Properties of Gases Mr. Shields Regents Chemistry U05 L01

What do we know about Gases What are the 4 states of matter? - Solids - Liquids - Gases - Plasmas So… GASES are a one of the 4 states of matter How many elements exist as a gas at STP? 11 How many are monoatomic and how many are diatomic? 6 monoatomic and 5 diatomic

What do we know about Gases What are the elemental monoatomic and diatomic gases? - Monoatomic: He Ne Ar Kr Xe Rn - Diatomic: H2 N2 O2 F2 CL2 Are there any other gases at STP? Yes! There’s lots of Compounds that are Gases at STP. What are some of these other gases? CO2 NH3 CO NO2 SO2 CH4 CH3CH2CH3 and there are many many others

What do we know about Gases What classification of matter do elements and compounds Belong to? Pure Substances Can you describe some properties of the gaseous state? They have volume (They occupy space) They completely occupy the container they’re in They assume the shape of the container 2. They are randomly organized and move about freely 3. They have mass

What do we know about Gases If two gases are mixed together what kind of mixture will they form? Homogeneous or heterogeneous ? two gases initially separated will quickly move through one another to form a homogeneous solution (gas in a gas)

What do we know about Gases If gases occupy volume and have mass what intensive property does this define? Density (d=m/v) Of all the states of matter which state has the lowest densities? Gases Why is that? In general: Densitygas < DensityLiq < Densitysolid

What do we know about Gases Since gas molecules have much less mass per unit volume Than solids or liquids there must be greater spacing between The molecules For example: He has 5600x less mass per unit volume than water. (1 g/cm3 / 1.79 x 10-4 g/cm3 = 5600) What two factors affect the density of gases? 1. Temperature 2. Pressure What’s the relationship between T,P and D?

What do we know about Gases As temperature increases gas density DECREASES As temperature decreases gas density INCREASES As pressure increases gas density INCREASES As pressure decreases gas density DECREASES What happens to the density of a gas as the temperature decreases from 30 deg. C to 20 deg. C? Density increases

What do we know about Gases We know D = m/v so … if Gas Density changes when either Temperature or Pressure Change but Mass remains constant then what else Changes? It has to be the volume that changes when T or P changes D = M/V

What do we know about Gases Gases exert pressure. As I push on a gas it resists my attempt to decrease volume. As pressure increases the volume of a gas DECREASES If pressure decreases the volume of the gas INCREASES EXAMPLE: The “Popping” of your ears as altitude changes is a result of the air inside of your ear trying to equalize its pressure with the pressure outside the ear. Gases continue expanding until the volume they are in is Completely and homogeneously occupied.

Pressure & the snow shoe The mathematical expression for pressure is…. Pressure = force / area So let’s consider 3 people trying to walk across snow. - One in high heels - One in boots - One wearing snow shoes Which one can walk across snow easier? OK. But Why?

Pressure & the snow shoe A 140 lb person in high heels (1 in x 1in); Pressure = (140lbs/1in2)/2 = 70 lbs/sq in A 140 lb person in flat boots (10in x 3in); Pressure = (140lbs/30in2)/2 = 2.3 lbs/sq in A 140 lb person in snow shoes (28 in x 8 in); Pressure = (140lbs/224in2)/2 = 0.31 lbs/sq in 0.31 lbs/sq in is much easier for the snow to support !

KMT We’ve discussed a number of the properties of gases and we’ve Discussed in general terms how gases react to various changes (for example pressure and temperature) The BIG question is WHY DO GASES BEHAVE THIS WAY? To understand the WHY we need to discuss something called… The KINETIC MOLECULAR THEORY (KMT)