Think about a world without intermolecular forces. Without these forces of attraction, nothing would be what it is! Hydrogen bonding, which is a type of intermolecular force, would not hold the double-helix of DNA together, plants would not be able to move water up the xylem tube and insects would not be able to stick to walls! Simply put without intermolecular forces there is no life at all!
- This article is abour the strength of intermolecular forces.
- First, we will define intermolecular forces and look at the strength of intermolecular forces in solids, liquids, and gases.
- Then, we will dive into some properties that affect intermolecular force strength.
- Lastly, we will look at the intermolecular forces present in acetone.
Strength of Intermolecular Forces in Solids, Liquids, and Gases
Intermolecular forces are attractive forces that hold neighboring molecules together. Intermolecular forces affect the physical properties of molecules.
Intermolecular forces are referred to as forces of attraction between particles of a substance.
There are four types of intermolecular forces you should be familiar with, as you will most likely see them in your AP exam!
- Ion-dipole forces: attractive forces that occur between an ion and a polar (dipole) molecule.
- Hydrogen Bonding: forces of attraction between a hydrogen atom covalently bonded to a highly electronegative atom (F, N or O) and the F, N or O of another molecule.
- Dipole-dipole forces: attractive forces that occur between the positive end of a polar molecule and the negative end of another polar molecule. In dipole-dipole forces, the larger the dipole moment, the greater the force.
- London dispersion forces: weak, attractive forces that is present in all molecules. It is also the only intermolecular force present in non-polar molecules. LDF is dependent on size and surface area. Heavier molecules (higher molecular weight) and also molecules with a larger surface area all result in higher London dispersion forces.
The relative strength of these intermolecular forces is shown below.
Fig. 1: Relative strength of intermolecular forces, Isadora Santos - StudySmarter Originals.
The state of matter of a substance is dependent on both the strength of intermolecular forces and the amount of kinetic energy a substance has. In general, Intermolecular forces decrease when you go from solids to liquids to gases. So, solids have strong intermolecular forces that hold particles together in place. Liquids have intermediate forces that are able to keep particles close while allowing them to move. Gases have the smallest amount of intermolecular forces present and these forces are said to be negligible.
You can learn more about the properties of gases by reading "Gases".
Effects of Intermolecular Forces on Physical Properties
Higher intermolecular forces result in:
- Greater viscosity
- Greater Surface Tension
- Increased solubility
- Higher melting point
- Higher boiling point
- Lower vapor pressure
First, let's talk about viscosity. Viscosity is a property seen in liquids, and it measures the resistance of a liquid to flow. Liquids that are considered polar or that are able to form hydrogen bonds have higher viscosity. The stronger the intermolecular force, the higher the viscosity of a liquid. So, liquids possessing strong intermolecular forces are said to be highly viscous.
Viscosity is referred to as a liquid's resistance to flow.
Think about it this way, a highly viscous liquid flows like honey and a barely viscous one flows like water.
For example, think about the structure of water and glycerol. Glycerol has three OH- groups that are able to undergo hydrogen bonding, compared to water which only has one OH- group that can form hydrogen bonding. Therefore, we can say that glycerol has a higher viscosity, and also a stronger intermolecular force.
Fig. 3: The structures of glycerol and water, Isadora Santos - StudySmarter Originals.
Next, we have surface tension. This property can be easily understood if we think about water molecules. Hydrogen bonding is present between neighboring water molecules, and this force exerts a downward force at the surface of the liquid, causing surface tension. The stronger the intermolecular force, the higher the surface tension of liquids.
Surface tension is referred to the amount of energy that is needed to increase the surface area of liquids.
Let's solve an example!
Why does 1-butanol have a higher surface tension compared to diethyl ether?
1-butanol contains hydrogen bonding, dipole-dipole, and London dispersion forces, whereas diethyl ether has dipole-dipole and London dispersion forces. We saw before that hydrogen bonding is stronger than dipole-dipole and London dispersion forces. Therefore, the presence of hydrogen bonding is what gives 1-butanol a higher surface tension, a, therefore, a stronger intermolecular force, than that of diethyl ether.
Fig. 4: Structures of 1-butanol and diethyl ether, Isadora Santos - StudySmarter Originals.
If you need to remember how to find out the types of intermolecular forces present in a molecule, check out "Intermolecular Forces"!
Another property that is affected by the strength of intermolecular forces is solubility. The solubility of solids is greatly affected by temperature. So, if temperature increases, the solubility of solids also increases. The solubility of gases in water is the opposite. It decreases with an increase in temperature.
Solubility is referred to as a measure of how much solute is able to dissolve in a given amount of solvent.
When it comes to relating solubility to intermolecular forces, we can say that As the intermolecular force between solvent and solute increases in strenght, solubility also increases!
Let's look at an example!
By looking at the following structures, which of them has the highest solubility in water?
Fig. 5: Structures of different compounds, Isadora Santos - StudySmarter Originals.
The key to solving this problem is knowing that the stronger the intermolecular forces between solvent and solute, the higher the solubility!
The substance with the strongest intermolecular force between solute and solvent will be the most soluble in water! In this case, compound C will have the strongest intermolecular force (hydrogen bonds) so it would also have the highest solubility in water!
- A is non-polar so it only possesses London dispersion forces.
- B is polar so it has dipole-dipole forces and London dispersion forces. However, hydrogen bonding is stronger than dipole-dipole interactions.
Effect of Intermolecular Forces on Melting Point
The melting points of substances depend on the strength of the intermolecular forces present between molecules. The general relationship between IMF and melting point is that the stronger the intermolecular force, the higher the melting point.
For example, a non-polar compound such as Br2 that only has London dispersion forces tend to have a low melting point because only a very small amount of energy is required to break its molecules apart. On the other hand, a high amount of energy is needed to melt a compound containing ion-dipole forces because these forces are very strong.
The strength of London dispersion forces is also affected by how heavy a substance is. This can be seen when we compare Br2 and F2. Br2 has a greater molar mass compared to F2 so Br2 will have a higher melting point and also a stronger London dispersion force than that of F2.
At room temperature, Cl2 is a gas, Br2 is a liquid, and I2 is solid. You can learn about this by reading "Solids, Liquids and Gases"!
Strength of Intermolecular Forces and Boiling point
When molecules change from a liquid to a gas phase, the temperature at which this occurs is known as the boiling point. The general rule relating IMF and boiling point is that the stronger the intermolecular force present, the greater the amount of energy needed to break them, so the higher the boiling point will be.
Let's look at an example!
Which of the following alkanes will have the higher boiling point?
Structures of Methane, Propane, and Butane - StudySmarter Originals.
These alkanes are non-polar, so the only intermolecular force present on them is London dispersion forces. Remember that, when dealing with non-polar molecules and LDF, the larger the surface area of a molecule, the stronger the intermolecular force.
In this case, the larger molecule is butane. So, butane will have the strongest IMF, and therefore, the highest boiling point!
This is actually true if you compare their actual boiling points!
- Methane has a boiling point of: 161.48 °C
- Propane has a boiling point of: 42.1 °C
- Butane has a boiling point of: 0.5 °C
If you a refresher on how to determine the intermolecular forces present in molecular, check out "Intermolecular Forces"!
Until now, we learned that increasing melting point, surface tension, viscosity, boiling point, and solubility lead to an increase in the strength of intermolecular forces of attraction. But, did you know that higher intermolecular forces result in lower vapor pressures?
Vapor pressure occurs when liquid molecules have enough kinetic energy to escape from the intermolecular forces and turn into a gas inside a closed container. Vapor pressure is inversely proportional to the strength of intermolecular forces. So, molecules with strong intermolecular forces have low vapor pressures!
Let's look at an example!
Which of the following would be expected to have the lower vapor pressure? CH3OH vs. CH3SH
Notice the OH bond in CH3OH. This means that it has the ability to form hydrogen bonding with neighboring molecules containing N, O, or F atoms. So, CH3OH has a stronger intermolecular force compared to CH3SH.
Since vapor pressure is inversely proportional to the strength of intermolecular forces, we can say that the substance with the strongest intermolecular force will have the lower vapor pressure. Therefore, the answer is CH3OH.
Strength of Intermolecular Forces on Acetone
A common question that you might encounter in your exam or while studying for AP chemistry is to analyze the strength of intermolecular forces on acetone, C3H6O. You have probably seen acetone before since acetone (also known as propanone or dimethyl ketone) is an organic compound widely used to remove nail polish and paint!
Fig. 7: Structure of Acetone, Isadora Santos - StudySmarter Originals
Acetone is a polar molecule so it contains dipole moments that do not cancel out due to symmetry. In polar molecules, the intermolecular forces present are dipole-dipole forces and London dispersion forces (remember that London dispersion forces are present in all molecules!). So, the strongest type of intermolecular interaction present in acetone is dipole-dipole forces.
Read "Dipoles" to learn more about bond polarity and dipole moments!
Determining the Strength of Intermolecular Forces
In AP chemistry exams, you might come across different problems asking you to determine the highest type of intermolecular force present in a molecule.
To be able to figure out the intermolecular forces present in a molecule, we can use the following rules:
- Ion-dipole forces will only be present if an ion and a dipole molecule are present.
- Hydrogen bonding will only be present if: no ions are present, the molecules involved are polar, and the hydrogen atoms are bonded to nitrogen (N), oxygen (O), or Fluorine (F).
- Dipole-dipole forces are only present if no ions are present and the molecules involved are polar. Also, if hydrogen atoms are present, they will not be bonded to N, O, or F.
- London dispersion forces are present in all molecules. But, LDF is the only intermolecular force present in non-polar and non-polarizable molecules.
What is the strongest intermolecular force present in ammonia (NH3) ?
First, we need to draw the structure of NH3. For this, let's look at the interaction between two NH3 molecules.
Fig. 8: Interaction between ammonia molecules - StudySmarter Originals.
Then, we need to ask the following questions:
- Are ions present? No
- Are the molecules involved polar or non-polar? Polar
- Are there any H-atoms bonded to nitrogen (N), oxygen (O) or fluorine (F)? Yes!
So, NH3 has London dispersion forces, dipole-dipole forces, and also hydrogen-bonding. Since hydrogen bonding is stronger than LDF and dipole-dipole forces, we can say that the highest intermolecular force present in NH3 is hydrogen bonding.
Now I hope that you are feeling more confident about the factors that increase and decrease the strength of intermolecular forces! And if you are still struggling with the basics of intermolecular forces, you should definitely take a look at "Intermolecular Forces" and "Dipoles".
Strength of Intermolecular Forces - Key takeaways
- Intermolecular forces are attractive forces that hold neighboring molecules together. Intermolecular forces affect the physical properties of molecules.
- The strength of attractive intermolecular forces increases with an increase in melting point, boiling point, viscosity, solubility, and surface tension.
- The strength of intermolecular forces decreases with an increase in vapor pressure.
References:
Hill, J. C., Brown, T. L., LeMay, H. E., Bursten, B. E., Murphy, C. J., Woodward, P. M., & Stoltzfus, M. (2015). Chemistry: The Central Science, 13th edition. Boston: Pearson.
Timberlake, K. C., & Orgill, M. (2020). General, organic, and Biological Chemistry: Structures Of Life. Upper Saddle River: Pearson.
Malone, L. J., Dolter, T. O., & Gentemann, S. (2013). Basic concepts of Chemistry (8th ed.). Hoboken, NJ: John Wiley & Sons.
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