partial negative (δ−, oxygen) end of the water dipole.

True or False? The magnitude of an ion-dipole interaction increases as the ionic charge increases.

True. A higher ionic charge creates a stronger electrostatic attraction with the dipole. Similarly, a larger dipole moment on the polar molecule also increases the interaction strength.

Why are ion-dipole interactions important in biological systems?

Many biological processes involve ions (such as Na + , K + , Ca 2+ ) interacting with polar water molecules or polar regions of biomolecules. Ion-dipole interactions govern how ions are hydrated, transported across membranes, and bound to protein active sites .

Define dipole moment .

A measure of the net polarity of a molecule, equal to the product of the charge magnitude at each end of the dipole and the distance between them; measured in Debyes (D).

The greater the difference in .......... between two bonded atoms, the greater the dipole moment of the bond.

The greater the difference in electronegativity between two bonded atoms, the greater the dipole moment of the bond.

In a dipole moment arrow, what does the arrowhead represent?

The arrowhead points toward the more electronegative (partial negative, δ−) atom, indicating the direction of electron density shift. The plus sign at the tail marks the less electronegative (partial positive, δ+) atom.

True or False? In the C—Cl bond of chloromethane, the chlorine atom develops a partial positive charge because it is bonded to carbon.

False. Chlorine is more electronegative than carbon, so it develops a partial negative (δ−) charge and the carbon develops the partial positive (δ+) charge. This creates the dipole moment in the C—Cl bond.

What does it mean for a molecule to have a net dipole moment of zero despite containing polar bonds?

If polar bonds are arranged symmetrically, their individual dipole vectors cancel out, giving a net molecular dipole moment of zero (e.g., CO 2 or CCl 4 ). The molecule is nonpolar overall even though its individual bonds are polar.

Define hydrogen bonding .

A strong type of dipole-dipole interaction that occurs when hydrogen is covalently bonded to a highly electronegative atom (N, O or F), creating a δ+ hydrogen that is attracted to the lone pair of a nearby N, O or F atom on another molecule.

True or False? Hydrogen bonds are stronger than typical dipole-dipole interactions and London dispersion forces.

True. Because N, O and F are highly electronegative, they create a very large partial positive charge on the bonded hydrogen, resulting in a stronger-than-typical dipole-dipole attraction called a hydrogen bond.

The three elements whose covalent bonds to hydrogen can produce hydrogen bonding are nitrogen, .......... and fluorine.

The three elements whose covalent bonds to hydrogen can produce hydrogen bonding are nitrogen, oxygen and fluorine.

Why does water have a higher boiling point than hydrogen fluoride, even though H—F forms a stronger individual hydrogen bond?

Each water molecule can form two hydrogen bonds (two δ+ H atoms and two lone pairs on oxygen), whereas HF can form only one (one δ+ H atom despite three lone pairs on F). The greater number of hydrogen bonds per molecule gives water a higher overall boiling point.

True or False? The order of hydrogen bond strength is H—F < H—O < H—N.

False. The correct order is H—N < H—O < H—F . Fluorine is the most electronegative element, making the H—F bond the most polar and forming the strongest individual hydrogen bond. Nitrogen is the least electronegative of the three, giving the weakest.

Acetic acid (CH 3 COOH, 60 amu) has a higher boiling point than propanol (CH 3 CH 2 CH 2 OH, 60 amu). Why?

Both molecules have the same molar mass and can form hydrogen bonds , but a pair of acetic acid molecules can form two hydrogen bonds whereas a pair of propanol molecules can form only one. More hydrogen bonds per pair requires more energy to separate the molecules.

A molecule that lacks a polar H atom cannot form hydrogen bonds with itself, but may form hydrogen bonds with water if water provides the .......... hydrogen.

A molecule that lacks a polar H atom cannot form hydrogen bonds with itself, but may form hydrogen bonds with water if water provides the δ+ hydrogen.

Acetic acid (CH 3 COOH, 60 amu) has a higher boiling point than propanol (CH 3 CH 2 CH 2 OH, 60 amu). Why?

The oxygen atom in acetaldehyde has lone pairs that can accept a hydrogen bond from a δ+ hydrogen on water. Because water supplies the δ+ hydrogen, acetaldehyde can form hydrogen bonds with water molecules even though it has no polar O—H or N—H bond of its own.

What types of intermolecular interactions are found within and between large biomolecules?

The same intermolecular forces found between small molecules — hydrogen bonding , dipole-dipole, and London dispersion forces — operate within and between large biomolecules such as proteins, carbohydrates and lipids.

The two strands of a DNA double helix are held together by .......... bonds between the nitrogenous base components.

The two strands of a DNA double helix are held together by hydrogen bonds between the nitrogenous base components.

True or False? Lipids are insoluble in water because their nonpolar tails cannot form hydrogen bonds with water molecules.

True. The hydrophobic (nonpolar) tails of lipids lack the polar bonds needed to form hydrogen bonds with water. Without this interaction, water molecules exclude lipid tails, making lipids insoluble.

Why are carbohydrates soluble in water while lipids are not?

Carbohydrates have many polar O—H groups that can form hydrogen bonds with water molecules ( hydrophilic interaction), making them soluble. Lipid tails are nonpolar and hydrophobic , so they cannot form hydrogen bonds with water and are therefore insoluble.

True or False? Intermolecular interactions in biomolecules can only occur between separate molecules, not within different regions of the same molecule.

False. Intermolecular interactions in biomolecules occur both between separate molecules and between different regions within the same large molecule (intramolecular secondary interactions), which is key to protein folding and DNA structure.

What term describes a molecule or molecular region that is attracted to water?

A hydrophilic ("water-loving") molecule or region is attracted to water. The attraction is driven by the ability to form hydrogen bonds with water molecules, as seen in the polar O—H groups of carbohydrates.

AP®ChemistryCollege BoardFlashcardsUnit 3: Properties of Substances & MixturesIntermolecular & Interparticle Forces

Intermolecular & Interparticle Forces (College Board AP® Chemistry): Flashcards

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Cards in this collection (42)

Define intermolecular forces.
Weak electrostatic forces that exist between atoms or molecules, responsible for physical properties such as boiling point, melting point, density, viscosity and solubility.
True or False?
Intermolecular forces are generally stronger than intramolecular forces such as covalent bonds.
False.
Intermolecular forces are generally weaker than intramolecular forces (ionic bonds, covalent bonds, metallic bonds). They act between molecules rather than within them.
Define London dispersion forces.
Weak intermolecular forces produced by instantaneous dipoles arising from the temporary uneven distribution of electrons. They exist in all atoms and molecules and are the only intermolecular force present in nonpolar molecules and noble gases.
London dispersion forces arise because electrons are in constant motion, creating an .......... that induces a dipole in a neighbouring molecule.
London dispersion forces arise because electrons are in constant motion, creating an instantaneous dipole that induces a dipole in a neighbouring molecule.
Why do London dispersion forces increase in strength as molecular size increases?
Larger molecules have more electrons, which increases polarizability — the ease with which the electron cloud can be distorted. Greater polarizability produces stronger instantaneous dipoles and therefore stronger London dispersion forces.
True or False?
A more branched molecule has stronger London dispersion forces than a less branched molecule of the same molecular mass.
False.
Greater branching makes a molecule more compact and less polarizable. This reduces the strength of London dispersion forces, which is why more branched isomers have lower boiling points.
Why do the boiling points of noble gases increase down Group 18?
Each successive noble gas has more electrons, increasing polarizability. This leads to stronger London dispersion forces between atoms, requiring more energy to overcome them and raising the boiling point.
Define dipole-induced dipole interaction.
A weak intermolecular attraction between a polar molecule and a nonpolar molecule, in which the permanent dipole of the polar molecule distorts the electron distribution of the nonpolar molecule, creating a temporary induced dipole.
How does iodine dissolve partially in water via a dipole-induced dipole interaction?
The partial negative charge on the oxygen atom of a polar water molecule distorts the electron cloud of the nonpolar iodine molecule, inducing a temporary induced dipole. The resulting weak attraction between the induced dipole and the water molecule allows limited dissolution.
The strength of a dipole-induced dipole interaction depends on the magnitude of the dipole moment in the polar molecule and the .......... of the nonpolar molecule.
The strength of a dipole-induced dipole interaction depends on the magnitude of the dipole moment in the polar molecule and the polarizability of the nonpolar molecule.
True or False?
Dipole-induced dipole interactions can occur between two nonpolar molecules.
False.
A dipole-induced dipole interaction requires one polar molecule to provide the permanent dipole that distorts the electron cloud of the nonpolar molecule. Two nonpolar molecules interact only via London dispersion forces.
Why is iodine only slightly soluble in water rather than freely soluble?
The dipole-induced dipole interaction between water and I₂ is weak. Because the induced dipole in I₂ is temporary and relatively small, the attractive force is insufficient to fully overcome iodine's tendency to associate with other I₂ molecules.
Define dipole-dipole interactions.
Intermolecular forces between polar molecules that arise from the electrostatic attraction between the permanent partial positive end of one molecule and the permanent partial negative end of a neighbouring molecule.
How are polar molecules arranged in the solid state compared to the liquid state?
In the solid state, polar molecules are arranged in an ordered way with the positive end of each molecule aligned with the negative end of its neighbour. In the liquid state, molecules move more freely, so the arrangement is disordered with both attractive and repulsive orientations present.
Dipole-dipole forces exist only in .......... molecules, whereas London dispersion forces are present in all covalent molecules.
Dipole-dipole forces exist only in polar molecules, whereas London dispersion forces are present in all covalent molecules.
True or False?
Acetonitrile (CH₃CN, 41 amu) has a higher boiling point than propane (C₃H₈, 44 amu) primarily because it has greater London dispersion forces.
False.
The two molecules have similar molecular masses, so their London dispersion forces are comparable. Acetonitrile's higher boiling point is due to the additional dipole-dipole interactions that arise from its permanent dipole moment.
What two factors determine the strength of a dipole-dipole interaction?
The magnitude of the molecular dipole moment and the proximity of the interacting molecules. Larger dipoles and closer molecular distances both produce stronger dipole-dipole interactions.
True or False?
Repulsive dipole-dipole interactions can occur when two polar molecules are oriented with their negative ends facing each other.
True.
When the same partial charges (both δ− or both δ+) face each other, the interaction is repulsive. In liquids, molecules adopt random orientations so both attractive and repulsive interactions occur simultaneously.
Define ion-dipole interaction.
An electrostatic force between an ion (cation or anion) and the oppositely charged end of a polar molecule's permanent dipole.
How do ion-dipole interactions explain why ionic compounds dissolve in polar solvents such as water?
Water molecules orient so that their δ− oxygen surrounds cations and their δ+ hydrogens surround anions. These electrostatic attractions between the ions and the molecular dipoles are strong enough to pull the ions apart from the crystal lattice, dissolving the compound.
When NaCl dissolves in water, the Na⁺ ion is attracted to the .......... end of the water dipole.
When NaCl dissolves in water, the Na⁺ ion is attracted to the partial negative (δ−, oxygen) end of the water dipole.
True or False?
The magnitude of an ion-dipole interaction increases as the ionic charge increases.
True.
A higher ionic charge creates a stronger electrostatic attraction with the dipole. Similarly, a larger dipole moment on the polar molecule also increases the interaction strength.
Why are ion-dipole interactions important in biological systems?
Many biological processes involve ions (such as Na⁺, K⁺, Ca²⁺) interacting with polar water molecules or polar regions of biomolecules. Ion-dipole interactions govern how ions are hydrated, transported across membranes, and bound to protein active sites.
Define dipole moment.
A measure of the net polarity of a molecule, equal to the product of the charge magnitude at each end of the dipole and the distance between them; measured in Debyes (D).
The greater the difference in .......... between two bonded atoms, the greater the dipole moment of the bond.
The greater the difference in electronegativity between two bonded atoms, the greater the dipole moment of the bond.
In a dipole moment arrow, what does the arrowhead represent?
The arrowhead points toward the more electronegative (partial negative, δ−) atom, indicating the direction of electron density shift. The plus sign at the tail marks the less electronegative (partial positive, δ+) atom.
True or False?
In the C—Cl bond of chloromethane, the chlorine atom develops a partial positive charge because it is bonded to carbon.
False.
Chlorine is more electronegative than carbon, so it develops a partial negative (δ−) charge and the carbon develops the partial positive (δ+) charge. This creates the dipole moment in the C—Cl bond.
What does it mean for a molecule to have a net dipole moment of zero despite containing polar bonds?
If polar bonds are arranged symmetrically, their individual dipole vectors cancel out, giving a net molecular dipole moment of zero (e.g., CO₂ or CCl₄). The molecule is nonpolar overall even though its individual bonds are polar.
Define hydrogen bonding.
A strong type of dipole-dipole interaction that occurs when hydrogen is covalently bonded to a highly electronegative atom (N, O or F), creating a δ+ hydrogen that is attracted to the lone pair of a nearby N, O or F atom on another molecule.
True or False?
Hydrogen bonds are stronger than typical dipole-dipole interactions and London dispersion forces.
True.
Because N, O and F are highly electronegative, they create a very large partial positive charge on the bonded hydrogen, resulting in a stronger-than-typical dipole-dipole attraction called a hydrogen bond.
The three elements whose covalent bonds to hydrogen can produce hydrogen bonding are nitrogen, .......... and fluorine.
The three elements whose covalent bonds to hydrogen can produce hydrogen bonding are nitrogen, oxygen and fluorine.
Why does water have a higher boiling point than hydrogen fluoride, even though H—F forms a stronger individual hydrogen bond?
Each water molecule can form two hydrogen bonds (two δ+ H atoms and two lone pairs on oxygen), whereas HF can form only one (one δ+ H atom despite three lone pairs on F). The greater number of hydrogen bonds per molecule gives water a higher overall boiling point.
True or False?
The order of hydrogen bond strength is H—F < H—O < H—N.
False.
The correct order is H—N < H—O < H—F. Fluorine is the most electronegative element, making the H—F bond the most polar and forming the strongest individual hydrogen bond. Nitrogen is the least electronegative of the three, giving the weakest.
Acetic acid (CH₃COOH, 60 amu) has a higher boiling point than propanol (CH₃CH₂CH₂OH, 60 amu). Why?
Both molecules have the same molar mass and can form hydrogen bonds, but a pair of acetic acid molecules can form two hydrogen bonds whereas a pair of propanol molecules can form only one. More hydrogen bonds per pair requires more energy to separate the molecules.
A molecule that lacks a polar H atom cannot form hydrogen bonds with itself, but may form hydrogen bonds with water if water provides the .......... hydrogen.
A molecule that lacks a polar H atom cannot form hydrogen bonds with itself, but may form hydrogen bonds with water if water provides the δ+ hydrogen.
Acetic acid (CH₃COOH, 60 amu) has a higher boiling point than propanol (CH₃CH₂CH₂OH, 60 amu). Why?
The oxygen atom in acetaldehyde has lone pairs that can accept a hydrogen bond from a δ+ hydrogen on water. Because water supplies the δ+ hydrogen, acetaldehyde can form hydrogen bonds with water molecules even though it has no polar O—H or N—H bond of its own.
What types of intermolecular interactions are found within and between large biomolecules?
The same intermolecular forces found between small molecules — hydrogen bonding, dipole-dipole, and London dispersion forces — operate within and between large biomolecules such as proteins, carbohydrates and lipids.
The two strands of a DNA double helix are held together by .......... bonds between the nitrogenous base components.
The two strands of a DNA double helix are held together by hydrogen bonds between the nitrogenous base components.
True or False?
Lipids are insoluble in water because their nonpolar tails cannot form hydrogen bonds with water molecules.
True.
The hydrophobic (nonpolar) tails of lipids lack the polar bonds needed to form hydrogen bonds with water. Without this interaction, water molecules exclude lipid tails, making lipids insoluble.
Why are carbohydrates soluble in water while lipids are not?
Carbohydrates have many polar O—H groups that can form hydrogen bonds with water molecules (hydrophilic interaction), making them soluble. Lipid tails are nonpolar and hydrophobic, so they cannot form hydrogen bonds with water and are therefore insoluble.
True or False?
Intermolecular interactions in biomolecules can only occur between separate molecules, not within different regions of the same molecule.
False.
Intermolecular interactions in biomolecules occur both between separate molecules and between different regions within the same large molecule (intramolecular secondary interactions), which is key to protein folding and DNA structure.
What term describes a molecule or molecular region that is attracted to water?
A hydrophilic ("water-loving") molecule or region is attracted to water. The attraction is driven by the ability to form hydrogen bonds with water molecules, as seen in the polar O—H groups of carbohydrates.

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