Announcements
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Text HW due tomorrow (Friday)
Online HW #3 (Type 1) due Monday,
September 17 by 7:00 p.m.
Online HW #3 (Type 2) due Wednesday,
September 19 by 7:00 p.m.
Lab write-up for Gases Lab due
Wednesday, September 19 at the start of
lab
Laws versus Theories
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Laws:
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Tell us what’s true
Observations
Facts
Theories:
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Tells us why (gives us an explanation)
a MODEL (not reality)
We like our theories to be simple
Gas Laws
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Think about what you know is true about
gases
Let’s make some observations of gases
and their behavior
Gas Laws
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First observation: gases take up the
volume of its container (has no definite
shape or volume)
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Fill a balloon with gas. What is the shape of
the gas?
Fill a soda bottle with gas. What is the shape
of the gas?
Gas Laws
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Second observation: gases mix well
(diffusion)
Constant random motion and collisions
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Water vapor mixes with air
Gas Laws
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Third observation: gases exert pressure
Collisions of gas particles with walls of its
container
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Why does a balloon stay inflated? Because
the gases inside are pushing on the walls of
the balloon.
A barometer works because of atmospheric
pressure
Gas Laws
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So what does this tell us?
For gases, we need to look at:
Pressure (P)
Volume (V)
Temperature (T)
Moles (n)
All of these factors matter with gases; these
aren’t as important when talking about liquids
and solids
V versus n
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Volume versus moles of gas (V vs. n)
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Take two balloons
Inflate them to different sizes
I blew more CO2 into the larger balloon, so
the volume got bigger
So as the number of moles increase, the
volume increases
Temperature is constant; but what about
pressure?
Clicker #1
Is the pressure in the larger balloon greater
than, less than, or equal to the smaller
balloon?
A) greater than the smaller balloon
B) less than the smaller balloon
C) equal to the smaller balloon
Clicker #1
Is the pressure in the larger balloon greater
than, less than, or equal to the smaller
balloon?
A) greater than the smaller balloon
B) less than the smaller balloon
C) equal to the smaller balloon
V versus n
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The pressure is equal because it has to be
about the same as atmospheric pressure,
otherwise it would expand more or deflate
n, V (at constant T and P)
P versus n
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Pressure versus moles of gas (P vs. n)
Carbon dioxide can sublimate:
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CO2 (s)  CO2 (g)
Place solid CO2 in bag
What happens as solid CO2 becomes gaseous
CO2?
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The bag inflates
The number of moles of gaseous CO2 increases
But what about the pressure?
Clicker #2
As the number of moles of carbon dioxide
increase, the pressure in the bag will
A) increase
B) decrease
C) stay the same
Clicker #2
As the number of moles of carbon dioxide
increase, the pressure in the bag will
A) increase
B) decrease
C) stay the same
P versus n
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So since the T and V (of the bag) are
constant, as the number of moles of CO2
increase, the pressure in bag will increase
n, P (at constant T and V)
P versus V
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Pressure versus volume (P vs. V)
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Syringe
If I push the plunger on a syringe,
the volume inside decreases
But what if I block the tip of the
syringe and push? Is it easier or
harder to push the plunger down?
What does this mean about the
pressure?
Clicker #3
If the volume of gas inside the syringe
decreases, the pressure will
A) increase
B) decrease
C) stay the same
Clicker #3
If the volume of gas inside the syringe
decreases, the pressure will
A) increase
B) decrease
C) stay the same
P versus V
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So as the volume of a container
decreases, the pressure inside the
container increases
V, P (at constant T and n)
V versus T
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Volume versus temperature (V vs. T)
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Liquid nitrogen boils at -196C
Air in balloon is at room temperature (25C)
What will happen to the temperature of the
gas inside the balloon if it is submerged in
liquid nitrogen?
Clicker #4
If the temperature of the gas inside the
balloon decreases, the volume will
A) increase
B) decrease
C) stay the same
Clicker #4
If the temperature of the gas inside the
balloon decreases, the volume will
A) increase
B) decrease
C) stay the same
V versus T
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As the temperature of a gas decreases,
the volume also decreases
T, V (constant n and P)
P versus T
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Pressure versus temperature (P vs. T)
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Aerosol can filled with gas
Can is sealed so volume is constant
What would happen if I heated the sealed
aerosol can?
Clicker #5
If the temperature of the gas inside the can
increases, the pressure will
A) increase
B) decrease
C) stay the same
Clicker #5
If the temperature of the gas inside the can
increases, the pressure will
A) increase
B) decrease
C) stay the same
P versus T
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As the temperature of a gas increases,
the pressure also increases
T, P (constant V and n)
P, V, n, T relationships
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We can also quantify these relationships
(think about these, don’t memorize!)
P, V, n, T relationships
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How are V and n related?
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Directly related
P, V, n, T relationships
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How are P and n related?
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Directly related
P, V, n, T relationships
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How are P and V related?
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Indirect and inverse relationship
Note: cannot go to zero because gas will always have some volume
and pressure!
P, V, n, T relationships
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How are P and T related?
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Directly related
P, V, n, T relationships
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How are V and T related?
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Directly related
V versus T
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But if T=0C or 0F  no temperature?
No! T can be lower than this!
If you lower the temperature enough, gas
will become a liquid so you have to
extrapolate
Shift to Kelvin scale: K = C + 273
Absolute scale
P, V, n and T
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To summarize:
V is directly proportional to n
V is directly proportional to T
P is directly proportional to n
P is directly proportional to T
P is inversely proportional to V
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P is proportional to 1/V
V is proportional to 1/P
Ideal Gas Law
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PV is directly proportional to nT
Turns out that PV/nT = R and for any gas
that is the same
PV = nRT
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If P is in atm, V in L, n in mol, T in K, then R
= 0.08206 L atm/mol K
All of the common sense things you know
about gases is in this law!
V versus T and Energy
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Temperature is related to the kinetic
energy of the gas molecules (measures
motion of the gas molecules)
At 0 Kelvin (absolute zero) there would be
no energy in the system
V versus T and Energy
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If I increase the temperature of a gas
from 20C to 40C, have I doubled the
kinetic energy?
No!
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But why not?
Celsius is not an absolute scale (0C is not
the lowest possible temperature)
V, T, and KE
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The kinetic energy does not double
because the temperature has not really
doubled!
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The difference between -273C and 20C is
not the same as the difference between 20C
and 40C
V, T, and KE
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But what if the temperature changed from
100K to 200K? Does the kinetic energy
double?
V, T, and KE
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What if you compare the kinetic energy of
the change between 0C to 20C and 0C
to 40C?