Describe the main properties of a gas. How are these predicted by kinetic molecular theory?
The kinetic theory describes a gas as a large number of submicroscopic particles (atoms or molecules), all of which are in constant rapid motion that has randomness arising from their many collisions with each other and with the walls of the container.
The main properties of a gas are
- One major characteristic of gases is that they expand on their own to completely fill their container. So, if you ever want to know the volume of a gas, all you need to know is the volume of the container that it's in.
- Another characteristic of gases is that they can be compressed. This is because the particles in gases are very far apart. The particles in solids and liquids are so close together that they are difficult (if not impossible) to compress. Brake hydraulics work so well because of this. If liquids were easy to compress, the brake fluid in your car would just 'squish together' instead of working to stop your car. So, the more distance between the particles of a substance, the easier it is to compress. And gas particles are very far apart, so they are easy to compress.
Kinetic theory makes many assumptions in order to explain the reasons gases act the way they do. According to kinetic theory:
- Gases consist of particles in constant, random motion. They continue in a straight line until they collide with something—usually each other or the walls of their container.
- Particles are point masses with no volume. The particles are so small compared to the space between them, that we do not consider their size in ideal gases.
- No molecular forces are at work. This means that there is no attraction or repulsion between the particles.
- Gas pressure is due to the molecules colliding with the walls of the container. All of these collisions are perfectly elastic, meaning that there is no change in energy of either the particles or the wall upon collision. No energy is lost or gained from collisions.
- The time it takes to collide is negligible compared with the time between collisions.
- The kinetic energy of a gas is a measure of its Kelvin temperature. Individual gas molecules have different speeds, but the temperature and kinetic energy of the gas refer to the average of these speeds.
- The average kinetic energy of a gas particle is directly proportional to the temperature. An increase in temperature increases the speed in which the gas molecules move.
- All gases at a given temperature have the same average kinetic energy.
- Lighter gas molecules move faster than heavier molecules.