Chapter 8: Gases

What does the Kinetic Molecular Theory describe?

Describes the motion of gasses

What do gasses consist of?

Small particles that exhibit certain behavior

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1. Move rapidly with high velocities(speeds)
2. Have very small attractive/repulsive forces between molecules
* Act like they’re alone
3. Occupy a much larger volume than the volume of the atoms alone
4. Are in constant motion, moving rapidly in straight lines
* When they hit the container walls, they exert force
5. Have a Kevin temperature proportionate to the average kinetic energy of the molecules

Kelvin temperature increases, kinetic energy ________

Kelvin temperature increases, kinetic energy ***increases***

What are the relating factors of gas (that we talked ab) and what are their abbreviations?

Temperature = T

Pressure = P

Volume = V

Amount of Gas = n

Volume of a gas

The space occupied by a gas

* Same as volume of container
* Increases w/ an increase in temperature at constant pressure

What units are used for volume of a gas?

Liters (L) or Milliliters (mL)

Temperature of a gas

The determining factor of the kinetic energy of gas particles

* Average kinetic energy of molecules (motion/speed → velocity)

What units are used for temperature of a gas?

Kelvin; others may be given but use K for all math

What happens when temperature is decreased vs increased? (molecule motion)

Increased: Molecules have more collisions

Decreased: Molecules have less collisions

Pressure of a gas

The force exerted by a gas against the walls of the container

* Force per unit area of surface area
* Measure of the gas particle collisions with the sides of the container

What units are used for pressure of a gas?

mm of mercury = mmHg or Torr (same value)

atmospheres = atm

pascals Pa or kilopascals

pounds per square inch Psi

atm → mmHg or Torr conversion

1 atm = 760 mmHg/Torr

* Exact

What do gas particles in the air around us exert?

atmospheric pressure

What is the air we breathe?

A mixture of gasses

* 21% oxygen
* 78% nitrogen
* 1% other gasses

Barometer

Measures the pressure exerted by the gasses in the atmosphere

How does a barometer work?

Using a dish of liquid mercury, place a tube on top of it and the air pressure will cause the liquid to fill the tube

* Indicates atmospheric pressure as the height in mm of the mercury column

What units does a barometer use?

mmHg/Torr

Who invented the barometer?

Evangelista Torricelli

What is the atmospheric pressure at sea level?

1 atm

Atmospheric Pressure

The pressure exerted by a column of air from the top of the atmosphere

* Decreases as altitude increases

What affects the atmospheric pressure? Examples.

Variations in weather and altitude

* Mountain = higher up = more altitude, less atm
* Hot sunny day = higher pressure = mercury column rises
* Cold rainy day = less pressure = mercury column falls

What does Boyle’s Law state?

* The pressure of a gas is inversely related to it’s volume when T is constant
* The product P x V is constant when temperature and amount of gas is held constant
* If the volume decreases, the pressure increases

Formula for Boyle’s Law

P₁V₁ = P₂V₂

Why does Boyle’s Law work?

As the pressure on a gas increases, the volume of the gas decreases because the gas particles are forced closer together. Conversely, as the pressure on a gas decreases, the gas volume increases because the gas particles can now move farther apart.

Boyle’s Law → Breathing

Inhalation

* Lungs expand = lung volume increase
* Lung pressure decreases
* Air flows toward lower pressure in the lungs
* Diaphragm contracts → moves down

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Exhalation

* Lungs contract = lung volume decrease
* Lung pressure increases
* Air flows from the higher pressure in the lungs to the outside
* Diaphragm relaxes → moves up

List the scientific notation equivalence of

* Kilo
* Deci
* Centi
* Milli
* Micro

Kilo = 10³

Deci = 10⁻¹

Centi = 10⁻²

Milli = 10⁻³

Micro = 10⁻⁶

What does Charles’s Law state?

* The Kelvin temperature is directly related to volume
* Pressure and amount of gas (moles) are constant
* When the temperature of a sample increases, it’s volume increases, at a constant pressure

What is the formula for Charles’s Law

V₁/T₁ = V₂/T₂

Why does Charles’s Law work?

The molecules' kinetic energy increases and they start to vibrate more and move around further from each other, therefore accounting for an increase in volume.

What does Gay-Lussac’s Law state?

* The pressure exerted by a gas is directly related to the Kelvin temperature of the gas
* Volume and amount of gas are constant

What is the formula for Gay-Lussac’s Law?

P₁/T₁ = P₂/T₂

Why does Gay-Lussac’s Law work?

If we increase the temperature of a gas sample, the particles will move faster. They hit the container walls with more pressure and more often.

What does Avogadro’s Law state?

* The volume of a gas is directly related to the number of moles of gas
* Temperature and pressure are constant

What is the formula for Avogadros’s Law?

V₁/n₁ = V₂/n₂

Why does Avogadro’s Law work?

If you increase the pressure of a system at equilibrium (typically by reducing the volume of the container), the stress will best be reduced by reaction that favors the side with the fewest moles of gas, since fewer moles will occupy the smallest volume.

* Decreased volume **shifts the system to the side of the reaction that has fewer moles of gas**.

Which of the laws will form a linear graph?

All except Boyle's

If both values for a problem are given in °C, should you use °C?

* or if the question is asking for °C

No, all math must be done in Kelvin, so convert

Vapor Pressure

When liquid molecules with sufficient kinetic energy break away from the surface of a liquid, they become vapor

What happens to liquid in a closed vs open container?

Open: All the liquid will eventually evaporate

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Closed: The vapor accumulates and creates pressure called vapor pressure

What is vapor pressure in terms of equillibrium?

When the entering condensation rate is equal to the exiting evaporation rate

* Constant pressure

Vapor Pressure and Boiling Point

* A liquid

The liquid exerts its own vapor pressure at a given temperature

* It boils when the vapor pressure is equal to the external pressure (atmospheric pressure)

Altitude and Boiling Point

* At high altitudes

* Atmospheric pressure is lower than 1 atm/760 mmHg
* The boiling point of water is less than 100° Celcius

Altitude and Boiling Point

* In a closed container such as a pressure cooker

* A pressure greater than 1 atm/760 mmHg can be obtained
* Water boils at a higher temperature than 100° Celcius

Combined Gas Laws Formula

P₁V₁/T₁ = P₂V₂/T₂

When using the combined gas laws should you cross multipy?

No, instead simply each side and use algebra to solve

Standard Temperature and Pressure

Allows the volumes of gasses to be compared at a specific temperature and pressure

* Ideal gasses will all have the same volume

What is the temperature and pressure for STP?

Temperature: 0°C = 273 K

Pressure: 1atm = 760 mmHg

What is the molar volume of a gas at STP roughly equal to?

The volume of 3 basketballs

At STP, Mole → L conversion

* What is it called?

1 mole = 22.4 L

* Molar volume

Ideal Gas Law Formula

PV = nRT

What is R in the Ideal Gas Law?

The constant

What are the units for each when using the Ideal Gas Law?

* Volume
* Temperature
* Amount of gas
* Pressure

Volume = Liters (L)

Temperature = Kelvin

Amount of gas = Moles

Pressure = atm OR mmHg

What is the R constant when pressure is in atm vs in mmHg

Atm = 0.0821

mmHg = 62.4

What does the Ideal Gas Law do?

Closely approximates the behavior of real gasses at typical conditions

* Real gasses show some deviations in behavior though

Partial Pressure

The partial pressure of a gas is the pressure that each gas in a mixture would exert if it were by itself in the container

* Gas molecules don’t affect each other and act like they’re alone

Dalton’s Law of Partial Pressure

* Pressure depends on the total number of gas particles not the type of particles
* The total pressure exerted by gasses in a mixture is the sum of the partial pressures of gasses
* P(total) = P₁ + P₂ + P₃ +…

Total Pressure

* At STP

1 mole of pure gas in a volume of 22.4 L will exert the same pressure as 1 mole of a gas mixture in 22.4 L

* 1 atm of N₂ = 1.0 atm
* 0.4 mol O₂. + 0.6 mole He = 1.0 atm
* 0.5 mol O₂. + 0.3 mole He + 0.2 mole Ar = 1.0 atm

Atmospheric Pressure

* In terms of partial pressure

The sum of the partial pressures of the gasses in the air