Question

What do we mean by intermolecular forces? What evidence is there that they exist?

Short answer

Intermolecular forces are the forces that exist between molecules, responsible for some physical properties of substances, such as boiling point, melting point, and viscosity. There are three main types: London dispersion forces (weak forces occurring between all molecules), dipole-dipole interactions (between polar molecules with permanent dipoles), and hydrogen bonding (a strong type of dipole-dipole interaction involving hydrogen). Evidence of intermolecular forces can be seen through properties like boiling and melting points, viscosity, and surface tension. For example, water has a high boiling point of 100°C due to hydrogen bonding.

Step-by-step solution

Definition of Intermolecular Forces

Intermolecular forces are the forces that exist between molecules, as opposed to intramolecular forces, which are the forces holding the atoms of a molecule together. These forces are responsible for some physical properties of substances, such as boiling point, melting point, and viscosity. There are three main types of intermolecular forces: London dispersion forces, dipole-dipole interactions, and hydrogen bonding. #Step 2: Explain London dispersion forces#

London Dispersion Forces

London dispersion forces, also known as van der Waals forces, are the weakest type of intermolecular forces and occur between all molecules, regardless of their polarity. These forces arise due to temporary, instantaneous dipoles that can form in molecules as electrons move randomly around the nuclei. An instantaneous dipole on one molecule induces a dipole on a neighboring molecule, creating a weak attractive force between them. The strength of London dispersion forces increases with the size of the molecules and the number of electrons present. #Step 3: Explain dipole-dipole interactions#

Dipole-Dipole Interactions

Dipole-dipole interactions occur between polar molecules, which have permanent dipoles due to differences in electronegativity between their atoms. The positive end of one polar molecule (the more electronegative atom) is attracted to the negative end of another polar molecule (the less electronegative atom). The strength of dipole-dipole interactions depends on the magnitude of the molecular dipoles and the distance between the molecules. #Step 4: Explain hydrogen bonding#

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction that occurs when a hydrogen atom is covalently bonded to a highly electronegative atom (such as N, O, or F) and is electrostatically attracted to an electronegative atom with a lone pair of electrons in an adjacent molecule. Hydrogen bonding is particularly strong due to the high electronegativity of these atoms and the small size of the hydrogen atom, which allows the molecules to come close together. #Step 5: Provide evidence for intermolecular forces#

Evidence of Intermolecular Forces

The existence of intermolecular forces can be demonstrated through their various effects on the properties of substances. Some examples include: 1. Boiling and Melting points: Substances with strong intermolecular forces typically have higher boiling and melting points, as more energy is required to overcome these forces. For example, water has a relatively high boiling point (100°C) due to hydrogen bonding, compared to other similar-sized molecules without hydrogen bonding. 2. Viscosity: The viscosity of liquids, or their resistance to flow, is related to the strength of the intermolecular forces between their molecules. Liquids with stronger intermolecular forces tend to have higher viscosity. 3. Surface tension: The presence of intermolecular forces can cause some substances to display a high surface tension, as molecules close to the surface experience an imbalance of forces that causes them to be attracted to their neighbors within the substance. This effect is seen in water, where hydrogen bonding leads to a high surface tension. Through these examples and explanations of the different types of intermolecular forces, we can confirm the existence and importance of intermolecular forces in determining the physical properties of substances.

Key concepts

London Dispersion Forces

Among the intermolecular forces, London dispersion forces are often the unsung heroes, quietly working behind the scenes in all molecules, whether they are polar or nonpolar. These forces arise because electrons are constantly in motion, and at any given moment, there can be an uneven distribution of them within a molecule. This fleeting imbalance creates what we call an instantaneous dipole that induces a similar dipole in a neighboring molecule, resulting in a temporary attraction.

For instance, noble gases like helium and neon remain as gases under standard conditions primarily because they are held together just by these weak London dispersion forces. However, don't let their weakness fool you; when molecular size and mass increase, these forces grow stronger. This is why larger molecules, such as those found in waxes and oils, can exist as solids or viscous liquids at room temperature. It's a classic case of 'the sum is greater than its parts' where a multitude of weak forces combine to have a substantial effect.

Dipole-Dipole Interactions

Imagine two magnets with their opposite poles facing each other – the attraction they exhibit is akin to dipole-dipole interactions. These forces occur in polar molecules that have a positive end and a negative end, much like a magnet. When polar molecules come into close proximity, the positive end of one molecule feels drawn to the negative end of another, creating this type of intermolecular attraction.

The molecule's shape and the strength of the dipole moment (a measure of polarity) influence the intensity of these interactions. Water, for example, is a classic polar molecule whose shape allows for potent dipole-dipole attractions, contributing to high boiling and melting points relative to its molecular weight. This kind of attraction has far-reaching consequences, not least in determining the states of matter and the way a substance behaves in different environments.

Hydrogen Bonding

Consider hydrogen bonding the VIP of intermolecular attractions, particularly potent and responsible for many of the quirks in water's character that life as we know it depends on. This type of bonding is a specific form of dipole-dipole interaction where a hydrogen atom covalently bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine is attracted to a nearby electronegative atom with a lone pair of electrons.

Hydrogen bonds are the reason why water has such a high boiling point for its size, why ice floats, and why DNA strands hold together in a double helix. Every time you see a drop of water bead up on a surface, you're witnessing hydrogen bonding in action – creating a molecular