Ideal Gases
Liceo Alfano
Reminder
Last week: Heat propagation
•
Conduction
•
Convection
•
Radiation
This week: Ideal gases
Concept Question
The molecules in a gas are
1)
Close together
2)
Far apart
Concept Question
The molecules in a gas are
1)
Close together
2)
Far apart
Concept Question
As you add heat to a gas, the molecules move
1)
Faster
2)
Slower
Concept Question
As you add heat to a gas, the molecules move
1)
Faster
2)
Slower
What is an Ideal Gas?
•
The movements of each molecule in a gas are so
random that it’s impossible to model them exactly
•
An ideal gas is a “perfect gas”– a gas where we
simplify the movements so we can make
calculations
•
In an ideal gas, we pretend the molecules are very
small and that they don’t interact with each other
•
This is true of most situations, so it is a valid
assumption
What is Pressure?
•
Pressure is a measure of how much individual
atoms are “pushing” on the sides of a container
in a gas
•
More collisions = higher pressure
•
If you pump air into a container that can’t get
any bigger, would the pressure increase or
decrease?
What is Pressure?
•
Pressure = Force/Area
•
We control the
pressure of a gas with
a piston
•
Measured in
atmospheres (atm) or
Pascales (Pa)
Concept Question
If I decrease the volume of an ideal gas container
(at constant temperature), the pressure will
1)
Increase
2)
Decrease
Concept Question
If I decrease the volume of an ideal gas container
(at constant temperature), the pressure will
1)
Increase
2)
Decrease
Video
As pressure decreases, volume does what?
Scuba Diving
•
Think of a scuba diver
coming up from the bottom
of the ocean
•
As he gets closer to the
surface, what happens to the
pressure?
•
As the pressure decreases,
what happens to the volume • If he comes up too
quickly,
his
lungs
of the air in his lungs?
might explode!
Concept Question
Now let’s look at pressure and temperature. If I
increase the temperature of an ideal gas (at
constant volume), the pressure will
1.Increase
2.Decrease
Concept Question
Now let’s look at pressure and temperature. If I
increase the temperature of an ideal gas (at
constant volume), the pressure will
1.Increase
2.Decrease
Video
When the temperature goes down after the fire
stops, what happens to the pressure in the bottle?
Car Tires
•
Think about the tires
on a car
•
If they get very hot,
what will happen to the
pressure of the air
inside?
•
The tires can’t get too
hot, or they’ll explode!
Concept Question
Now let’s look at volume and temperature. If I
increase the temperature of an ideal gas (at
constant pressure), the volume will
1.Increase
2.Decrease
Concept Question
Now let’s look at volume and temperature. If I
increase the temperature of an ideal gas (at
constant pressure), the volume will
1.Increase
2.Decrease
Video
As the temperature drops, what happens to the
volume of the air in the balloons?
Hot Air Balloon
•
Think about when they
are inflating a hot air
balloon. What are they
doing?
•
They make the air
inside the balloon hot,
making it get bigger!
Football
•
If you inflate a football
inside then go play
outside on a cold day,
what happens to the
volume of the football?
•
The football actually
gets a bit smaller!
Combined Gas Law
THE EQUATION: P1V1/T1 = P2V2/T2
•This
is for ONE GAS in TWO STATES
•Does
this agree with the conditions we just
established?
•Let’s
keep temperature constant
•Now
let’s keep volume constant
•Now
let’s keep pressure constant
Ideal Gas Law
Taking the combined gas law and plugging in STP
(standard temperature and pressure) conditions, we get a
new equation
THE EQUATION: PV = nRT
•
P = pressure (atm)
•
V = volume (L)
•
n = number of moles of the gas
•
R = ideal gas constant (0.0821 L*atm/mol*K or
8.31m3*Pa/K*mol)
•
T = temperature (K)
Experiments
•
Collapsing can (collasso della lattina)
•
Dancing cap (Il tappo ballerino)
•
Boiling syringe (La siringa ad ebollizione)
•
“Pee-pee” boy
Practice Problems
Remember, PV = nRT (R=0.0821 L*atm/mol*K)
and P1V1/T1 = P2V2/T2
If I have 21 mol of nitrogen gas at a pressure of 78
atm and a temperature of 900K, what is the
volume of the gas?
A container filled with 0.25 mol of oxygen gas has a
volume of 2.35L at a temperature of 45C. What is
the pressure of the gas?
Practice Problems
Remember, PV = nRT (R=0.0821 L*atm/mol*K)
and P1V1/T1 = P2V2/T2
An air bubble at the bottom of a lake, at a pressure of
120kPa, has a volume of 1cm3. If the temperature at the
bottom of the lake is 5.5C and the temperature at the top
is 21C, what is the volume of the bubble just before it
reaches the surface?
•
1 atm = 101kPa
•
1L = 1000cm3
Practice Problems
Remember, PV = nRT (R=0.0821 L*atm/mol*K)
and P1V1/T1 = P2V2/T2
I have 21.6kg of neon at a pressure of 3.65 atm. What
will the pressure be if the nitrogen is replaced by
21.6kg of carbon gas? (volume and temperature don’t
change)
•Molecular
mass of neon= 0.020 kg/mole
•Molecular
mass of carbon= 0.012 kg/mole