Objectives:
1) Introduce the empirical properties of gases
2) Describe STP and SATP in terms of pressure and temperature
3) Convert between Celsius and Kelvin temperature scales

All gases have different chemical
properties but similar physical properties

Chemical Properties:
› Gases are either: very reactive (ex. F, Cl, O)
slightly reactive (ex. N)
or inert (ex. noble gases)

Physical Properties:
› Regardless of chemical properties, all gases:
1) Always fill their container
(have no definite shape or volume)
2) Are highly compressible
(increase pressure = decrease volume)
3) Diffuse through any available space
4) Have volume and/or pressure affected by temperature
temperature
volume
(if container can expand)
temperature
pressure
(if the container is a set volume)

Gas – empirically, it is a substance that:
› Fills the shape of any container
› Diffuses rapidly
› Mixes with other gases easily

›
in pressure and/or volume when heated
›
in pressure with increase in volume
Pressure – force per unit area
› Unit: kPa (kilopascal) or atm (atmosphere)
› caused by the collisions of gas particles with the
sides of the container

When you wear snowshoes, the
force is distributed over the
surface area of the snowshoes,
so you exert less pressure on
the ground than you would if
you wearing regular shoes.

This allows you to walk over
snow instead of sinking into it.

Pressure exerted by air on all objects
Standard Temperature and Pressure (STP)
= 101.325 kPa (use 101 kPa) and 0°C


But laboratory temperatures are not at 0°C
So scientists agreed on another set of conditions...
Standard Ambient Temperature and Pressure (SATP)
= 100 kPa and 25°C

Much closer to lab conditions – so scientists don’t freeze

Units of Pressure

What happens when you need to
convert between units of pressure
?????

Convert 5 torr to atm

Convert 690 mm Hg to kPa and atm

At sea level, average atmospheric
pressure is about 101kPa.

Scientists used this value to define
one standard atmosphere (1 atm)
as 101.325kPa

Scientists have also used a mercury
barometer to measure atmospheric
pressure so standard pressure is also
defined at 760 mm Hg

101.325 kPa = 1 atm = 760 mm Hg
• When a tube filled with mercury is
inverted, the weight of the column of
mercury pulls it toward Earth.
• However, the weight of the air
directly above the open dish pushes
down on the surface of the mercury
and prevents all of the mercury from
falling out of the tube.
• The two opposing forces balance each
other when the height of mercury is about
760 mm.
• If the vertical mercury filled tube is longer
than 760 mm, the mercury drops to 760 mm.

A student took a photo
of an empty soda
bottle at a rest area in
the Rocky Mountains.

Their GPS helped
determine the altitude.

When they returned to
sea level the bottled
collapsed because of
the air pressure. WHY?

Marnie & Deanna are collecting air pressure readings for
a class project.

Marnie is recording her data near sea level. Deanna
chose to hike to the top of a nearby mountain.

Where is the air pressure greater?
• The picture shows how
much more air is in the
column above Marnie
than above Deanna.
• The air pressure at sea
level is greater than at
Deanna's elevation.

How hot or cold an object is? No, not really...
› It’s the average kinetic energy of the particles of
a substance
› Absolute zero – the lowest temperature that can
be obtained; the kinetic energy of all entities of
solids, liquids or gases would become zero
› Kelvin temperature scale - absolute zero is zero
Kelvin (0 K = -273 °C)
› Celsius temperature scale – zero is when water
freezes (273 K = 0 °C)

To convert degrees Celsius to
Kelvin , you add 273.
K = °C + 273

To convert Kelvin to degrees
Celsius, you subtract 273.
°C = K - 273

Examples:
› What is 254 K in °C ?
-19°C
› What is -34°C in K ?
239K

Yes – using Kelvin you have different numbers to use:
Standard Temperature and Pressure (STP)
= 101 kPa and 0°C
= 101 kPa and 273.15K (use 273K)
Standard Ambient Temperature and Pressure (SATP)
= 100 kPa and 25°C
= 100 kPa and 298.15 K (use 298K)

Much closer to lab conditions – so scientists don’t freeze

Pg. 150 #1-3
› Remember: 101kPa =1 atm =760 mm Hg

Pg. 154 # 11-13
› Remember: T (K) = t (°C) + 273