The Gas Laws

What is a gas? As a substance changes from solid to liquid to gas, it has more entropy (randomness). It will also have more kinetic energy. Can you think of an exception? (hint: water) Golf Ball Demo

Kinetic Theory of Gases The kinetic theory of gases says that moving gas molecules collide with the container they are in. These collisions cause gas pressure.

Kinetic Theory of Gases Basis: all matter is composed of tiny particles called molecules that are in constant motion. Gas molecules: are small compared with the average distances between them collide without loss of kinetic energy exert almost no forces on one another outside of collisions Thus, a gas is mostly an empty space The absolute temperature of a gas is proportional to the average kinetic energy of its molecules

Boyle’s Law Boyle’s law relates gaseous volume and pressure under constant temperature P1 V2 --- = ---- P2 V1

Boyle’s Law

Charles Law Charles Law relates V (volume) and T (temperature). As pressure increases, temperature _____? Both Charles and Boyles Laws assume something is constant. What is it?

Charles Law Each of these balloons is at a different temperature. Find the one placed in the freezer, the one outdoors in the sun and the one in the classroom.

Combined Gas Law Combined Boyle’s and Charles’ laws give the ideal gas law p1 V1 p2 V2 ------- = ------- T1 T2 At constant T (T1 = T2) we have Boyle’s law At constant p (p1 = p2) we have Charles’ law p V ----- = const T

Ideal Gas Law An ideal gas has particles that do not have any mass or size. The collisions of particles with one another are elastic (no energy is lost) Real gases are not ideal, but ideal gas laws do approximate true conditions except at very high pressure and temperature.

Ideal Gas Law PV = nRT P pressure V volume n number of moles R gas constant (8.31 J K-1mol-1 or 8.205 X 10-2 L atm K-1mol-1 T temperature in Kelvin (Kelvin = 273 + Celsius

What is the volume of 1 mole of gas? Volume is related between all gases by Avogadro's hypothesis, which states: Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules

So, a mole of gas has a volume of 22. 4 L So, a mole of gas has a volume of 22.4 L. Any gas will do—as long at it is at STP

What is STP? Standard Temperature and Pressure T = 25oC (273 + 25 = 298 K) P = 1 atm

Summary The concept of ideal gas is a good model to study properties of substances The absolute zero temperature corresponds to the absence of molecular motion and cannot be reached. Heat cannot be effectively converted into other forms of energy. Entropy or disorder in isolated systems can only be increased.

How do hot air balloons work? Which of the gas laws apply?

Building Your Own Balloon