Intermolecular Forces (Simplified): Videos & Practice Problems

Intermolecular forces are crucial for understanding the physical properties of substances. They include ion-dipole forces, the strongest type, which occur between ions and polar compounds, such as sodium chloride in water. Hydrogen bonding, the second strongest, involves hydrogen connected to fluorine, oxygen, or nitrogen. Dipole-dipole forces arise between polar covalent compounds, while London dispersion forces, the weakest, occur in nonpolar compounds. Despite their strength, London dispersion forces are present in all substances, highlighting their significance in molecular interactions.

Intermolecular Forces (Simplified) Concept 1

We can talk about the intermolecular forces, realizing that it is one of two major electrostatic forces, the other one being intramolecular forces. Alright? So we have these two major electrostatic forces, i.e., attractive forces that exist. So intramolecular exists within a molecule, and they bond atoms together and influence the chemical properties of those molecules. We're going to say here this influences chemical bonds, which we break down into ionic and covalent, and realize that when it comes to intramolecular forces, they are stronger than intermolecular forces. Now, what exactly are intermolecular forces then? Well, these are the attractive forces that exist between molecules and they influence the physical properties. Now, they hold liquid and solid molecules together so they are important.

So if we take a look at the visual representation of an intramolecular force versus an intermolecular force, 'intra' is inside the molecule, so here we're talking about the actual chemical bond between an oxygen atom and a hydrogen atom within water. 'Inter' is always between molecules. So here we're talking about the attractive forces between two water molecules. And in this image, why are they attracted to one another? Well, remember electronegativity. Hydrogens are less electronegative so it's partially positive. Oxygen is more electronegative, so it's partially negative. Their differences in polarity make them attracted to one another, and this dotted line shows that attractive force. So realize that an intramolecular force is the literal bond between atoms. An intermolecular force is not an actual bond. You'll see that it's shown as a dotted line, symbolizing an attraction between molecules, not a literal bond between molecules.

Intermolecular Forces (Simplified) Example 1

In this example question, it asks to identify the type of force involved in the situations below as either intermolecular or intramolecular. Remember, intermolecular forces are the non-bonding forces that exist. They're really just the attractive forces that exist between molecules, not the actual bonds. Intramolecular forces deal with the bonding forces. So we're talking about atoms connecting to each other.

First, we have the condensation of water vapor. This just means that water is going from its gaseous phase to its liquid phase. Here, we have closer interactions between different water molecules with each other. We have closer attractive forces for one another, but we're not really making new bonds here. Thus, this would be an intermolecular force.

Next, we have the formation of ammonia through the combination of nitrogen and hydrogen. In this combination, we're forming something new by taking N₂ and H₂ to make NH₃. So, we're making new bonds. This would be an intramolecular force.

Next, sugar dissolves in water. Here, sugar is polar, water is polar, but we're not really forming a brand-new bond between sugar and water. They are dissolving into each other to make a solution here. So this would be an intermolecular force.

Finally, water flowing up the veins of a plant due to capillary action. Capillary action is actually a property that's the result of an intermolecular force. This would be intermolecular. The hydrogen bonding that exists in water allows it to perform this type of phenomenon where it's actually going to move up the roots and the stems of a plant so it can reach the top part of the plant and feed its leaves and the top portion of it. So just remember, capillary action is one of those special properties that's a result of intermolecular forces.

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)

Intermolecular Forces (Simplified) Concept 2

When talking about the types of intermolecular forces, realize that there are 4 types of them, and they are the forces that attract molecules together. Now here we're going to say the polarity of compounds plays a big role in the type of force present. So let's go through this. We have our types of forces, then we have the they exist between, we talk about their strengths, and then we give an example. So the first type we have is ion-dipole. This is the force that exists between ions, so positive and negative ions, and polar compounds. They are the strongest of the intermolecular forces. So anytime you see an ionic compound, you can assume that water is the solvent that it can be dissolved in. Usually, you'll see an ionic compound with water, but let's say you see only an ionic compound and they ask what the intermolecular force is, just assume it's dissolved in water. When we dissolve sodium chloride in water, we have ions and Na⁺ aqueous and Cl^- aqueous. Remember, aqueous just means that the ion is submerged in water. And what's actually happening is that we have our sodium ion here which is positive, the oxygen end of water which is partially negative, would be attracted to it because opposite charges attract. The attractive force would be illustrated by this dotted line. Cl^- is negative, so it would be the hydrogen end of water that would be attracted to it because water would be partially positive here, and the attractive force between chlorine and the hydrogen will be symbolized by that dotted line. So this is what we mean when we say an ion is aqueous in solution. It's water molecules surrounding it, basically dissolving it. We're gonna say solvating it. These are terms they usually use. So ion-dipole is the strongest force. It's indicative of an ionic compound being dissolved within a polar substance, such as water.

The next one is hydrogen bonding. This is when we have compounds containing hydrogen directly connected to F, O, N, fluorine, oxygen, and nitrogen. So here, this one is the second strongest after ion-dipole. Here we have water and we have NH₄⁺. NH₄⁺ is a polyatomic ion, it's the ammonium ion. We could show water molecules being attracted to one another, and the dotted lines show that attraction, or we could show NH₄⁺ molecules being attracted to the water, so hydrogen here is partially positive. Oxygen and its lone pair would be attracted to one of these hydrogens. So it's those dotted lines between these molecules that represent the hydrogen bonding. But hydrogen bonding itself can't exist unless a molecule is showing an attractive force between hydrogen, fluorine, oxygen, or nitrogen.

The next one is dipole-dipole. Now, before we talk about dipole-dipole, realize that hydrogen bonding is a special type of dipole-dipole. It's also dipole-dipole itself; it's just a hydrogen bonding type of dipole-dipole. This is when you have 2 polar covalent compounds. This one here is the 3rd strongest of all the intermolecular forces. So here we have HCl, which is polar, and SO₂, which is polar. And here we could show the attraction between them. So remember chlorine has its lone pairs. It's more electronegative than hydrogen, so it would be partially negative. Hydrogen will be partially positive. Within SO₂, you could show the sulfur with its lone pairs, oxygen with its lone pairs. Oxygen is more electronegative, so it'd be partially negative. It would form an attractive connection to the hydrogen of HCl. That dotted line would be the dipole-dipole interaction between these two molecules, and it could only happen because both molecules are polar.

Finally, we have the last and the weakest intermolecular force, which is called London dispersion, and sometimes it's called dispersion forces, and sometimes it's called van der Waals forces. This is the dominant force between 2 nonpolar covalent compounds. Here we have CH₄, which is a hydrocarbon, and here we have CCl₄, which is nonpolar as well. Now, if you don't know this, make sure you go back and take a look at my videos on molecular polarity, where we show how to draw these structures and explain why exactly they're nonpolar. So here, we have CH₄ and here we have CCl₄. Now both of these are nonpolar, so why exactly would they all of a sudden be attracted to one another? Well, that's because when molecules come too close together, there is an instance because of their close proximity to one another that they developed a slight polarity. That slight polarity between each other causes them to become attracted to one another. So we'd say that there is some type of attraction between this molecule and this molecule because of that instance, that moment where one being too close to another makes them both slightly polar for a moment. Overall, they're still nonpolar though. We're just talking about that split moment. Now these are our rankings of the intermolecular forces and what's important to know here is that the London dispersion forces, although they're the weakest, they are actually present in all types of compounds. So water, which has its main intermolecular forces being hydrogen bonding, actually has some London dispersion forces in it as well. This ionic compound being dissolved in water, its major force is ion-dipole, but it also has a little bit of London dispersion forces in it as well. So just realize that although dispersion forces are the weakest, they are present in all compounds. So they serve an important purpose.

Intermolecular Forces (Simplified) Example 2

Here it says to identify the major type of intermolecular force between the particles of each of the following. So first, we're starting out with N₂. N₂ is two nitrogens bonded together, and if we assume we are in a container filled with N₂ and they are all N₂, they are all nonpolar because they are the same exact element bonded to each other. They share electrons equally and perfectly with one another. Because they are nonpolar, their major force is London dispersion, aka dispersion forces, aka van der Waals forces.

The next one, CH₃OH. The important thing here is that we have an O here, and right next to it is an H. When you have hydrogen connected to F, O, or N, it is hydrogen bonded.

The next one, CH₃Cl. If we were to draw this, we have carbon in the center connected to 3 hydrogens and then connected to a chlorine. For those of you who remember my videos on molecular polarity, we would know that this is not a perfect shape. A perfect shape would mean that all the surrounding elements are the same, but they are not. Three are hydrogens and one is a chlorine. Because it's not a perfect shape, it is a polar molecule. It's a polar covalent molecule, so its major force is dipole-dipole.

And then finally, we have KCl, which is an ionic compound. And we just said that this CH₃OH, which is called methanol, had hydrogen bonding, which is a polar force. So you have an ionic compound with a polar compound. So this would be ion-dipole.

So, remember, to be able to successfully identify all the intermolecular forces, it's going to require you remember some of our earlier topics when it comes to molecular polarity, when it comes to dipole moments. So if you haven't watched those videos or it's been a while, I suggest you go back and take a look because knowing the polarity of these compounds is sometimes key to determining the correct intermolecular force.

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. London dispersion forces

Answer: (E)

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

• A.
• B.
• C.

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

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 are the main types of intermolecular forces?

The main types of intermolecular forces are ion-dipole, hydrogen bonding, dipole-dipole, and London dispersion forces. Ion-dipole forces occur between ions and polar compounds and are the strongest. Hydrogen bonding, the second strongest, involves hydrogen atoms bonded to fluorine, oxygen, or nitrogen. Dipole-dipole forces arise between polar covalent compounds, while London dispersion forces, the weakest, occur in nonpolar compounds. Despite their relative weakness, London dispersion forces are present in all substances, contributing to molecular interactions.

How do ion-dipole forces work?

Ion-dipole forces occur between ions and polar molecules. For example, when sodium chloride (NaCl) dissolves in water, the Na⁺ ions are attracted to the partially negative oxygen atoms in water molecules, while the Cl⁻ ions are attracted to the partially positive hydrogen atoms. This attraction is represented by dotted lines and is the strongest type of intermolecular force. These forces are crucial for the solvation process, where ions are surrounded and stabilized by water molecules.

What is hydrogen bonding and why is it important?

Hydrogen bonding is a type of intermolecular force that occurs when hydrogen is directly bonded to fluorine, oxygen, or nitrogen. This bond is highly polar, making the hydrogen partially positive and the other atom partially negative. This polarity leads to strong attractions between molecules, such as in water (H₂O) and ammonia (NH₃). Hydrogen bonding is crucial for many biological processes, including the structure of DNA and the properties of water, such as its high boiling point and surface tension.

What are London dispersion forces and where do they occur?

London dispersion forces, also known as van der Waals forces, are the weakest type of intermolecular force. They occur in all molecules, but are the dominant force in nonpolar compounds. These forces arise due to temporary dipoles created when electrons in adjacent molecules are distributed asymmetrically. This temporary polarity induces a similar polarity in neighboring molecules, leading to an attraction. Despite their weakness, London dispersion forces are significant because they are present in all substances, contributing to the overall intermolecular interactions.

How do dipole-dipole forces differ from hydrogen bonding?

Dipole-dipole forces occur between polar covalent molecules where the positive end of one molecule is attracted to the negative end of another. Hydrogen bonding is a specific, stronger type of dipole-dipole interaction that occurs only when hydrogen is bonded to fluorine, oxygen, or nitrogen. While both involve attractions between polar molecules, hydrogen bonds are significantly stronger due to the high polarity and small size of the hydrogen atom, which allows for closer interactions.