Gases

Gas Animations

Kinetic Molecular Theory

• Particles in an ideal gas…– have no volume.– have elastic collisions. – are in constant, random, straight-line

motion.– don’t attract or repel each other.– have an avg. KE directly related to Kelvin

temperature.

Real Gases• Particles in a REAL gas…

– have their own volume– attract and repel each other

• Gas behavior is most ideal…– at low pressures– at high temperatures– in nonpolar atoms/molecules

• ***Most real gases act like ideal gases except under high pressure and low temperature.

• **Most real gases act like ideal gases except under high pressure and low temperature.– Under these conditions the assumptions

of the kinetic theory are too far from reality. This is when the gas laws (based on an ideal gas) will not accurately model real gas behavior.

– Conditions where a gas has particles that are very close together.

Characteristics of Gases• Gases expand to fill any container.

– Take the shape and volume of their container.

• Gases are fluids (like liquids).– Little to no attraction between the particles

• Gases have very low densities.= lots of empty space between the particles

Characteristics of Gases• Gases can be compressed.

– lots of empty space between the particles– Indefinite density

• Gases undergo diffusion & effusion.– random motion – scatter in all directions

C. Johannesson

Temperature= how fast the molecules are moving

ºF

ºC

K

-459 32 212

-273 0 100

0 273 373

32FC 95 K = ºC + 273

• Always use absolute temperature (Kelvin) when working with gases.

Pressure

area

forcepressure

Which shoes create the most pressure?

Pressure• Barometer

– measures atmospheric pressure

Mercury Barometer

Aneroid Barometer

C. Johannesson

Pressure• Manometer

– measures contained gas pressure

U-tube Manometer Bourdon-tube gauge

Pressure- how much a gas is pushing

on a container.

2m

NkPa

• Atmospheric pressure- atmospheric gases push on everything on Earth

• UNITS AT SEA LEVEL

1 atm =101.3 kPa (kilopascal)= 760 mmHg =760 torr

=14.7 psi

STP

Standard Temperature & PressureStandard Temperature & Pressure

0°C 273 K

1 atm 101.3 kPa-OR-

STP

V = volume = how much space a gas occupies

Units– L, mL, cm3

– 1000 mL = 1 L, 1 mL = 1 cm3

n = moles = how much gas there is

R = ideal gas constant • = 0.0821 (L*atm) (mol*K) • = 8.31 (L*kPa) (mol*K)

BASIC GAS LAWS

Charles’ Law

• T V (temperature is directly proportional to volume)• T ↑ V↑ & T↓ V↓

• V1 = V2

T1 T2 T is always in K– P and n = constant

• Ex) A 25 L balloon is released into the air on a warm afternoon (42º C). The next morning the balloon is recovered on the ground. It is a very cold morning and the balloon has shrunk to 22 L. What is the temperature?

V

T

CharlesLaw.exe

• Charles’ Law Video

• Link to video

• A gas occupies 473 cm3 at 36°C. Find its volume at 94°C.

Boyle’s Law• P↓ V ↑ & P↑ V ↓• P 1/V (pressure is inversely proportional to volume)

• P1V1 = P2V2– T and n = constant

Ex: Pressure: 98.6 kPa 0.92 atmVolume: ? mL 8.0 L

P

V

Boyle'sLaw.exe

• A gas occupies 100. mL at 150. kPa. Find its volume at 200. kPa.

AVOGADRO’S LAW• Vn Vn • V n (direct)

• V1 = V2

n1 n2

– T & P Constant

EX: A 3 liter sample of gas contains 3 moles. How much gas will there be, in order for the sample to be 2.3 liters? P & T do not change

Avogadro'sLaw.exe

Gay-Lussac’s Law

• P1 = P2

T1 T2

– V & n constant

• Direct relationship

• PT PT

P

T

Gay-Lussac'sLaw.exe

Example: A can of Dust Off is sitting next to my computer at 25°C and 3.5 atm. I flip the can over and spray some air out. The room has a pressure of 1.0 atm. What is the temperature of the air as it escapes the container?

http://www.youtube.com/watch?v=4qe1Ueifekg2.06 min

COMBINED IDEAL GAS LAW

• P1V1 = P2V2

n1T1 n2T2

• If P, V, n, or T are constant then they cancel out of the equation.

• n usually constant (unless you add or remove gas), so

• P1V1 = P2V2

T1 T2

• A gas occupies 7.84 cm3 at 71.8 kPa & 25°C. Find its volume at STP.

• Nitrogen gas is in a 7.51 cubic centimeter container at 5C and 0.59 atm. What is the new volume of the gas at standard temperature and pressure?

• A sample of chlorine gas has a pressure of 7.25 kPa at 20.0C and a volume of 16.0 mL. What will the pressure be at 60.0C if its volume does not change?

Ideal Gas Law (“Pivnert”)• PV = nRT

• R = ideal gas constant • = 0.0821 (L*atm)

(mol*K)

• = 8.31 (L*kPa)

(mol*K)

Ideal Gas Law (“Pivnert”)

PV=nRT R = The Ideal Gas Constant (memorize)R = 0.0821 (L*atm) (mol*K) R = 8.31 (L*kPa) (mol*K)* Choose which R to used based on the units of your pressure. If you have mmHg change it to atm.* V has to be in Liters, n in Moles, T in Kelvin, P can be in atm or kPa

P V = n R T (atm) (L) = (moles) (L*atm/mol*K) (K)(kPa) (L) = (moles) (L*kPa/mol*K) (K)

• A balloon contains 2.00 mol of nitrogen at a pressure of 0.980 atm and a temperature of 37C. What is the volume of the balloon?

• Calculate the pressure (atm) of 2.79 g of F2 that occupies 5.00 L at 44.2C.

Dalton’s Law of Partial Pressure• The total pressure of a mixture of gases is

equal to the sum of the partial pressures of the component gases.

• Ptotal = Pgas 1 + Pgas 2 + P gas 3 + …

• Example: Find the total pressure for a mixture that contains three gases. The partial pressure of nitrogen is 15.75 kPa, helium is 47.25 KPa, and oxygen is 18.43 kPa.

Gases are collected over waterWhen a H2 gas is collected by water displacement, the gas in the collection bottle is actually a mixture of H2 and water vapor. EX: Hydrogen gas is collected over water at 22.5°C. Find the

pressure of the dry gas if the atmospheric pressure is 94.4 kPa. PH2O = 2.72 kPa

• When dealing with a mixture of gases we will often have to account for the different amounts of each gas in order to determine the partial pressure of each gas. Many times you will be given the total pressure and the percent composition of the gas mixture. To find the pressure of the individual gases: For each gas, multiply the total pressure by the percent for that gas.

• Example: A mixture of gases is 15.00% nitrogen, 45.00% oxygen, and 40.00% neon at 105.0kPa (OR SET IT UP AS A PROPORTION)

• To find the partial pressure when not given % composition. • If you are not given the % composition of each gas, you can find it

easily when dealing with volume in liters or moles of each gas. It will simply be moles of part/ total moles or liters of part/total liters X 100.

• Example:A tank contains a mixture of 5.0 mol N2, 2.5 mol O2, and 3.0 mol of CO2 at 38C and a total pressure of 5.5 atm. Calculate the partial pressure of each gas (in torr).

• When given mole or volume amounts of each gas in the mixture: Set it up as a proportion.

• Moles of part = Pressure of part • Total moles Total pressure

• Volume of part = Pressure of part• Total volume Total pressure

   

 

• Example: A tank contains a mixture of 5.0 mol N2, 2.5 mol O2, and 3.0 mol of CO2 at 38C and a total pressure of 5.5 atm. Calculate the partial pressure of each gas (in torr).

• Diffusion is the tendency of molecules to move toward areas of lower concentration until the concentration is uniform throughout. = Spreading of gas molecules throughout a container until evenly distributed.

•Effusion = gas escapes through a tiny hole in its container

Graham’s Law Graham’s Law Rate of diffusion of a gas is inversely related to the square root of its molar mass.

= Gases of lower molar mass diffuse and effuse faster than gases of higher molar mass.

Effusion VS Diffusion