Properties of Gases

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Properties of Gases
Mr. Shields Regents Chemistry U05 L01
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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
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How many elements exist as a gas at STP?
How many are monoatomic and how many are diatomic?
6 monoatomic and 5 diatomic
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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! Theres 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
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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 theyre in
• They assume the shape of the container
• 2. They are randomly organized and move about
  freely
• 3. They have mass

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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)
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What do we know about Gases
If gases occupy volume and have mass what
intensive property does this define? Density
(dm/v)
Of all the states of matter which state has the
lowest densities?
Gases Why is that?
In general Densitygas lt DensityLiq lt
Densitysolid
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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 Whats the
relationship between T,P and D?
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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
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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
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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.
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Pressure the snow shoe
The mathematical expression for pressure
is. Pressure force / area
So lets 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?
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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 !
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KMT
Weve discussed a number of the properties of
gases and weve 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)