kinetic theory of gases

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Kinetic Theory Model 1

Gases are composed of particles that are in continuous random motion

Kinetic Theory Model 2

Attraction and repulsion between gas particles are negligible

Kinetic Theory Model 3

The particles of a gas have kinetic energy (1/2mv^2)

Kinetic Theory Model 4

The average kinetic energy of the particles of a gas is proportional to its temperature and is the same for all gases at the same temperature

Kinetic Theory Model 5

Particle collisions are elastic

Maxwell-Boltzmann distribution curves

shows how the kinetic energies of gas particles are distributed in a sample of gas

Axes of Maxwell-Boltzmann

• X-axis: Kinetic energy of gas particles.

  • Particles on the left have low energy.

  • Particles on the right have high energy.

• Y-axis: Proportion of particles with a particular kinetic energy.

  • A higher point on the graph means more particles have that energy.

Shape of Maxwell-Boltzmann

• Starts just above the origin (0,0) as  There is never zero movement of gas particles

• Rises to a peak and then falls away gradually.

Area under the MB curve

The total area under the curve represents all the particles  in the gas sample (100%).

Most probable energy MB curve

• The peak of the curve is the most probable energy

• This is the energy possessed by the greatest number of particles.

 

Lowest temp on an MB curve

• Higher peak

• Moves to the left

• Must have the same area as the original!!

High temp on an MB curve

• lower peak

• Moves to the right

• Must have the same area as the original!!

What does having heavier molecules do? eg. oxygen and nitrogen at the same temp

The heavier molecule (Oxygen) has a lower temp

Gas property: Take the shape of their container / Low density

• The particles of a gas show negligible forces of attraction and are in constant random motion.

• Therefore gas particles spread out as far as possible occupying the entire volume and shape of their container

Gas property: Can be compressed

• The particles of a gas have negligible volume and they are widely spaced

• So there is room for them to compressed into a smaller volume

Gas property: Readily diffuse through other gases

• The particles of a gas are in constant random motion, have negligible forces of attraction and are widely spaced

• Therefore, the molecules of any gas can diffuse, between the molecules of any other gas

Gas property: exert pressure

• Particles in the gas phase do not attract one another but move randomly in straight lines.

• When they collide with their container walls they exert pressure on the walls.

• The pressure increases with the frequency and force of these collisions.

Temp affect on pressure

Pressure increases with increasing gas temperature

• gas temp up, as does average KE

• greater particle speed which results in greater rate and force of particle collisions with the container walls.

• Increased force and rate of collisions with the container walls results in an increased pressure.

Amount of gas effect on pressure

Pressure increases with the amount of gas

• More gas means more gas  particles in the container

• there will be a greater rate of collisions between gas particles and their container walls

• This causes the increased pressure

Volume of the container effect on pressure

Pressure decreases with increasing gas volume

• Increasing the container volume means particles on average have a greater distance to travel before colliding with their container walls.

• This leads to a decreased rate of collisions and hence a decreased pressure.

• The force of particle collisions with the container walls remains unchanged.

List and explain the conditions under which a real gas behaves like an ideal gas: Low Pressure

• Gas particles are far apart.

• The volume of the particles themselves is negligible compared to the volume of the container.

• Intermolecular forces have very little effect because particles rarely interact.

List and explain the conditions under which a real gas behaves like an ideal gas: High Temperature

• Gas particles have high kinetic energy.

• Particles move rapidly and collide frequently.

• Intermolecular forces become insignificant compared to the particles' motion.

List and explain the conditions under which a real gas behaves like an ideal gas: Small, Non-Polar Molecules

• Examples include helium (He) and hydrogen (H₂).

• These gases have very weak intermolecular forces.

• As a result, their behaviour is closer to that predicted by the ideal gas model.

Differences between an ideal gas and a real gas

• An ideal gas assumes that its particles have no volume, whereas a real gas consists of particles that have a finite volume.

• An ideal gas assumes that there are no intermolecular forces between particles, while real gases experience intermolecular attractions and repulsions.

• In an ideal gas, collisions between particles are perfectly elastic and are not affected by intermolecular forces; however, in a real gas, collisions may be influenced by intermolecular forces.