Gas laws

• Description of the motion of gas molecules

  Gas molecules move with Brownian motion, which is the random motion of molecules caused by collisions with larger particles
• How does a gas exert a force on its container

  • Molecules collide with the walls of their container
  • Collisions cause a change in momentum
  • A change in momentum produces a force equal to the rate of change of momentum
• Boyles law

  When a gas is it a constant temperature, pressure and volume are inversely proportional to each other
• Boyles law explanation

  • When the volume of a gas increases, the space between molecules increases and so the time between collisions in larger
  • This causes the rate of collisions and so the rate of change of momentum to decrease
  • This means the force is exerted lower, causing a decrease in pressure
• Charles law

  When a gas is at a constant pressure, the volume is directly proportional to the absolute temperature
• Charles law explanation

  • As temperature increases, the average kinetic energy of the molecules increases
  • Pressure is constant so the force and also the rate of change of momentum must remain constant
  • To achieve this, volume increases so the faster speed of the molecules is compensated by there being larger gaps between them
• Pressure law

  When a gas has a fixed volume, pressure is directly proportional to the absolute temperature
• Pressure law explanation

  • As temperature increases, the average kinetic energy, and so the speed of the molecules increases
  • This increases the rate of collisions, and so produces a larger rate of change of momentum
  • This leads to a greater force exerted and so an increase in pressure
• Assumption of gases in kinetic theory

  • The gas contains a large number of molecules
  • The molecules are identical to each other
  • All collisions between molecules and the walls of their container are perfectly elastic
  • The time taken for collisions is negligible compared to the time between collisions
  • There are no intermolecular forces between molecules
  • Molecules are in constant random motion
  • The gas particles obey Newtons laws of motion
• Ideal gas assumptions

  • The particles obey newtons laws of motion
  • They travel in straight lines
  • They are in random motion