Kinetic molecular theory

Ideal gasesReal gasesProperties of liquidsProperties of solids

Lisa Allen

Stonington High School

What are the properties of an “Ideal Gas?”

•The size of the gas particles is negligible compared to the distances between them. As the particle size is below the threshold of significance, we consider it to be zero.

• All collisions between particles are elastic – that means they don’t lose energy when they hit.

•Gas particles are in constant rapid motion.

•Gas particles do not stick to each other.

•Temperature is a measure of the average kinetic energy of a sample. Increased temperature means faster velocities of particles.

How does an ideal gas behave?

• Gases expand to fill their containers.

Gases are FLUIDS

• That just means they can flow. Liquids can also flow. If the particles are free to move around each other, they are fluids.

Gases have low density and can be compressed to smaller volume.

• Compare the size of a mole of water molecules in liquid form with the size of a mole of water molecules in gaseous form.

• Scuba tanks put lots of air in a smaller space. Liquids and solids do not compress!

Gases effuse and diffuse

Diffusion of gases is why the whole freshman hall smells like Axe…

Effusion refers to the escape of gas particles through a small opening

This is how the Axe gets out of the can onto the freshman. Diffusion is how it gets off the freshman and fills the hallway!

Real gases do not always behave like perfect ideal gases!

• If the temperature is low and the pressure is high, gas particles get mashed together, and forces of attraction between particles can be significant.

• Sticky gases – polar substances – are less likely to behave like ideal gases.

Liquid: an unusual state of matter• Liquids are able to flow.• Liquid particles are in constant motion, but most

do not have the energy to escape from the forces of attraction to other liquid particles. In a sample of a liquid, a particle that happens to have enough kinetic energy may evaporate from the surface of the liquid. (different than boiling!)

• Liquids can’t be compressed. You can’t fit 2 gallons of water in a 1 gallon bottle!

• Diffusion happens within a liquid.• Liquids exhibit surface tension. Polar liquids

have very high surface tension.

SolidsSolids have a definite shape and volume.

They don’t “flow.”

Particles in a solid are in fixed locations relative to each other.

Glass and some plastics are not considered true solids because they don’t have a clear melting point. Even though they seem pretty solid to us, they flow verrrrrrryyyyy slllloooowwwly! Real solids have a definite melting point.

Solids don’t compress or diffuse well.

Some solids are crystals. The stronger the bond between the particles, the higher the melting point of the crystal.