GAS PROPERTIES AND GAS LAWS

Gases may be…•Monoatomic Gas: Made of one atom, like He

•Diatomic Gas: Made of two atoms, like Cl2 or H2

*This is the case for Br, I, N, Cl, H, O, F

Also known as Miss BrINClHOF (Brinklehoff)

•Polyatomic Gas: Made up of more than two atoms, like CO2, NO2, or CH4 (methane)

Some unique properties of gases:

Gases have mass It is easy to compress a gas Gases fill their containers

completely Gases diffuse through

each other easily Gases exert pressure on

their surroundings The pressure of a gas

depends on its temperature

Gases Behaviors:

Gases are composed of a large number of particles that behave like hard, spherical objects in a state of constant, random motion.

Basically, gas particles are like billiard balls in a 3-D pool table, and they are all moving all over the place all the time in different directions

Gas Molecule Movements These particles move in a straight

line until they collide with another particle or the walls of the container.

Then they bounce off, and move again: these collisions are elastic

Gas Molecule Motion

Other Things to Consider about Gas Molecule Movement

There is no force of attraction between gas particles or between the particles and the walls of the container.

Keep in mind that things aren’t repelled, either….no attraction, no repulsion…

Other Things to Consider about Gas Molecule Movement

Collisions between gas particles or collisions with the walls of the container are perfectly elastic. None of the energy of a gas particle is lost when it collides with another particle or with the walls of the container.

This is why gases take the shape of their container!

The energy of the system is constant as long as the pressure and temperature remain constant.

Kinetic Energy and Gas

The average kinetic energy of a collection of gas particles depends on the temperature of the gas and nothing else.

Think back to the fact that temperature is a measure of Kinetic Energy (the random motion of molecules)

Gas Volume These particles are much smaller

than the distance between particles. Most of the volume of a gas is therefore empty space.

Compared to the container, the volume of the gas is zero. So, we say the volume of the container is the volume of the gas

The volume of the gas in an empty soda bottle is?

Kinetic Molecular Theory of Gases (KMT)

1. Gases consist of very small particles , each of which has a mass.

2. The distance separating gas particles is very large so much so that we say the volume of the gas is negligible as compared to the volume of the container (the gas itself has no volume)

3. Gases exert no force on one another 4. Gas particles are in random, rapid, constant

motion5. Collisions with other gas particles or the walls of

the container are elastic (no energy lost)6. The average KE of a gas depends on the

temperature of the gas

Measuring Gases

We measure gases in several ways…

Volume =V Temperature =T Pressure =P Number of Moles =n

What is Temperature? Temperature is a measure of heat; more specifically it

is the (average) measure of the random kinetic energy of the molecules in an object

Kinetic energy: Energy of motion

More motion = More KE = Higher temperatures

Less Motion= Lower KE = Lower temperatures

Temperature (T)

For most scientists, the Celsius scale is used

However, we need to use the Kelvin scale for gas laws

Why? Why this strange and bizarre scale that uses a boiling

point of 373K and a freezing point of 273K for water?

Well, it’s because our “normal” temperature scales are based upon numbers that make sense to use (or not) (like freezing at 0°C and boiling at 100 °C) or are a bit

more convoluted (like the Fahrenheit scale, where zero comes from the temperature of ice, water and NH4Cl and body temperature was 98 °F and still water with ice was 32 °F. )

An Absolute Temperature Scale

The Kelvin scale bases temperature on an absolute scale, where temperatures correspond to the amount of motion of the particles

Absolute zero (O K) is when there is no molecular motion (at all) Scientists have gotten close to zero K, but not quite there.

It exists naturally in deep space

So why do we need to use Kelvin again?

Calculations using temperature of zero could be undefined or have no value (which really can’t be)

Can’t have negative volumes in our equations, because that is impossible (from using negative temperatures values in the variables)

The Kelvin scale avoids all of these issues

The Kelvin Scale

Is based in Absolute Zero, which is -273°C

0K= -273°C 273K=0°C

To convert between K and °C, °C + 273 =K

or K -273 = °C

It’s that simple, which is good since no gas laws calculations can use °C

Volume (V)

We usually measure volume in Liters (L), but sometimes in other metric units

1L = 1000mL1L = .001m3

We will use these conversions- be sure to know them!

Pressure (P)

Gas 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. This idea of gas molecules hitting the wall will be used often.

Pressure is force measured over an areaP=Force/ area

and yes, Physics children, Force = mass (acceleration)

Units of Pressure

atmospheres (atm) millimeters of mercury (mm Hg) Pascals (= Pa) kiloPascals (= kPa) Standard pressure is defined as:

1 atm1atm =760.0 mm Hg1 atm=101.325 kPa

Measuring Pressure of Gases

Manometers- Measure the pressure of a gas as compared to the outside world

Moles (n) We’ve been here before. Calm down about

it. Remember that 1 mole is 6.02E23 pieces of

something- in this case, usually molecules of gas (but sometimes atoms, if not a diatomic gas).

Also, 1 mol gas at STP (Standard Temp/Pressure) = 22.4L

“= “means occupies, takes up, etc

Boyle’s Law

Relates pressure and volume, while temperature and number of moles are constant (so they do not appear in the equation)

Boyle’s Law:

P1V1=P2V2 In a closed rigid container of a gas at a

constant temperature, the pressure times the volume remains constant (P1V1=k)

Pressure and volume are inversely related The P1 is the pressure at the first volume

(V1), while P2 is the pressure at the second volume (V 2).

Boyle’s Law: P1V1=P2V2

The product of pressure and volume remains constant as long as the temperature remains constant. (The number of moles must also remain constant.)

• When volume is high, pressure is low

• When the volume is low, pressure is high

Pressure

Volume

Using Boyle’s Law

If a balloon with a volume of 3L is under a pressure of 1 atmosphere, determine the new volume if the pressure is changed to .8 atm.

We are given P1, V1, and P2. We are asked to find V2

Two key words here are new and changed- to ID these measurements as linked (having the same subscript)

What is the new volume?

Charles’ Law Charles’ law

relates volume and temperature, at a constant pressure and number of moles in a flexible container. Since the pressure and number of moles are constant, they do not appear in the equation.

Charles’ law

V1/T1=V2/T2 In a closed container of a gas at

a constant temperature, the pressure times the volume remains constant (P1V1=k)

The P1 is the pressure at the first volume (V1), while P2 is the pressure at the second volume (V 2).

Charles’ Law

V1/T1=V2/T2 can be rearranged to read

V1T2=V2T1

Why would we care to rearrange this? This means no division in the equation. You can use it either way, just remember that they are DIRECTLY PROPORTIONAL.

Charles’ Law As the temperature increases, the

volume increases. As the temperature decreases, the

volume decreases. Temperature and volume are

proportionally related.

Chuck’s law (still…)

Think again about temperature- it is the KE of the gas particles.

Think about what the volume is a result of: the force that the gas molecules are exerting of the container

Think about how if something hits another thing at a higher speed- it hits with more force. More force is pushing harder. Pushing harder means further. This means greater volume when we are dealing with a flexible container!

Gay Lussac’s Law

Relates pressure and temperature, when volume is kept constant (P1/T1=k)

P1/T1= P2/T2 Or

P1T2=P2T1

Combined Gas Law (Putting it all together)

(P1V1)/ T1=(P2V2)/T2

Takes all other gas laws into account, even if you can’t see them here (they cross out of the equation)

When in doubt about most of the guy’s laws, you can use this one, because when the pressure, volume, or temperature is constant, you have the law you need.