What is the primary difference between intermolecular and intramolecular forces?

Intramolecular forces are strong chemical bonds (like covalent bonds) that hold atoms together within a molecule. Intermolecular forces are much weaker attractions that exist between separate molecules.

How does the boiling point of a straight-chain alkane compare to its branched isomer?

The straight-chain alkane has a higher boiling point. This is because it has a larger surface area, allowing for more points of contact and stronger London dispersion forces compared to the compact branched isomer.

Why does water ($H_2O$) have a significantly higher boiling point than hydrogen sulfide ($H_2S$)?

Water can form strong hydrogen bonds due to the high electronegativity of oxygen. Hydrogen sulfide only experiences weaker permanent dipole-dipole and London forces because sulfur is not electronegative enough to support hydrogen bonding.

What is a common mistake when identifying molecules that can form hydrogen bonds?

A common error is assuming any molecule with hydrogen can hydrogen bond. It specifically requires hydrogen to be directly bonded to N, O, or F, and the presence of a lone pair on a neighboring electronegative atom.

Why is it incorrect to say that non-polar molecules like $CH_4$ have no attractive forces?

All molecules, including non-polar ones, possess electrons that are in constant motion. This motion creates temporary dipoles that result in London dispersion forces, which allow even non-polar gases to liquefy at low temperatures.

What error is made when drawing a hydrogen bond as a solid line?

A solid line represents a covalent bond (intramolecular). Intermolecular forces like hydrogen bonds must be drawn as dashed or dotted lines to indicate they are weaker, non-bonding interactions.

Define London Dispersion Forces.

Temporary attractive forces that result when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles. They are present in all atoms and molecules.

What are the three specific elements that must be bonded to Hydrogen for a Hydrogen bond to occur?

Nitrogen (N), Oxygen (O), and Fluorine (F). These are the three most electronegative elements, which create the necessary polarity for the bond.

Define 'Polarizability' in the context of intermolecular forces.

Polarizability is the ease with which the electron cloud of an atom or molecule can be distorted to form a dipole. Larger electron clouds (more electrons) are more polarizable and lead to stronger London forces.

Why does the boiling point of noble gases increase as you move down the group?

As you move down the group, the number of electrons increases. This increases the polarizability of the atoms, leading to stronger London dispersion forces that require more energy to overcome.

Intermolecular Forces | Pearson Edexcel A-Level Chemistry

1. Physical Chemistry

Intermolecular Forces

Summary

Intermolecular forces (IMFs) are the attractive forces that exist between molecules, determining the physical state and properties of substances. Unlike intramolecular forces (covalent bonds) which hold atoms together within a molecule, IMFs are significantly weaker but dictate boiling points, melting points, and solubility patterns.

1. Definition & Core Concepts

Intermolecular forces are electrostatic attractions between neighboring molecules, whereas intramolecular forces are the chemical bonds (like covalent or ionic) that hold atoms together inside a single molecule.
The strength of these forces is generally much lower than covalent bonds; for instance, a hydrogen bond is typically only about one-tenth as strong as a standard covalent bond.
These forces are responsible for the existence of condensed phases (liquids and solids) and directly influence physical properties such as viscosity, surface tension, and volatility.

Diagram comparing intramolecular covalent bonds within a water molecule to intermolecular hydrogen bonds between two water molecules.

2. London Dispersion Forces (Induced Dipole-Dipole)

London dispersion forces are the weakest type of IMF and exist between all atoms and molecules, regardless of polarity.
They arise from the constant motion of electrons, which can create a temporary dipole when more electrons happen to be on one side of an atom than the other.
This temporary dipole induces a corresponding dipole in a neighboring molecule, leading to a short-lived attraction.
The strength of London forces increases with the number of electrons (larger electron clouds are more polarizable) and the surface area of the molecule (more points of contact).

3. Permanent Dipole-Dipole Forces

4. Hydrogen Bonding

Hydrogen bonding is a particularly strong and specific type of permanent dipole-dipole attraction that occurs when hydrogen is covalently bonded to highly electronegative atoms: Nitrogen (N), Oxygen (O), or Fluorine (F).
The large electronegativity difference creates a highly polarized bond, leaving the hydrogen atom with a significant partial positive charge and no inner-shell electrons to shield its nucleus.
This "exposed" proton is strongly attracted to the lone pair of electrons on an N, O, or F atom of a neighboring molecule.

5. Key Distinctions

6. Exam Strategy & Tips