The Ideal Gas Equation (AQA A Level Chemistry): Revision Note

The Ideal Gas Equation

Kinetic theory of gases

• The kinetic theory of gases describes the behaviour of gas particles and forms the basis of the ideal gas model

• The main assumptions of the kinetic theory are:

  • Gas particles move rapidly and randomly in straight lines

  • The volume of the gas particles is negligible compared with the volume of the container

  • There are no intermolecular forces between the particles

  • Collisions between particles, and between particles and the container walls, are perfectly elastic (no kinetic energy is lost)

  • The average kinetic energy of the particles is directly proportional to the absolute temperature

• Gases that obey all of these assumptions are called ideal gases

• In reality, real gases do not behave exactly like this, although at high temperature and low pressure, they may behave similarly to ideal gases

Ideal gases

• The volume of an ideal gas depends on its pressure and temperature

• When a gas is heated at constant pressure, the particles gain kinetic energy and collide more frequently and with greater force with the walls of the container

• To keep the pressure constant, the particles must move further apart, so the volume increases

• At constant pressure, the volume of a gas is directly proportional to its absolute temperature (in kelvin)

Ideal gas equation

• The ideal gas equation shows the relationship between pressure, volume, temperature, and number of moles of an ideal gas:

PV = nRT

P = pressure (pascals, Pa)

V = volume (m³)

n = number of moles of gas (mol)

R = gas constant (8.31 J K^-1 mol^-1)

T = temperature (kelvin, K)

Limitations of the ideal gas equation

• At very low temperatures and high pressures, real gases do not behave ideally

• Under these conditions, gas particles are much closer together

  • As a result, intermolecular forces become significant

  • These may include instantaneous dipole–induced dipole (London dispersion) forces or permanent dipole–permanent dipole attractions

  • These attractive forces pull particles slightly away from the container walls, reducing the force of collisions

  • This means the measured pressure is lower than that predicted for an ideal gas

• In addition, at high pressures, the volume of the gas particles is no longer negligible compared with the volume of the container

  • As a result, the actual volume available for particle movement is smaller than assumed in the ideal gas model

• Therefore, real gases deviate from the kinetic theory assumptions that there are no intermolecular forces and that the volume of gas particles can be ignored