Let’s Learn the Gas Laws
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Transcript Let’s Learn the Gas Laws
Let’s Learn the
Gas Laws
Some definitions first…
Temperature
Measure of the average kinetic energy of the particles
in a substance
Kinetic energy – energy of motion
Volume
The amount of space occupied by a sample or
substance
Pressure
Force divided by area
A “push” what we will use
What is a gas?
Gas – no definite shape or volume
It will take shape of container it is contained in
Gases have 3 properties we can measure:
Temperature (T)
Volume (V)
Pressure (P)
If there is one constant, there is a relationship between the
other 2!
Now to Gas Laws…
Jacques Charles
1746 – 1823
Volume and Temperature
Charles Law
The relationship between volume and
temperature
Pressure is constant
Temperature vs. Volume Graph
30
25
15
10
5
0
– 273
Temperature (C)
100
Volume (mL)
20
Absolute Zero
If a volume vs. temperature graph is plotted for gases,
most lines can be extrapolated downward so that
when volume is 0 the temperature is -273 C.
Obviously naturally, gases don’t really reach a 0
volume, but the spaces between molecules approach 0.
At this point all molecular movement stops.
–273C is known as “absolute zero” (0 K)
no KE- energy of motion
Therefore we get the Kelvin scale…
Kelvin Scale
Lord Kelvin suggested that a reasonable temperature
scale should start at a true zero value.
He kept the convenient units of C, but started at absolute
zero.
Therefore since at 0 volume, temperature is -273 C:
K = C + 273
Practice: 62C = ? K
From now on, we will be using Kelvin!!
Back to Charles's Law…
Your data shows that…
• As T increases, V increases (↑T = ↑V )
• As T decreases, V decreases (↓T = ↓V)
Therefore this is a direct relationship
But why is this
important??
My experience with a hot air balloon
The balloon and basket were placed in a field.
The burner was ignited and a huge fan blew the
heated air into the balloon on the ground.
The balloon started to fill up and lift off of the
ground.
When the balloon was
completely filled with hot
air where would it be??????
A balloon and liquid nitrogen
http://www.youtube.com/watch?v=al5f9q845q0
What is happening to the balloon?
What is happening to the temperature? Volume?
Joseph Louis Gay-Lussac.
1778-1850
Pressure and Temperature
Pressure
Lets look at our data…
Temp
Gay-Lussac’s work
Determined the relationship between temperature and
pressure of a gas.
Measured the temperature of air at different pressures,
and observed a pattern of behavior which led to his
mathematical law.
During his experiments volume of the system and amount of gas
were held constant.
He found: ↑T = ↑P
↓T = ↓P
Therefore this is a direct relationship.
What is pressure?
The pressure of a gas is the push exerted on the wall
of the container a gas is trapped in.
There are several units for pressure depending on
the instrument used to measure it including:
Atmosphere (atm)
Millimeters of mercury (mmHg)
Kilopascals (kPa)
Torr (torr)
We will only be using atm and torr
Why is this important?
Car tires… what happens to the pressure
inside the tire during the winter (when
it is cold)?
As temperature decreases, pressure
decreases
This is why we have to add air to the
tires during winter
Some practice problems…
What is the relationship between T and P?
Whose law is this?
What is the relationship between T and V?
Whose law is this?
Lets try some calculations
Always set up:
P1 =
P2 =
T1 =
T2 =
** Remember temperature has to be in K and pressure in atm **
You will have one unknown
Write down the starting temperature or pressure…
What is the relationship?
If you want a bigger number
If you want a smaller number
Lets try some calculations
Always set up:
V1 =
V2 =
T1 =
T2 =
You will have one unknown
Write down the starting temperature or volume…
What is the relationship?
If you want a bigger number
If you want a smaller number
Robert Boyle
1627 – 1631
Volume and Pressure
Lets look at our data…
Boyle’s Law
Boyle’s law states that pressure and the volume of a
gas are inversely proportional.
At constant temperature for a fixed mass.
Therefore: ↑V = ↓ P
↓V = ↑ P
Therefore this is a indirect relationship.
Why is this important?
Let’s try making straw snap…
Hold both ends and flick it…
What happened?
Now reduce the volume and
flick it…
What happened?
Okay, but what else?
Making the best water balloons…
What happens if you keep filling the balloon?
What is the best water balloon for a water balloon toss?
• Smaller – will not pop as easily
What is the best water balloon to throw at someone?
• Larger – will pop very easily
Lets try some calculations
Always set up:
V1 =
V2 =
P1 =
P2 =
** Remember pressure in atm **
You will have one unknown
Write down the starting volume or pressure…
What is the relationship?
If you want a bigger number
If you want a smaller number
Gas Laws… Continued
The Combined Gas Law
Lets Review…
Charles’ Law – volume and temperature
Boyle’s Law – pressure and volume
Charles’ Law & Boyle’s Law
Charles’ Law - Temperature and gas have a
direct relationship.
T↑ V↑ and T↓ V↓
Boyle’s Law - describes the indirect
relationship between the pressure and
volume of a gas.
P↑ V↓ and P↓ V ↑
The Combined Gas Law
The combined gas law is a gas
law which combines Charles‘ law
and Boyle's law.
In the combined gas law, the
volume of gas is directly
proportional to the absolute
temperature and inversely
proportional to the pressure.
The Combined Gas Law
Equation:
P1V1 = P2V2
T1
T2
Where:
P1 is the initial pressure P2 is the final pressure
V1 is the initial volume
V2 is the final volume
T1 is the initial
T2 is the final
temperature (in Kelvin)
temperature (in Kelvin)
Let’s Practice…
A gas takes up a volume of 17 liters, has a
pressure of 2.3 atm, and a temperature of 299 K. If
I raise the temperature to 350 K and lower the
pressure to 1.5 atm, what is the new volume of the
gas?
P1 =
P2 =
V1 =
V2 =
T1 =
T2 =
Ideal Gas Law
STP
Standard Temperature and Pressure
l T = 273K
l P = 1 atm, 101.3 kPa, 760 mmHg, 760 torr
Ideal Gas
An Ideal Gas (perfect gas) is one which obeys the
ideal gas law exactly.
Ideal Gas
An Ideal Gas is modeled on the Kinetic Theory of
Gases which has 4 basic postulates:
Gases consist of small particles (molecules) which are
in continuous random motion
The volume of the molecules present is negligible
compared to the total volume occupied by the gas
Intermolecular forces are negligible
Pressure is due to the gas molecules colliding with the
walls of the container
Real Gases
Real Gases deviate from Ideal Gas Behavior
because:
at low temperatures the gas molecules have less kinetic
energy (move around less) so they do attract each other
at high pressures the gas molecules are forced closer
together so that the volume of the gas molecules
becomes significant compared to the volume the gas
occupies
Under ordinary conditions, deviations from Ideal
Gas behavior are so slight that they can be neglected
A gas which deviates from Ideal Gas behavior is
called a non-ideal gas.
Ideal Gas Laws
The ideal gas law is a combination of all the gas laws:
Boyle’s, Charles’, Gay-Lussac’s
The ideal gas law can be expressed as
PV = nRT
Where:
P is the pressure in atm
V is the volume in liters
n is the number of moles
R is a constant (0.0821 L atm /mol K)
T is the temperature in Kelvin
• How do you calculate Kelvin?
Lets Practice…
Number 1 on Homework worksheet
If I have 4 moles of a gas at a pressure of 5.6 atm
and a volume of 12 liters, what is the temperature?
P=
n=
V=
R=
T=
PV = nRT