Sparks
CH301
DIFFUSION AND EFFUSION
MIXTURES – Day 6
HW Posted
What did we learn last time?
Ideal Gas is amazing – empirically derived and also
theoretically derived.
We now know how to relate rms velocity to both
temperature and mass
Finally, there is a distribution of velocities. This will have
huge implications for future understanding of chemistry!
Demo
Let’s think about our demo. What is the ratio
of the speeds of the two molecules in our
demo? NH3 : HCl
Numerical Question: Give an answer to one decimal place
Standard Molar Volume
One mole of any ideal gas occupies 22.4 L at STP
Most real gases
have standard
molar volumes
within 2% of this
value.
Try it Out!
Avogadro Example 1
What is the density of CO2 gas at STP?
A. 1.96 g/L
B. 44 g/L
C. 0.05 g/L
D. 1050 g/L
Engaging in practice matters…
Illusion of Understanding
•Watching isn’t the same as doing
Maximize Learning Opportunities
•test your understanding
•make mistakes
•receive coaching
What are we going to learn today?
EFFECT OF VELOCITY ON DIFFUSION AND EFFUSION
REPRESENTING GAS MIXTURES
Concept of Partial Pressures
Diffusion and Effusion
Diffusion:
Spread of particles due to random motion
(perfume “smell” wander across the room)
Effusion:
Loss of gas from a container through a small pore.
(He balloon that deflates slowly)
Both directly related to the velocity of the gas particles
Effusion
POLL: CLICKER QUESTION
You have two gases under identical conditions.
One gas has a density that is double that of the
other gas. What is the ratio of the rate of diffusion
of the high density gas compared lower density gas
A.
B.
C.
D.
E.
2 times less
Sqrt(2) times less
2 times faster
sqrt(2) times faster
they are identical
Figure 5.20: Uranium-enrichment converters from the Paducah gaseous diffusion plant in Kentucky.
Actually- this is technically effusion...!
MASS DENSITY FOR GASES
THINK BACK TO BALLOONS
SAME T & P
DIFFERENT DENSITIES WERE DUE TO THE
DIFFERENT MASSES OF THE GAS
“PARTICLES”
WHAT ABOUT MIXTURES?
How to describe a mixture
Two containers of equal volume separated by a wall
nHe= mole He
T = 300K
P = 1 bar
nAr = mole Ar
T = 300K
P = 1 bar
Mixtures
Groupwork quiz
Remove the wall. What is the total pressure?
Partial Pressure
Total pressure is still 1 bar
Where does the pressure come from?
We can think of dividing it up into the
pressure from the He and the pressure from the Ar.
Mixtures
Half the particles are Ar so half the pressure should be from Ar
Mixtures
The same is true for He
PHe = nHeRT/V
nHe is half the number of total moles
So PHe is half the total pressure
Partial Pressure
This is what we call “partial pressure”
In a mixture, the partial pressure of gas “i”
Pi = niRT/V
Dalton’s Law
The sum of all partial pressure must be the
total pressure
Mole Fraction Percentage
What fraction of the particles are gas “i”?
Mole Fraction Percentage
What fraction of the particles are gas “i”?
Mole fraction Xi is the number of moles i
divided by the total number of moles
Mole Fraction Percentage
What fraction of the particles are gas “i”?
Mole fraction Xi is the number of moles i
divided by the total number of moles
Mole Fraction Percentage
What fraction of the particles are gas “i”?
Mole fraction Xi is the number of moles i
divided by the total number of moles
Mole Fraction Percentage
What fraction of the particles are gas “i”?
Mole fraction Xi is the number of moles i
divided by the total number of moles
Mole Fraction Percentage
What fraction of the particles are gas “i”?
Mole fraction Xi is the number of moles i
divided by the total number of moles
Two tanks are connected by a closed valve. Tank
A, filled with O2, has a volume of 2 L and a
pressure of 5 atm. Tank B, filled with N2, has a
volume of 4 L and a pressure of 10 atm. The
valve is opened. What is
1. The partial pressure exerted by oxygen?
2. The total pressure exerted by the mixture?
3. What is the mole fraction of nitrogen?
Report each answer to one decimal place
What did we learn today?
We can apply our knowledge of velocities to diffusion
and effusion of gases
We can utilize the idea of partial pressures and mole
fractions to understand more about a gas sample.
DAY 6 LEARNING OUTCOMES
Perform calculations to determine the mole fractions of gases
within and gas mixture and relate mole fraction to the partial
pressure of a gas within a gas mixture.
Describe the relationship between partial pressures and the
total pressure as described in Dalton’s Law of Partial
Pressure.