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# Gases

GCI Chapter 5
by

## Joe Christopher

on 28 September 2016

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#### Transcript of Gases

Mixtures of Gases
Kinetic Molecular Theory
Diffusion and Effusion
Gas Laws
3 Laws
Boyle's
Charles's
Properties of a gas
Pressure (P)
Volume (V)
Temperature (T)
Moles (n)
Applying the Ideal Gas Law
Standard Conditions (STP)
1.0 atm, 273K
Standard Molar Volume
22.4 L (1 mol of gas at STP)
Gas doesn't matter
Gas Pressure
Created by gas molecules as they strike the surfaces around them.
Number of gas particles in a given volume (Concentration)
Volume of the container
Average speed of the gas particles (Temp)
Gases
Ideal Gas Law
By combing the gas laws we can write a general equation
Allows calculation of any variable if other 3 known

Density of a Gas
Density is ratio mass/volume
Density of a gas is generally given in g/L
Atmospheric Pressure
Differences in air pressure result in weather and wind patterns
The higher in the atmosphere you climb, the lower the atmospheric pressure.
at the surface the atmospheric pressure is 14.7 psi (1 atm), but at 10,000 ft it is only 10.0 psi
Barometer
ear popping
Wind
Breathing
Straws
Density
Collisions
Boyle's Law
Pressure
is inversely related to
Volume
Graph of P vs. 1/V is linear
P x V = constant
Charles's Law
Volume
is directly related to
Temperature
Graph of V vs. T is linear
V/T = constant (if T measured in Kelvin)
Volume
directly proportional to the number of gas molecules (
Moles
)
V/n = constant
Equal volumes of gases contain equal numbers of molecules.
The gas doesn't matter
.
R
is called the
gas constant
Density at Standard Conditions
Density at Other Conditions
Density is directly proportional to molar mass
Easy to compare
Molar Mass of a Gas
One of the methods chemists use to determine the molar mass of an unknown substance is to heat a
weighed
sample until it becomes a gas; measure the temperature, pressure, and volume; and use the ideal gas law to calculate
moles of gas
.
What is different??
Combined Gas Law
Dynamic problems??
Practice Problem....
Procedure
1. Cancel out constant variables
2. Solve for unknown value
Dalton's Law of Partial Pressures
Each pressure value can be calculated using the ideal gas law
Mole Fractions can also be used
78%
21%
0.9%
Practice Problem???
The particles of the gas (either atoms or molecules) are constantly moving.
The attraction between particles is negligible.
When the moving gas particles hit another gas particle or the container, they do not stick;
elastic collision
.
There is a lot of empty space between the gas particles compared to the size of the particles.
The average kinetic energy of the particles is directly proportional to the Kelvin temperature
When gases are mixed together, their molecules behave independent of each other
All the gases have the same volume
fill the container
All gases at the same temperature
Same average kinetic energy
Can often be thought of as one gas
The pressure of a single gas in a mixture of gases is called its
partial pressure
The sum of the partial pressures equals the total pressure
For gases in reactions we use the ideal gas law...
to find moles of a component from P, V, and T.
to find Volume from moles of a component.
Stoichiometry of Gases
Simplest model for behavior of gases
Molecular Velocities
Since all the gas molecules in a sample can travel at different speeds we talk about the “average velocity” of the particles
however, the distribution of speeds follows a pattern called a Boltzman distribution
the method of choice for our average velocity is called the
root-mean-square
(u)
In a gas mixture at a given temperature lighter particles travel faster, on average, than heavier ones
KE directly proportional to Kelvin Temperature
The process of a collection of molecules spreading out from high concentration to low concentration is called
diffusion
.
The process by which a collection of molecules escapes through a small hole into a vacuum is called
effusion
.
Both rates are related to the average rms velocity
lighter gases both diffuse and effuse faster
Graham's law of Effusion
Full transcript