Structure 1.5.1—An ideal gas consists of moving particles with negligible volume and no intermolecular forces. All collisions between particles are considered elastic.

Structure 1.5.1—An ideal gas consists of moving particles with negligible volume and no intermolecular forces. All collisions between particles are considered elastic.

What You’ll Learn:

  • Recognize the key assumptions in the ideal gas model.


ideal gas, moving particles, negligible volume, no intermolecular forces, elastic collisions, real gases, deviation, low temperature, high pressure, molar volume, constant, specific temperature, specific pressure, pressure, volume, temperature, amount, ideal gas equation, PV=nRT, combined gas law.

Syllabus Links


Ideal Gas Assumptions

An ideal gas is a hypothetical concept in the field of thermodynamics and gas behavior. It is based on a set of simplifying assumptions that help to model and analyze the behavior of real gases under different conditions. The description provided can be broken down into several key components:

  1. Moving particles: An ideal gas consists of a large number of particles (atoms or molecules) in constant motion. These particles have kinetic energy as they move around, and their motion gives rise to the macroscopic properties of the gas, such as pressure and temperature.
  2. Negligible volume: In an ideal gas, the volume occupied by the individual gas particles is considered to be negligible compared to the total volume of the gas. This assumption means that the size of the particles is so small that it doesn’t significantly affect the behavior of the gas, allowing for simpler mathematical models.
  3. No intermolecular forces: An ideal gas assumes that there are no attractive or repulsive forces between the gas particles. This means that the particles don’t interact with each other, except during collisions. In reality, real gases do experience intermolecular forces, but these forces are often negligible under certain conditions (e.g., high temperatures or low pressures).
  4. Elastic collisions: Collisions between the gas particles, as well as collisions with the walls of the container, are considered to be perfectly elastic. In an elastic collision, the total kinetic energy of the colliding particles is conserved. This means that no energy is lost to heat or other forms of energy during the collision.

These simplifying assumptions allow scientists and engineers to derive mathematical equations, such as the ideal gas law (PV=nRT), which can be used to predict and analyze the behavior of real gases under certain conditions. However, it is important to note that the ideal gas model has its limitations, and deviations from the ideal behavior may occur, especially at very high pressures or very low temperatures.

Intermolecular forces recap

Intermolecular forces are the forces that exist between molecules or particles in a substance. They play a crucial role in determining the physical and chemical properties of substances, such as boiling point, melting point, and viscosity. There are three primary types of intermolecular forces: London dispersion forces, permanent dipole-dipole interactions, and hydrogen bonding. Here’s a detailed explanation of each type:

  1. London dispersion forces (also known as van der Waals forces or induced dipole-induced dipole interactions): These are the weakest type of intermolecular forces and occur between all molecules, whether polar or nonpolar. London dispersion forces result from temporary fluctuations in electron distribution around a molecule, which create an instantaneous dipole. This temporary dipole can induce a dipole in neighboring molecules, leading to an attractive force between them. The strength of London dispersion forces increases with the size and shape of the molecules, as larger molecules with more electrons have greater fluctuations in electron distribution.
  2. Permanent dipole-dipole interactions: These forces occur between polar molecules, which have a permanent separation of positive and negative charges due to differences in electronegativity between the atoms within the molecule. This results in a positive end (partial positive charge) and a negative end (partial negative charge) in the molecule, creating a permanent dipole. The positive end of one polar molecule is attracted to the negative end of a neighboring polar molecule, leading to an attractive force between them. Dipole-dipole interactions are generally stronger than London dispersion forces but weaker than hydrogen bonding.
  3. Hydrogen bonding: This is a special type of dipole-dipole interaction that occurs between molecules containing a hydrogen atom bonded to a highly electronegative atom (usually nitrogen, oxygen, or fluorine). Due to the high electronegativity difference, the hydrogen atom develops a significant positive charge, while the electronegative atom acquires a significant negative charge. The hydrogen atom in one molecule is then attracted to the electronegative atom in a neighboring molecule, forming a relatively strong intermolecular force called a hydrogen bond. Hydrogen bonding is responsible for many unique properties of substances, such as the high boiling point of water compared to other similarly sized molecules.
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