Got Gas?

What gases are important for each of the following: O2, CO2

and/or He?

A. B. C. D.

What gases are important for each of the following: O2, CO2

and/or He?

A. CO2 B. O2/CO2 C. O2 D. He

Gases are made up of molecules. Air consists of a mixture of gases, including nitrogen N2, Oxygen O2, and

carbon dioxide CO2. Gases exert pressure on any surface with

which they come in to contact. The gases in our atmosphere are subject to the Earth’s

gravitational pull therefore the atmosphere is much denser nearer the Earths surface and less dense as we travel away

from the planet.

Some Gases in Our Lives

oxygen O2 nitrogen N2

ozone O3

argon Ar carbon dioxide CO2

water H2O

Noble gases:

helium He neon Ne krypton Kr xenon XeOther gases:

fluorine F2 chlorine Cl2

ammonia NH3

methane CH4 carbon monoxide CO

nitrogen dioxide NO2 sulfur

dioxide SO2

Pressure is one of the most easily measured properties of a gas. We define pressure as the force per unit area.

P= Force/Area.

Pascal: The SI unit of pressure.

1 Pascal = 1 N/m2 = 1 kg x m/s2 x 1/m2

Named after Blaise Pascal

Ice skates are familiar examples of the effects of pressure. The area of the blades of a skate are much smaller than, say, the soles of your feet. So if you strap on ice skates, your weight will act on an area much smaller than it would if you were wearing normal shoes.

Since A decreases while F stays the same, the pressure you exert on the ice will be much greater if you're wearing skates. This pressure is often enough to melt a layer of ice, which allows your skate to

glide smoothly across an ice rink. If you try the same maneuver with normal shoes, you will not generate enough pressure to melt the ice and

won't get anywhere fast.

Air pressure is the force exerted on you by the weight of tiny particles of air (air

molecules). Although air molecules are invisible, they still have weight and take up space. We are usually unaware of the air pressure around us, probably because we are used to it. Since there's a lot of

"empty" space between air molecules, air can be

compressed to fit in a smaller volume.

How much pressure are you under?

Earth's atmosphere is pressing against each

square inch of you with a force of 1 kilogram per square centimeter (14.7 pounds per square inch). The force on 1,000 square

centimeters (a little larger than a square

foot) is about a ton!

Why doesn't all that pressure squash me?

Remember that you have air inside your body too, that air balances out the pressure outside so you stay nice and firm and

not squishy. Well…some of us are more squishy than

others

Run for Cover! Air pressure can tell us

about what kind of weather to expect as

well. When it's compressed, air is said

to be "under high pressure". If a high

pressure system is on its way, often you can expect cooler temperatures and clear skies. If a low pressure system is

coming, then look for warmer weather, storms

and rain.

Some important pressure terms

• Atmosphere - A unit of measurement defined as 101,325 Pascals. The typical pressure at sea level varies around one standard atmosphere (atm). Atmospheric pressure P can be calculated via the following equation:

P = ghρWhere g = acceleration due to gravity

h = height of liquid in barometer (usually Hg)

ρ = density of liquid in barometer

WRITE THIS IN YOUR NOTES NOW

Look a barometer!!

Barometer?Weather forecasters measure air

pressure with a barometer . Barometers are used to measure the current air pressure at a

particular location in "inches of mercury" or in "millibars" (mb). A measurement of 29.92 inches of mercury is equivalent to 1013.25

millibars. Crazy huh?

Bar - A unit of measurement equivalent to 1×105 Pascals.

More pressure terminology

• mm Hg- A unit of pressure commonly used with the barometer. It corresponds to 1 torr and 1/760 atm at 0o Celsius only.

• Torr- A unit of pressure closely related to mm Hg, but more convenient and absolute. 1 Torr = 1 mm Hg at 00 Celsius. 1 Torr always equals 1/760 atm, irrespective of the temperature.

• Pounds per Square Inch- A unit of pressure commonly used in the United States. 1 psi = 1 lb in-2. 14.6960 psi corresponds to one atmosphere.

Why do my ears pop as I

increase altitude? As the number of molecules of air around you decreases, the air pressure decreases. This causes your ears to pop in

order to balance the pressure between the outside and

inside of your ear. Since you are breathing fewer molecules

of oxygen, you need to breathe faster to bring the few molecules there are into your lungs to make up for the

deficit. Air pressure on Mt. Everest is 670 mb less than on the

Cayman Islands.

1. If you were on a mountain, would the weight of the air above you (air

pressure) be greater than or less than it

is now?

What do you think causes wind? Don’t say

beans!!!

If you lived in Kansas and observed a sudden drop in air pressure, what kind of weather would you expect to

see soon?

Why do hot air balloons rise?

Air weighs less than water, would you

expect the pressure exerted by water to be greater or less than the pressure

exerted by the same volume of air?

The Gas Laws:The volume of a gas means

nothing unless the conditions under which it was collected are known.

A temperature and pressure are needed to describe any

volume of gas

Temperature - Temperature changes

cause particle motion changes which cause a

volume change.

Pressure - Gases can be compressed, or

squeezed, causing a change in the gas

volume.

The Four Gas Law Variables:

1. Temperature, 2. Pressure, 3. Volume, 4. Moles

Volume

All gases must be enclosed in a container

that, if there are openings, can be sealed

with no leaks. The three-dimensional space

enclosed by the container walls is called volume. When the generalized variable of volume is

discussed, the symbol V is used.

Volume in chemistry is usually measured in liters (symbol = L)

ormilliliters

(symbol = mL). A mL is also called a cubic

centimetercm3

A liter is also called a cubic decimeter dm3

TemperatureAll gases have a

temperature, usually measured in degrees

Celsius (symbol = °C). Note that Celsius is

capitalized since this was the name of a person (Anders Celsius). When

the generalized variable of temperature is

discussed, the symbol T is used.

There is another temperature scale which is very important in gas behavior. It is called

the Kelvin scale (symbol = K). Note that K does not have a degree sign

and Kelvin is captalized because this was a

person's title (Lord Kelvin, his real name was

William Thomson).

All gas law problems will be done with Kelvin temperatures.

You can convert between Celsius and Kelvin like

this:Kelvin = Celsius + 273

For example, 25 °C = 298 K, because

25 + 273 = 298.

Standard temperature is defined as

zero degrees Celsius or

273 K.

PressureGas pressure is created by the molecules of gas hitting the walls of the container.

This concept is very important in helping you to understand gas behavior. Keep it solidly in mind. I will

use this idea of gas molecules hitting the wall often. When the generalized variable of pressure is

discussed, the symbol P is used.

Three different units of pressure used in chemistry.

• atmospheres (symbol = atm) • millimeters of mercury (symbol = mm Hg)

• Pascals (symbol = Pa) or, more commonly, kiloPascals (symbol = kPa)

When working with gas laws, all pressure units must be the same

in any calculation.1 atm =

760 mm Hg = 101 kPa =

101,300 Pascals = 1013 mb = 29.92 in Hg

If needed, be able to convert from one pressure unit to

another.

Do the following conversions as practice:

1. 27 K to Celcius

2. 1450 0C to K

3. 1.25 atmospheres to inches of Hg

4. 500 millimeters of Hg to kPa

Standard temperature and pressure =STP:

Standard temperature is 00 Celcius.

ButAll gas calculations

must use Kelvin temperatures.K = 0C + 273

Standard pressure is 1 ATM (at sea-level).

mm Hg is millimeters of mercury. This describes the height of a vertical column of

mercury that the pressure will support against gravity. The unit Torr can be used to

indicate mm Hg. The barometric pressure reported in U.S. weather reports is usually expressed as inches of mercury. There are

25.4 millimeters in one inch.kPa is kiloPascals. A Pascal is a unit of

force equal to a N / m2. It is describing the pressure exerted by the molecules of the gas striking a surface.mb is millibars. One millibar is equal to

100 N / m2. Crazy huh?

The last gas law variable

Amount of GasDon’t “freak out” but the amount of gas present is measured in moles (symbol = mol) or in grams (symbol = g or gm).

Typically, if grams are used, you will need to convert to moles at some point. When the generalized variable of amount in moles is discussed, the letter "n" is used as the symbol (note: the letter is in lowercase. The others discussed above

are all caps.).

To be successful working gas law problems, do the

following:

First read the question to see what question is being asked.

What do you know?

Write and label all variables given in the

problem.

What are you trying to find?

Label the variable the question is asking you to

find.

Reread the question to see what conditions

change.Label all measurements before the change as P1,V1, and T 1.

Label all measurements after the change as P2, V2, and T2.

Find the gas equation that connects the

variables used in the problem.

BoylesCharles

Gay/Lussac

Plug the variables into the gas equation

you’ve chosen

Rearrange the gas equation to solve for the “Find” variable.

VTP

Plug and Chug•Plug numbers and units into the equation•Pickup your calculator and punch buttons. •Solve. •Check your units, do they make sense? •If so,•Write the answer to the problem and circle it.

Avagadro’s Law

Equal volumes of gasses, at the same temperature and pressure, have equal numbers of particles.

Important points to note….

• All gasses have the same physical behavior

• Increase volume of gas = increase number of particles (if temp and pressure are constant)

• Volume is directly proportional to number of particles

• V = n (n = number of moles)

Molar volume = volume of 1 mole of gas

At STP, (273k, 1atm) 1 mole of any gas = 22.4

Liters

What is the molar mass of…

22.40 Liters of H2 (g)

What is the molar mass of…..

22.40 Liters of N2 (g)

What is the molar mass of…..

22. 40 liters of O2 (g)

Calculate the number of liters

of occupied by 2.5 moles of N2 gas

Calculate the number of liters of occupied by 0.350 moles O2 gas

The universal gas constant = R

This is a constant that lets us solve “ideal gas”

equation problemsThink of it as a correction factor

Boyles law Pressure and volume are inversely proportional

P = V PV = ______

Charles LawVolume & Temperature are directly

proportional

V = T V =____T

Avagadro’s LawVolume & moles are directly

proportional

V = nV = _______n

If we rearrange…..

PV = RnT

At STP what is the gas constant ???(watch your units)