Intermolecular forces are the forces of attraction between molecules. These forces are important because they help chemists determine the physical properties of a substance, such as its state, its melting and boiling point, etc. The stronger the intermolecular forces, the higher the melting and boiling points are.
Intermolecular forces are categorized into three major types, Van der Waal forces, dipole-dipole forces, and hydrogen bonding. Van der Waals forces are weak intermolecular forces present in between neutral or nonpolar molecules. These forces exist in almost all molecules to some extent.
Dipole-Dipole forces are a property of polar molecules. In polar molecules, one end is slightly negatively charged and the other is slightly positively charged due to the electronegativity difference between the two bonding atoms. This polarized molecule is called a dipole. The dipole can be permanent or induced due to the presence of another ionic molecule. The force of attraction between these molecules is termed dipole-dipole interaction.
Hydrogen Bonding, as its name suggests, is a bond between hydrogen molecules. However, hydrogen alone is insufficient for hydrogen bonding. First, it must form a strong molecular dipole by a covalent bond with either oxygen, fluorine, or nitrogen. Second, this dipole must approach another atom of oxygen, fluorine, or nitrogen to form a hydrogen bond. Hydrogen bonding is the most powerful intermolecular force. It falls under the category of permanent dipole-dipole forces as it is an attractive force between permanent dipoles.
Image from Labster's lab simulation on Intermolecular Forces.
Intermolecular force is one of the fundamental concepts that you will have to teach in an introductory chemistry course. As a teacher, you may find it hard to explain it to students. So, read on to learn five ways through which you can make intermolecular forces an interesting topic to teach.
First, let’s explore the three reasons why students find the concept of intermolecular tricky.
The types of intermolecular forces are often confusing to students because so many identical terms are used to describe them. For instance, students often mix the two terms; intermolecular and intermolecular forces.
Intermolecular forces are often very weak and exist within a microscopic molecule. So, first, they can be difficult to observe and measure. Second, since these forces are abstract in nature, it’s hard for students to visualize them.
There are three intermolecular forces, each with its own examples, behavior, and bonding structure. Memorizing each one with its related examples can be hard for students in a single go.
Keeping in view the problems faced by students, here are five ways to familiarize students with intermolecular forces.
Whenever teaching intricate topics like intermolecular forces, you will need to start with the very basics. This will give students a foundation to build their understanding of intermolecular forces and clear their prior misconceptions.
For instance, to explain this concept, first, you need to explain atoms, then molecules, and then how they are arranged to form a substance.
Atom and Molecules
The smallest individual unit of a substance is called an atom. Atom's independent existence is very rare. It is mostly bonded with another atom/s. This combination of atoms is called a molecule. Within its structure, a substance is composed of millions of such molecules.
Intermolecular vs. Intramolecular forces
Students often confuse intermolecular and intramolecular forces. “Intra” means within a molecule. Since within a molecule we have a group of atoms. The forces in between the atoms are called intramolecular forces or chemical bonds. There are mainly three types of intermolecular forces, ionic, covalent, and metallic.
“Inter” means in-between. So, such forces, that exist in between the molecule are called intermolecular forces such as ion-dipole, dipole-dipole, London dispersion, and hydrogen bonding. These forces are much weaker compared to intramolecular forces but they dictate the physical properties of a substance.
It has been observed that students learn when they are asked to physically manipulate things. So, give them opportunities to experiment with intermolecular forces.
Many simple experiments can be done with common household materials. For instance, students can test the relative strengths of different intermolecular forces by seeing how far a balloon will travel when blown up in different liquids.
Similarly, they can test out the physical properties of different substances and see which one has the strongest intermolecular forces based on their results.
You can mention some interesting facts about intermolecular forces to engage students and energize them. A couple of them are mentioned below:
The intermolecular forces govern the physical state of a substance. If the intermolecular forces are very strong, that matter will exist as solid. If they are relatively weak, then that compound exists as a liquid. If they are weakest, then the compound exists as a gas.
IMF is also responsible for other physical properties of substances, such as boiling point, melting point, and solubility.
Intermolecular forces are constantly at work all around us and play a role in many everyday phenomena, such as the formation of clouds and the movement of fluids.
Although they are often very weak, intermolecular forces can significantly impact the behavior of substances. For instance, the relatively weak van der Waals forces are responsible for water vapor being a gas at room temperature.
Chemistry is generally considered a dry concept where educators teach about atoms, molecules, compounds, reactions, and equations. To students, these things don’t seem fascinating. They are interested in learning the use case of content being taught to them. So, it is one of the best teaching practices to relate your topic with its practical applications.
For instance, you can explain the importance of intermolecular forces in compounds we use in our daily lives, or which constitute our life.
About three-fourths of Earth is covered with water. It is a unique liquid that we use daily for different purposes. If we look at the molecular size of water, that’s very small. However, its boiling point and specific heat capacity are anomalously higher compared to other liquids of its size.
These characteristics are all brought about by hydrogen bonds. Because hydrogen bonds are so strong, it takes a lot of power to break them, which is why water has a comparatively high boiling point. However, hydrogen bonding may also separate molecules, meaning that it can also set them apart. For instance, when water changes to ice, the crystal structure is governed by hydrogen bonding. In this case, since the molecules are more dispersed than they are in the liquid, ice floats atop water.
DNA (Deoxyribonucleic Acid)
Nearly every cell in our body has DNA, which is a necessary component of life. The primary function of DNA is to store biological code (bases), within its double helix so that the equipment can read the code when necessary.
In order to preserve DNA from harm while yet allowing for readability when necessary, DNA bases must be buried and shielded within the double helix. For that, we use hydrogen bonding. The hydrogen connections between the bases in the helix are strong enough to hold them together and protect them. At the same time, they are also delicate enough to break when necessary and enable access to the bases.
One way to explain the concept is to ask students to compare the physical properties of various substances based on the intermolecular forces in between. This may seem tough and inconvenient at times. The alternative and convenient way to get students energized about intermolecular forces are through virtual lab simulations.
At Labster, we are committed to engaging students in science through interactive simulations. Through our simulations, we aim to teach them theory visually and let them perform all lab experiments virtually.
In this case, you can check our lab simulation on Intermolecular Forces. In this simulation, students comprehend the importance of intermolecular force. They are tasked to rebuild the world in which the intermolecular forces have vanished.
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