Download 5.9 Diffusion of Gases and Daltons Law

Unit 5: States of Matter
Content Outline: Diffusion of Gases and Dalton’s Law (5.9)
I.
II.
The Ideal Gas according to the Kinetic Molecular Theory of Matter
A. Gases consist of large numbers of tiny particles that are far apart relative to their size.
B. Collisions between gas particles and/or between particles and the container walls are Elastic
collisions.
C. Gas particles are in continuous, rapid, random motion. They possess Kinetic Energy.
D. The Kinetic Energy allows the individual atoms/molecules to overcome all attractive forces.
E. The temperature of a gas depends on the mass and velocity(speed) of the atoms/molecules in the gas.
The Ideal Gas Laws mathematical equation is:
A. PV = nRT
1. R is the Ideal Gas Constant – its value is (0.0821 L*Atm/Mol* K)
2. V is volume – It is measured in Liters.
3. T is Temperature – It is measured in Kelvins
4. P is pressure – It is measured in Atmospheres
5. n is amount of a substance measured in moles
III. Kinetic-Molecular Theory of Real Gases
A. Diffusion (the ability at atoms/molecules to “mix” together over time)
1. The atoms/molecules of a gas move randomly and spontaneously (because of the high Kinetic
Energy they possess) allowing for different gases within the same container to easily mix into one
combined composition.
2. The diffusion is always from an area of high concentration to an area of low concentration.
Factors that affect the rate of diffusion include:
a. Temperature – colder things move slower than warmer things
b. Mass – heavier things move slower than lighter things
c. Size of the container – the bigger the container the longer it takes to completely diffuse
d. Agitation – “stirring” the environment Like a fan stirs the air or you stir a liquid in a beaker.
IV. Measuring Pressure of gases
A. Pressure (P) is the unit force exerted on a surface (container or atmosphere).
1. So far we know that Pressure (P) is expressed in Atmospheres, using a Barometer.
a. 1 Atmosphere = 760 mm Hg (Mercury) at 0OC and sea level.
b. 1 mm Hg = 1 torr
2. The barometer was developed by the Italian Physicist, Evangelista Torricelli in 1628 to
measure Atmospheric pressure.
a. It works using a fluid such as Mercury (a toxic metal), a sealed glass tube, and a pan. How
simple, don’t you think?
b. How it works:
c. Step 1: Fill the glass tube with the liquid. Torricelli used Mercury (not smart, but he did.)
Step 2: Fill the pan with the same liquid.
Step 3: Using your thumb as a “cork”, invert (turn upside down) the glass tube so that the
open end is on the bottom.
Step 4: Put your thumb and glass tube end into and under the surface of the liquid in the
pan.
Step 5: Release and wait. (They liquid, in the tube, will begin to move (up or down) to
equate with the atmospheric pressure. The atmospheric pressure is “pushing” down on the
fluid surface in the pan.
 If Atmospheric pressure is greater (pushing down)… the fluid will rise.
 If the atmospheric pressure is less… the fluid will go down because of gravity
force being greater.

At equilibrium, both the pressure in the tube and the atmospheric pressure
are equal. Can you “see” the definition in the word?
Step 6: Measure the change that took place.
 If we used Mercury (Hg), the liquid would be 760 mm (7.6 cm) above the
surface in the pan.
 Watch the weather report tonight and they will talk about pressure.
If the pressure is rising, it will be a sunny day.
If the pressure is falling, it will be a rainy day.
If the pressure is really low and falling, there may be a hurricane. It could
also be a tornado. The low pressure is why the houses explode into small
pieces. You should open the windows to reduce the pressure difference
between the house and outside… then go seek shelter in the interior lowest
part of the house.
3. To measure the Pressure of a gas, we would use a different piece of equipment called a
manometer.
a. All gases exert pressure by their continuous colliding with the container wall.
b. This works much the same way as the barometer, but we need an enclosed structure,
instead of an open pan, to contain the gas. The other end of the glass tube may be sealed
(called a closed-arm manometer) or it may be open (called an open-arm manometer).
 Closed –arm used to compare just a single gas or gas to gas.
 Open-arm used to compare a gas to the atmospheric pressure.
c. How it works:
Step 1: Fill the manometer glass tube with a liquid, such as Hg or water.
Step 2: Fill the gas chamber with the pure gas.
Step 3: Connect the manometer and gas chamber through a closed cock lever.
Step 4: Open and allow the gas to travel into the glass tube.
Step 5: Record the change in fluid level.
 If the gas has greater force than the liquid, the liquid will rise.
 If the gas has less force than the liquid, the liquid will fall.
4. Pressure can also be measured in Pascal(Pa)
a. A French Physicist named Blaise Pascal in 1646 proposed this unit.
b. This is mostly a Physics unit with gases, but it can be used in Chemistry.
c. It relates pressure in Newtons to Surface Area (meter2)
 1 Newton/1m2 = 1 Pascal
 1 Atmosphere = 1.013 X 105 Pa = 101.3 kPa (kiloPascals)
V. Dalton’s Law of Partial Pressures
A. This Law was proposed by the English Chemist John Dalton (same one who proposed the Atomic
Theory in Unit 1) in early 1800’s.
B. The law states: The total pressure of an unreactive gas mixture (such as air) is the sum of the
partial(individual) pressures of the component gases in the container.
C. Mathematically expressed as:
Ptotal = P1 + P2 + P3… (This is easy…just adding and subtracting.)
VI. Solving Dalton’s Law Problems:
A. Step 1: Check your units – Are you given Atmospheres, Torr, or a percentage?
B. Step 2: Are you being asked for Atmospheres or Torr?
C. Step 3: Perform necessary conversions
For example Dry Air at Sea Level and 0OC based upon percentage composition – 78.08% N2, 20.95%
O2, and 0.93 Ar. Calculate the partial and total pressures of each?
We can “think” of this as a composition problem:
Step 1: We are given %, so we have to convert the percentage to a part of 1 atm or 760 torr
Step 2: Being asked for Atmosphere or Torr, so let’s use torr.
Step 3: Start with Nitrogen.
Nitrogen = 78.08% X 760 torr = 593.4 torr
Step 4: Go to Oxygen.
Oxygen = 20.95% X 760 torr = 159.2 torr
Step 5: Lastly Argon.
Argon = 0.93% X 760 torr = 7.1 torr
Step 6: Use Dalton’s Law to add to get the sum (total).
N2 + O2 + Ar = 593.4 torr + 159.2 torr + 7.1 torr = 769.7 torr or just over 1 atm.