Intermolecular Forces: Videos & Practice Problems

Understanding the distinction between intramolecular and intermolecular forces is crucial for grasping the fundamentals of chemistry. Intramolecular forces are the strong bonds that hold atoms together within a molecule, influencing its chemical properties. These include ionic and covalent bonds. In contrast, intermolecular forces are weaker and occur between molecules, affecting their physical properties and keeping liquids and solids intact. These forces arise due to differences in electronegativity, resulting in partial charges that attract molecules to each other.

There are four main types of intermolecular forces, each with varying strengths and characteristics. Ion-dipole forces are the strongest, occurring between ions and polar molecules, typically seen when ionic compounds dissolve in water. Hydrogen bonding, the second strongest, happens when hydrogen is directly bonded to fluorine, oxygen, or nitrogen. Dipole-dipole interactions are the third in strength and occur between two polar molecules. Lastly, London dispersion forces are the weakest and exist in all compounds, including nonpolar molecules, which can momentarily induce dipoles due to close proximity. Despite their relative weakness, London dispersion forces are universally present and play a significant role in molecular interactions.

Intermolecular vs Intramolecular Forces

Intermolecular forces and intramolecular forces are two fundamental types of electrostatic forces that play crucial roles in the behavior of substances. Intramolecular forces are the attractive forces that exist within a molecule, bonding atoms together and significantly influencing the chemical properties of that molecule. These forces are categorized into two main types: ionic and covalent bonds. It is important to note that intramolecular forces are generally stronger than intermolecular forces.

In contrast, intermolecular forces are the attractive forces that occur between separate molecules. These forces are essential for holding together the molecules in liquids and solids, thereby influencing their physical properties. For example, in water (H₂O), the intramolecular force is the bond between the oxygen atom and the hydrogen atoms, which is a strong covalent bond. On the other hand, the intermolecular forces are the attractions between different water molecules.

The concept of electronegativity is key to understanding these intermolecular attractions. In water, oxygen is more electronegative than hydrogen, resulting in a partial negative charge on the oxygen atom and a partial positive charge on the hydrogen atoms. This difference in polarity leads to the attraction between water molecules, which is represented by a dotted line in diagrams, indicating that it is an attractive force rather than a true bond.

In summary, while intramolecular forces create the bonds that define the structure of a molecule, intermolecular forces govern the interactions between molecules, affecting the physical state and properties of substances.

Intermolecular vs Intramolecular Example

Understanding the distinction between intermolecular and intramolecular forces is crucial in chemistry. Intermolecular forces are the attractive forces that occur between molecules, while intramolecular forces refer to the bonds that hold atoms together within a molecule.

For instance, consider the condensation of water vapor, where water transitions from a gaseous to a liquid state. In this process, water molecules come closer together, exhibiting attractive forces without forming new bonds. Therefore, this scenario involves intermolecular forces.

In contrast, the formation of ammonia (NH₃) from nitrogen (N₂) and hydrogen (H₂) is an example of intramolecular forces at work. Here, new bonds are created as the atoms combine to form a new substance.

When sugar dissolves in water, both substances are polar, but they do not create new bonds. Instead, they interact to form a solution, which is another example of intermolecular forces. Similarly, the phenomenon of capillary action, where water moves up the veins of a plant, is driven by intermolecular forces, specifically hydrogen bonding. This property allows water to travel from the roots to the leaves, demonstrating the significance of intermolecular interactions in biological systems.

In summary, recognizing whether a situation involves intermolecular or intramolecular forces helps in understanding the behavior of substances in different states and interactions.

The dominant forces between molecules (intermolecular forces) are ____ in origin.

• A. electrostatic
• B. electrodynamic
• C. electromagnetic
• D. gravitational
• E. magnetic

Answer: (A)

Intermolecular forces are: 

• A. between molecules and weaker than a chemical bond
• B. between two atoms within a molecule and weaker than a chemical bond
• C. between molecules and stronger than a chemical bond
• D. between two atoms within a molecule and stronger than a chemical bond.
• E. between electrons within an atom and stronger than a chemical bond

Answer: (A)

Types of Intermolecular Forces

Intermolecular forces are crucial in understanding how molecules interact with one another. There are four primary types of intermolecular forces, each varying in strength and characteristics, significantly influenced by the polarity of the compounds involved.

The strongest intermolecular force is the ion-dipole force, which occurs between ions and polar molecules. This force is particularly evident when ionic compounds, such as sodium chloride (NaCl), dissolve in water. In this scenario, the positive sodium ions (Na⁺) are attracted to the partially negative oxygen atoms of water, while the negative chloride ions (Cl^-) are attracted to the partially positive hydrogen atoms of water. This interaction is often represented by dotted lines indicating attraction, illustrating how water molecules surround and solvate the ions.

Next in strength is hydrogen bonding, a specific type of dipole-dipole interaction that occurs when hydrogen is directly bonded to highly electronegative atoms like fluorine (F), oxygen (O), or nitrogen (N). For example, in a solution containing ammonium ions (NH₄⁺) and water, the hydrogen atoms of water can form hydrogen bonds with the lone pairs of oxygen in the ammonium ion. This type of bonding is essential for the unique properties of water and many biological molecules.

The third type of intermolecular force is dipole-dipole interactions, which occur between two polar covalent molecules. For instance, in a mixture of hydrogen chloride (HCl) and sulfur dioxide (SO₂), the positive end of HCl can attract the negative end of SO₂, leading to an attractive force between the two polar molecules. This interaction is only possible due to the presence of permanent dipoles in both molecules.

Finally, the weakest intermolecular force is known as London dispersion forces, also referred to as dispersion forces or van der Waals forces. These forces are predominant in nonpolar covalent compounds, such as methane (CH₄) and carbon tetrachloride (CCl₄). Although these molecules are nonpolar, they can develop temporary dipoles when they come close to each other, leading to a momentary attraction. Despite being the weakest, London dispersion forces are present in all types of compounds, including those with stronger intermolecular forces, and play a significant role in molecular interactions.

In summary, understanding these four types of intermolecular forces—ion-dipole, hydrogen bonding, dipole-dipole, and London dispersion forces—provides insight into the behavior of substances in different states and their interactions in various chemical contexts. Each type of force contributes to the overall properties of substances, influencing boiling points, solubility, and molecular stability.

Intermolecular Forces Example

Understanding intermolecular forces is crucial for predicting the behavior of substances. Intermolecular forces are the forces of attraction or repulsion between neighboring particles (atoms, molecules, or ions). Here, we will explore the major types of intermolecular forces for various compounds.

Starting with nitrogen gas (N₂), it consists of two nitrogen atoms bonded together. Since both atoms have the same electronegativity, the bond is nonpolar covalent. The primary intermolecular force present in N₂ is London dispersion forces, also known as dispersion forces or van der Waals forces. These forces arise due to temporary dipoles that occur when electron distributions around atoms fluctuate.

Next, consider methanol (CH₃

When examining methane (CH₄) in the presence of water (H₂O), we note that water is a polar solvent while methane is a nonpolar hydrocarbon. The interaction between a polar and a nonpolar molecule leads to dipole-induced dipole forces, where the polar molecule induces a temporary dipole in the nonpolar molecule.

In the case of chloromethane (CH₃

Finally, potassium chloride (KCl) is an ionic compound formed from a metal (potassium) and a nonmetal (chlorine). When mixed with methanol, which exhibits hydrogen bonding, the interaction between the ionic compound and the polar solvent results in ion-dipole forces. This type of force is significant in solutions where ionic compounds dissolve in polar solvents.

In summary, the major types of intermolecular forces identified include:

• N₂: London dispersion forces
• CH₃OH: Hydrogen bonding
• CH₄ and H₂O: Dipole-induced dipole
• CH₃Cl: Dipole-dipole
• KCl and CH₃OH: Ion-dipole

Which of the following intermolecular forces are found in ALL molecules?

• A. Ionic forces
• B. Ion-dipole forces
• C. Hydrogen bond forces
• D. Dipole-dipole forces
• E. Dipole/Induced-Dipole forces
• F. London dispersion forces

Answer: (F)

Which of these molecules exhibit the highest number of different intermolecular forces?

• A. CH₃CH₂CH₃
• B. CH₃CH₂OH
• C. CH₂O
• D. Li₂CO₃

Answer: (B)

It is common to add Epson salts to bath water when one has been over exercising and has sore muscles. What is the primary intermolecular force that exists between magnesium sulfate, the primary in Epson salts, and the water in the bathtub?

• A. dipole-dipole forces
• B. ion-dipole forces
• C. London forces
• D. Hydrogen bonding
• E. ionic forces
• F. dipole/induced-dipole

Answer: (B)

Which species is expected to have the largest dispersion forces?

• A. CH₄
• B. CH₃CH₃
• C. CH₃CH₂CH₃
• D. C₁₂H₂₆

Answer: (D)

Here’s what students ask on this topic:

What is the difference between intermolecular forces and intramolecular forces?

Intermolecular forces and intramolecular forces are two distinct types of forces that dictate how molecules interact and bond with each other.

Intermolecular forces are the forces of attraction or repulsion that act between neighboring particles (atoms, molecules, or ions). They are responsible for holding substances together and determine many of their physical properties, such as boiling points, melting points, and solubility. Examples of intermolecular forces include hydrogen bonds, London dispersion forces, and dipole-dipole interactions.

In contrast, intramolecular forces are the forces that hold the atoms within a molecule together. They are the chemical bonds that involve the sharing or transfer of electrons between atoms. These forces include covalent bonds, ionic bonds, and metallic bonds. In comparison to intermolecular forces, intramolecular forces are generally much stronger and are responsible for the actual structure of molecules.

Which substance is most likely to have the strongest intermolecular forces?

The strength of intermolecular forces in a substance is determined by the type of interactions that occur between its molecules. Generally, substances with hydrogen bonding tend to have the strongest intermolecular forces. Hydrogen bonding occurs when a hydrogen atom is covalently bonded to a highly electronegative atom such as nitrogen, oxygen, or fluorine, and this hydrogen atom is also attracted to an electronegative atom in a nearby molecule.

Substances with dipole-dipole interactions also have strong intermolecular forces, but these are generally weaker than hydrogen bonds. Dipole-dipole interactions occur between polar molecules, where the positive end of one molecule is attracted to the negative end of another.

Finally, London dispersion forces are the weakest intermolecular forces and are present in all molecules, particularly in nonpolar molecules. These forces are due to temporary dipoles that occur when electron distribution within a molecule is momentarily uneven.

Therefore, a substance with hydrogen bonding is most likely to have the strongest intermolecular forces compared to substances with only dipole-dipole interactions or London dispersion forces.

How do you determine intermolecular forces?

To determine intermolecular forces, you first need to consider the types of molecules involved and their respective properties. There are three main types of intermolecular forces:

1. London Dispersion Forces (LDF): These are the weakest and occur in all molecules, whether they are polar or nonpolar. They are caused by the temporary dipoles that occur when electron density within a molecule fluctuates. Larger and more polarizable molecules will have stronger London dispersion forces.
2. Dipole-Dipole Interactions: These occur between polar molecules, where the positive end of one molecule is attracted to the negative end of another. The strength of these forces increases with the polarity of the molecule.
3. Hydrogen Bonds: These are a special type of dipole-dipole interaction that occurs when hydrogen is bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine. The hydrogen atom carries a partial positive charge and is attracted to the lone pair electrons of electronegative atoms in nearby molecules.

To identify which forces are at play, look at the molecular structure and electronegativity differences. Nonpolar molecules only exhibit London dispersion forces. Polar molecules exhibit both London dispersion forces and dipole-dipole interactions. If the molecule has hydrogen directly bonded to N, O, or F, hydrogen bonding.

What are intermolecular forces?

Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles (atoms, molecules, or ions). These forces are not as strong as intramolecular forces, such as covalent or ionic bonds within a molecule, but they are critical for determining the physical properties of substances.

There are several types of intermolecular forces:

1. Van der Waals Forces: These are the weakest intermolecular forces and include:
   • London Dispersion Forces: Temporary, instantaneous dipoles that occur in all atoms and molecules, causing a transient attraction.
   • Dipole-Dipole Interactions: Occur between polar molecules with permanent dipole moments.
   • Hydrogen Bonds: A special type of dipole-dipole interaction that occurs when a hydrogen atom is bonded to a highly electronegative atom (like oxygen, nitrogen, or fluorine) and is attracted to another electronegative atom.
2. Ion-Dipole Forces: These are stronger than van der Waals forces and occur when an ion is attracted to a polar molecule.

These intermolecular forces are essential for understanding boiling and melting points, solubility, viscosity, and surface tension among other physical properties of substances.