Particle model & pressure

• Gases:


  consist of a large number of small particles, which are molecules or atoms.

  • These particles are in CONSTANT RANDOM MOTION & move in straight lines, until they collide with either the container walls / other particles.

  
• As the TEMPERATURE of a gas rises, the SPEED of its molecules increases.

  • As the temperature is a measure of the AVERAGE KINETIC energy of the molecules in a substance.
• When VOLUME of the gas is constant & increase the temperature, the kinetic energy will increase meaning the molecules move faster.
• PRESSURE in a gas is the result of COLLISIONS between the gas molecules & the container walls.

  The UNITS for pressure can be measured in:

  • Pascals (Pa) or Atmospheres (atm.)

  
• The MORE MOLECULES in a given volume, the HIGHER the pressure.

  • As gas molecules collide MORE FREQUENTLY & with GREATER FORCE against the container walls.

  
• If we heat a gas up, the molecules:

  • gain KINETIC ENERGY
  • move at HIGHER SPEEDS
  • leading to MORE FREQUENT & FORCEFUL collisions with the container walls.
  • which INCREASES the PRESSURE.

  
• At a CONSTANT VOLUME, an INCREASE in TEMPERATURE will lead to an INCREASE in PRESSURE.

  
• Pressure in gases:


  Caused by gas particles colliding with the walls of the container.

  
• Increase pressure by:


  • increasing the temperature
  • increasing the number of particles
  • decreasing the volume

  
• When kept at a constant temperature, the pressure & volume of a gas are inversely proportional.

  
• Pressure (pascals)  x volume = constant value (same value)
• For a gas at constant temperature, pressure x volume always equals the same constant. Therefore:

  p1V1 = p2V2

  
• p1V1 = p2V2, Where:

  • p1 is the initial pressure
  • V1 is the initial volume
  • p2 is the pressure after a change occurs
  • V2 is the volume after a change occurs

  
• An unknown gas occupies a volume of 1.5m^3, at a pressure of 100Pa.

  Calculate the pressure exerted by the gas, if it’s compressed to a volume of 0.3m^3

  (assume that the temperature & mass of the gas stay the same)

  pV = constant

  • 100Pa x 1.5m^3 = 150
  • P x 0.3m^3 = 150
  • P = 150 / 0.3 = 500

  
• To prepare for a dive, 1800 litres of air from the atmosphere is compressed into a 12-litre gas cylinder.

  Calculate the pressure of air in the cylinder

  (assume the temp. Remains constant & that atmospheric pressure is 101kPa)


  101kPa x 1800L = p x 12L

  P = (101kP x 1800L) / 12L = 15150kPa
• A gas occupies a volume of 0.5 m3 at a pressure of 150 Pa.

  Calculate the pressure exerted by the gas if the volume is compressed to 0.2 m3?

  (Assume constant temperature)

  375Pa
• A gas occupies a volume of 0.3 m3 at a pressure of 50 Pa.

  The pressure increased to 150 Pa, what is the new volume?

  (Assume constant temperature)

  0.1m^3
• A gas initially occupied a volume of 10 L at an unknown pressure.

  The gas was then compressed to 2 L & measured as having a pressure of 20 kPa.

  Calculate the original pressure before the gas was compressed.

  (Assume constant temperature)

  4kPa