Ideal Gas Molecules
Gases behave in predictable ways - their pressure increases as we heat them up and their pressure decreases when we increase the volume of the container that they are held in. Gas molecules move faster as they are heated and slow down when they are cooled, until they stop moving altogether. The temperature at which particles stop moving is called absolute zero.
Pressure and temperature of a gas
Gas molecules are constantly moving in random directions and they are forever bumping into things - with other gas molecules and into the sides of the container that is holding them. When gas molecules bounce against the walls of the container they exert a force, and hence a pressure, on the sides of the container. If the gas molecules are moving more vigorously, they will bounce harder against the walls of the container therefore they are exerting more pressure.
As temperature increases, the gas molecules have more kinetic energy. This is a proportional relationship which means that if we double the temperature, the average kinetic energy of the gas molecules also doubles. The gas particles are moving faster and will collide more frequently against the container walls and will do so with a greater force. This means that as temperature increases, the pressure of a gas also increases. Again, temperature and pressure are proportional, so if I double the temperature I will double the pressure.
The relationship between pressure and temperature when volume is kept constant is described in the equation:
Where P1 is the initial pressure and T1 is the initial temperature. P2 is the final pressure and T2 is the final temperature.
Worked example:
A sample of neon gas has a pressure of 101 kPa at 298 K. What is its pressure if the temperature is increased to 348 K?
- Initial pressure / initial temp = final pressure / final temp (P1/T1 = P2/T2)
- 101 / 298 = ? / 348
- 0.34 = ? / 348
- 0.34 x 348 = 118 kPa
Pressure and volume
If we take a gas and place it into a smaller container, the gas molecules are going to hit the wall more frequently, increasing the pressure. On the other hand, if the gas is placed into a much larger container, there is more space for the gas molecules to move before they collide with the container walls. This reduces the pressure.
The relationship between pressure and volume is inversely proportional. This means that if we double the volume, the pressure of a gas is halved.
The relationship between pressure and volume when the temperature is kept constant is described in the equation:
Where P1 is the initial pressure and V1 is the initial volume. P2 is the final pressure and V2 is the final volume.
Worked example:
A sample of helium gas with a pressure of 150 kPa is inside a container with a volume of 500 cm3. What is the new pressure of the sample of helium when it is transferred to a container with a volume of 750 cm
- Initial pressure x initial volume = final pressure x final volume (P1V1 = P2V2)
- 150 x 500 = 750 x ?
- 75 000 = 750 x ?
- 75 000 / 750 = 100 kPa
Kelvin scale and absolute zero
When referring to the temperature of gas molecules, we use the units Kelvin. To convert from Celsius to Kelvin you need to add 273 and to convert Kelvin into Celsius, subtract 273. Zero Kelvin, which we refer to as absolute zero is the coldest temperature that molecules can reach. At this temperature, particles completely stop moving since they have no kinetic energy whatsoever. Zero K (or absolute zero) is equal to -273 Celsius.
Did you know..
In practice it is impossible to reach absolute zero. The closest scientists have managed is 0.45 nanokelvin above absolute zero, using laser which removed heat energy from the sample. The problem is that our surroundings our constantly emitting heat energy. The only way to achieve absolute zero only when the sample is entirely removed from the universe.
Download worksheet: Ideal Gas Molecules / Answers Next Page: Magnetism