9.6 - Factors Affecting Gas Pressure

Gas pressure comes from particle collisions

• The particles in a gas move constantly in random directions at a range of speeds.
• As they move, they collide with each other and with the walls of their container.
• A gas's particles take up very little space - most of a gas is empty space - so particles travel far between collisions.

How collisions create pressure

• When a particle collides with a surface it changes direction, so it changes momentum.
• A change in momentum means a force is exerted on the surface.
• Each collision is in a random direction, but together the collisions give a net force at right angles to the wall.
• Pressure is the force acting per unit area of a surface. Gas pressure is the total force of all colliding particles on each unit area of the walls.
• The more often particles hit the walls, and the harder they hit, the greater the pressure.

Absolute zero and the Kelvin temperature scale

Absolute zero

• The coldest temperature possible is -273 degrees C, called absolute zero.
• At absolute zero the particles have as little energy in their kinetic stores as possible - they are very nearly still.
• A gas cooled towards absolute zero exerts less and less pressure, because its particles strike the walls less often and less hard; at absolute zero an ideal gas would exert no pressure at all.

The Kelvin scale

• Absolute zero is the starting point of the Kelvin scale, where it is 0 kelvin (0 K).
• A change of 1 degree C is the same size as a change of 1 K - the scales differ only in where their zero sits.

To convert between them:

• From degrees Celsius to kelvin, add 273.
• From kelvin to degrees Celsius, subtract 273.

Temperature and average kinetic energy of gas particles

• The temperature of a gas measures the average energy in the kinetic energy stores of its particles.
• The hotter the gas, the more energy its particles have on average, so the faster they move on average.
• The Kelvin temperature of a gas is directly proportional to the average kinetic energy of its particles - double the temperature in kelvins and you double the average kinetic energy.

Heating and cooling a gas

• Heating transfers energy into the kinetic energy stores of the particles, increasing their average speed.
• Cooling transfers energy out of their kinetic stores, so they move more slowly on average.
• Particles always have a range of speeds, so this is an average across all particles, not a single speed shared by every particle.

How temperature and volume change gas pressure

• Two things change the pressure of a fixed mass of gas: its temperature and its volume.
• Both act by changing how often particles hit the walls and how hard they hit.

Changing the temperature (at constant volume)

• Heating makes particles move faster on average - they hit the walls more often and with greater force each time, so the pressure increases.
• Cooling makes particles move slower - they hit the walls less often and with less force, so the pressure decreases.

Changing the volume (at constant temperature)

• Squeezing the gas into a smaller volume packs the same particles into less space - they hit the walls more often, so the pressure increases.
• Letting the gas expand into a larger volume spreads the particles out - they hit the walls less often, so the pressure decreases.