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STATES OF MATTER - properties of gases and liquids (fluids) and solids

3. Simple kinetic particle model of a gas - pressure and temperature effects

Doc Brown's chemistry revision notes: basic school chemistry science GCSE chemistry, IGCSE chemistry, O level and ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old science students for national examinations in chemistry and also helpful for UK advanced level chemistry students aged ~16-18 and US grades 11-12 K12 honors.

(c) doc b 3. The particle model of a GAS

  • How does the kinetic particle theory of gases explain the properties of gases?
  • A gas has no fixed shape or volume, but always spreads out to fill any container - the gas molecules will diffuse into any space available.
  • There are almost no forces of attraction between the particles so they are completely free of each other.
  • The particles are widely spaced and scattered and always moving rapidly at random throughout the container so there is no order in the system.
  • The particles move linearly and rapidly in all directions, and frequently collide with each other and the side of the container.
  • The collision of gas particles with the surface of a container causes gas pressure, on bouncing off a surface they exert a force in doing so.
  • With increase in temperature, the particles move faster as they gain kinetic energy, the rate of collisions between the particles themselves and the container surface increases and this increases gas pressure eg in a steam locomotive or the volume of the container if it can expand eg like a balloon.

Using the particle model to explain the properties of a Gas

  • Gases have a very low density (‘light’) because the particles are so spaced out in the container (density = mass / volume).
    • Density order: solid > liquid >>> gases
  • Gases flow freely because there are no effective forces of attraction between the gaseous particles – molecules.
    • Ease of flow order: gases > liquids >>> solids (no real flow in solid unless you finely powder it!)
    • Because of this gases and liquids are described as fluids.
  • Gases have no surface, and no fixed shape or volume, and because of lack of particle attraction, they always spread out and fill any container (so gas volume = container volume).
  • Gases are readily compressed because of the ‘empty’ space between the particles.
    • Ease of compression order: gases >>> liquids > solids (almost impossible to compress a solid)
  • Gas pressure
    • When a gas is confined in a container the particles will cause and exert a gas pressure which is measured in atmospheres (atm) or Pascals (1.0 Pa = 1.0 N/m2), pressure is force/area i.e. the effect of all the collisions on the surface of the container.
      • All particles have mass and their movement gives them kinetic energy and momentum.
      • The gas pressure is caused by the force created by millions of impacts of the tiny individual gas particles on the sides of a container.
      • For example – if the number of gaseous particles in a container is doubled, the gas pressure is doubled because doubling the number of molecules doubles the number of impacts (collisions) on the side of the container so the total impact force per unit area is also doubled.
      • This doubling of the particle impacts doubling the pressure is pictured in diagrams below.
  • Effect of temperature change
  • If the volume of a sealed container is kept constant and the gas inside is heated to a higher temperature, the gas pressure increases.
    • The reason for this is that as the particles are heated they gain kinetic energy and on average move faster.
    • Therefore they will collide with the sides of the container with a greater force of impact, so increasing the pressure.
    • There is also a greater frequency of collision with the sides of the container BUT this is a minor factor compared to the effect of increased kinetic energy and the increase in the average force of impact.
    • Therefore a fixed amount of gas in a sealed container of constant volume, the higher the temperature the greater the pressure and the lower the temperature the lesser the pressure.
    • See Part 19. Particle model - gas pressure-volume-temperature calculations

    • See Part 20.The Combined gas law equation - more complicated calculations involving P-V-T

  • If the ‘container’ volume can change, gases readily expand* on heating because of the lack of particle attraction, and readily contract on cooling.
    • On heating, gas particles gain kinetic energy, move faster and hit the sides of the container more frequently, and significantly, they hit with a greater force.
    • Depending on the container situation, either or both of the pressure or volume will increase (reverse on cooling).
    • Note: * It is the gas volume that expands NOT the molecules, they stay the same size!
    • If there is no volume restriction the expansion on heating is much greater for gases than liquids or solids because there is no significant attraction between gaseous particles. The increased average kinetic energy will make the gas pressure rise and so the gas will try to expand in volume if allowed to e.g. balloons in a warm room are significantly bigger than the same balloon in a cold room!
  • Thermal energy (heat energy) conduction in gases
    • All gases are very poor conductors of thermal energy, energy which is due to the kinetic energy of the moving particles.
    • Heat energy is transferred by 'hotter' higher kinetic energy gas particles colliding with 'cooler' lower kinetic energy particles so raising their kinetic energy and spreading the heat energy.
    • However, the density of gases is very low, so the density or rate of 'collision transfer' is quite low.
    • Therefore gases are very good insulators e.g. think of their used in house insulation where air is trapped in various ways like foam or fibre glass loft insulation.
  • Electrical conduction in gases
    • Electrical conduction requires the presence of free IONS or free ELECTRONS i.e. particles that can carry an electrical charge.
    • Gases are poor conductors of electricity because they are usually not in an ionic or ionised form.
    • However, applying a very high potential difference of thousands of volts, especially with a low gas pressure, can cause the formation of free ions and electrons and electrical conduction can happen.
    • Strip lighting and neon signs use this effect.
  • Diffusion in gases
  • Brownian motion in gases
    • When smoke particles suspended in air are viewed under a microscope they appear to 'dance around' when illuminated with a light beam at 90o to the viewing direction.
    • This is because the pollen grains show up by reflected light and 'dance' due to the millions of random hits from the fast moving air molecules.
    • This phenomenon is called 'Brownian motion' after a botanist called Brown first described the effect (see Part 5 for evidence for Brownian motion in gases).
    • At any given instant of time, the particle hits will not be even all round the surface of the smoke particles, so they get a greater number of hits in a random direction and then another, hence the smoke particles zig-zag around in all directions with random movement.

Learning objectives for using the particle model of a gas to explain its properties

Image adapted from Wikipedia, but in reality the 'red and blue' particles are moving much faster (e.g. 200 m/s) and colliding millions of time a second.

Be able to describe the kinetic particle model of a gas.

Be able to draw particle pictures to illustrate and explain the physical properties of gases.

Know, and why, for the same substance, gases have a much lower density than liquids or solids.

Know and explain that gases have no shape or fixed volume - the particles move and spread out to fill any container.

Know and explain why gases are much more easily compressed to a smaller volume compared to solids and liquids because of the relatively large space between gas particles.

Be able to explain pressure in terms of the kinetic particle model of a gas i.e. the collective effect of many impacts of gas molecules on the enclosing surface..

Know and explain that very weak forces between gas particles allows a gas to be a fluid and flow freely e.g. under pressure in a gas pipe.

Explain using the particle model that increasing the number of gas particles in a container, increases the number of collisions of gas particles with the surface and so increases pressure.

Be able to explain the effect of changing temperature in terms of the kinetic particle model of a gas e.g. increasing temperature can increase volume or pressure depending on conditions and explain this is due to the increase in the average kinetic energy of the particles.

Know that the thermal energy, the internal energy of a gas is the kinetic energy of the particles.

Know that gas particles are not usually electrically charged, gases are very poor conductors of electricity (but a high p.d. may produce ions and electrical conduction).

Be able to describe and explain microscope observations of Brownian motion in gases e.g. the motion of suspended smoke particles in air.

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