Gas ChemistryGas ChemistryFeel the Pressure?Feel the Pressure?

Nature of GasesNature of Gases

Expansion: no definite shape/volumeExpansion: no definite shape/volume particles travel in all directions to fill spaceparticles travel in all directions to fill space

Fluidity: particles easily “slide” past each Fluidity: particles easily “slide” past each other (both liquids and gases are referred other (both liquids and gases are referred to as fluids)to as fluids)

Low Density: 1/1000Low Density: 1/1000thth density of density of solid/liquidsolid/liquid

Nature of Gases (cont.)Nature of Gases (cont.)

Compressible: particles can be forced together, Compressible: particles can be forced together, reducing empty space in betweenreducing empty space in between

Diffusion: spontaneous mixing of gases due to Diffusion: spontaneous mixing of gases due to random motionrandom motion rate depends on speed, diameter, attractive forcesrate depends on speed, diameter, attractive forces

Effusion: pass through small opening under Effusion: pass through small opening under pressurepressure

Variables Describing a GasVariables Describing a Gas

(P) Pressure(P) Pressure

(V) Volume(V) Volume

(T) Temperature(T) Temperature

(n) Amount of gas in moles(n) Amount of gas in moles

Brain Starter!!Brain Starter!!

1. Convert 3 moles of hydrogen into liters.1. Convert 3 moles of hydrogen into liters.

2. What is -32 degrees Celsius in Kelvin?2. What is -32 degrees Celsius in Kelvin?

3. Convert 2.5 atm into mm Hg.3. Convert 2.5 atm into mm Hg.

Kinetic Molecular Theory (KMT)Kinetic Molecular Theory (KMT)

1.1. Gases are tiny particles separated by vast Gases are tiny particles separated by vast empty space (assumed zero volume)empty space (assumed zero volume)

2.2. Gas particles move is rapid, straight line Gas particles move is rapid, straight line motion (possess kinetic energy)motion (possess kinetic energy)

3.3. Gas particle collisions are perfectly elastic (no Gas particle collisions are perfectly elastic (no net loss of kinetic energy)net loss of kinetic energy)

4.4. No attractive or repulsive forces between gas No attractive or repulsive forces between gas particlesparticles

5.5. Average kinetic energy is directly proportional Average kinetic energy is directly proportional to the Kelvin temperature.to the Kelvin temperature.

Ideal GasIdeal Gas

Ideal Gas - imaginary gas that perfectly Ideal Gas - imaginary gas that perfectly conforms to all assumptions of the KMTconforms to all assumptions of the KMT

Real gases behave like ideal gases at lower Real gases behave like ideal gases at lower pressures and higher temperatures.pressures and higher temperatures.

Deviations:Deviations: High pressure or Low temp. – attractive forces cause High pressure or Low temp. – attractive forces cause

condensation to liquidscondensation to liquids Polar molecules behave less ideal than nonpolar Polar molecules behave less ideal than nonpolar

moleculesmolecules

Charles’ LawCharles’ Law

Observed that raising the temp. 1 Observed that raising the temp. 1 ºC ºC increased the volume 1/273 of original increased the volume 1/273 of original volumevolume

Observed that raising the temp. Observed that raising the temp. 10 10 ºC ºC increased the volume 10/273 of original increased the volume 10/273 of original volumevolume

Decreasing temp caused same decrease Decreasing temp caused same decrease in volumein volume

Kelvin Scale and Absolute ZeroKelvin Scale and Absolute Zero

Charles noted that volume and temperature are Charles noted that volume and temperature are only directly proportional if using the Kelvin only directly proportional if using the Kelvin temp. scaletemp. scale

Absolute zero ( 0 K) is the lowest possible Absolute zero ( 0 K) is the lowest possible temperaturetemperature 0 K means no kinetic energy (no atom/molecule 0 K means no kinetic energy (no atom/molecule

motion)motion) theoretically: zero gas volume at zero Kelvintheoretically: zero gas volume at zero Kelvin

Why won’t gases disappear at zero Kelvin?Why won’t gases disappear at zero Kelvin?

Temperature UnitsTemperature Units

Degrees CelsiusDegrees Celsius 0°C = Freezing point0°C = Freezing point 100°C = Boiling point100°C = Boiling point

Conversions:Conversions: K (Kelvin) = °C + 273K (Kelvin) = °C + 273 °F (Fahrenheit) = (9/5)°C + 32°F (Fahrenheit) = (9/5)°C + 32 °C = K – 273 or °C = (°F – 32) x (5/9)°C = K – 273 or °C = (°F – 32) x (5/9)

PressurePressure

Pressure = force / areaPressure = force / area

Gas particle collisions with a surface Gas particle collisions with a surface cause pressurecause pressure

Atmospheric pressure (sea level): Atmospheric pressure (sea level): 1 atmosphere 1 atmosphere

(atm)(atm)

Air PressureAir Pressure

Atmospheric Pressure- force per unit area Atmospheric Pressure- force per unit area exerted against a surface by the weight of exerted against a surface by the weight of air above that surface.air above that surface.

Higher altitude = lower pressure because Higher altitude = lower pressure because of less air!of less air!

Breathing issues?Breathing issues?

How it affects you!How it affects you!

Boiling water:Boiling water: Water boils whenWater boils when

vapor pressure meets/vapor pressure meets/

exceeds air pressure.exceeds air pressure. If lower pressure,If lower pressure,

water boils at lowerwater boils at lower

temperature.temperature. Issues??Issues??

OOPS!!!OOPS!!!

How could this happen?How could this happen?

Wow, really !?!Wow, really !?!

1.1. Tanker was steam Tanker was steam cleaned on the insidecleaned on the inside

2.2. The Tanker was sealed The Tanker was sealed and allowed to cooland allowed to cool

3.3. Internal pressure was Internal pressure was lower than external lower than external pressure.pressure.

4.4. Atmospheric pressure Atmospheric pressure collapsed the tanker car.collapsed the tanker car.

Pressure UnitsPressure Units

1 atm (atmosphere) = average P at sea 1 atm (atmosphere) = average P at sea levellevel

1atm = 760 mm Hg1atm = 760 mm Hg

= 760 torr= 760 torr

= 101.325 kPa (kilo Pascal)= 101.325 kPa (kilo Pascal)

= 14.7 psi (pounds per square inch)= 14.7 psi (pounds per square inch)

STPSTP

STP = standard temperature and pressureSTP = standard temperature and pressure

exactly 1 atm and 0 exactly 1 atm and 0 ºCºC

Volume of a gas depends upon T & P so a Volume of a gas depends upon T & P so a standard is required for comparisonstandard is required for comparison

Charles’ LawCharles’ Law

V = kT or V/T = kV = kT or V/T = k

If VIf V11/T/T11 = k and V = k and V22/T/T22 = k = k

ThenThen

2

2

1

1

T

V

T

V=

Charles Law ExampleCharles Law Example

A gas sample at 25 A gas sample at 25 ºC and 752 mL is heated to ºC and 752 mL is heated to 50 ºC, what is the new volume?50 ºC, what is the new volume?

T1 = 25 ºC = 298 KT1 = 25 ºC = 298 K

T2 = 50 T2 = 50 ºC = 323 KºC = 323 K

V1 = 752 mLV1 = 752 mL

mLK

KmL

T

TVV 815

298

)323)(752(

1

212 ===

Boyle’s LawBoyle’s Law

Observed that doubling volume of sample Observed that doubling volume of sample of gas reduced pressure by ½of gas reduced pressure by ½

Observed that tripling volume of a sample Observed that tripling volume of a sample of gas reduced pressure by 1/3of gas reduced pressure by 1/3

Inverse relationship between P & VInverse relationship between P & V

Boyle’s Law Eqns.Boyle’s Law Eqns.

PV = k or V= k/PPV = k or V= k/P

k is a constant regardless of gask is a constant regardless of gas

If PIf P11VV11 = k and P = k and P22VV22 = k = k

then Pthen P11VV11 = P = P22VV22 Useful version of Boyles Law

Boyle’s Law ExampleBoyle’s Law Example

A 1.0 L sample of gas at 1.0 atm is A 1.0 L sample of gas at 1.0 atm is allowed to expand at constant temperature allowed to expand at constant temperature to a volume of 5.0 L, what is the new to a volume of 5.0 L, what is the new pressure?pressure?

(1.0 L)(1.0 atm) = x atm (5.0 L)(1.0 L)(1.0 atm) = x atm (5.0 L)

x = 0.2 atm x = 0.2 atm

Gay-Lussac’s LawGay-Lussac’s Law

P = kT or P/T = kP = kT or P/T = k

If PIf P11/T/T11 = k and P = k and P22/T/T22 = k = k

ThenThen

2

2

1

1

T

P

T

P=

What about moles of gas?What about moles of gas?

(glad you asked)(glad you asked)

Dalton’s LawDalton’s Law

P = kn or P/n = kP = kn or P/n = k

If PIf P11/n/n11 = k and P = k and P22/n/n22 = k = k

ThenThen

2

2

1

1

n

P

n

P=

Avogadro’s LawAvogadro’s Law

V = kn or V/n = kV = kn or V/n = k

If VIf V11/n/n11 = k and V = k and V22/n/n22 = k = k

ThenThen

2

2

1

1

n

V

n

V=

Combined Gas LawCombined Gas Law

PV/Tn = kPV/Tn = k

If PIf P11VV11/T/T11nn11 = k and P = k and P22VV22/T/T22nn22 = k = k

Then Then movie clip

22

22

11

11

nT

VP

nT

VP=

Gas Law SimulationGas Law Simulation

Click Here to launch the gas law simulatorto launch the gas law simulator

OrOr

http://www.colorado.edu/physics/phet/web-pages/simulations-base.html

Simulations – Heat and Thermo – Gas Properties Simulations – Heat and Thermo – Gas Properties

Dalton’s Law of Partial PressuresDalton’s Law of Partial Pressures

The total pressure of a mixture of gases is equal The total pressure of a mixture of gases is equal to the sum of the partial pressures of each to the sum of the partial pressures of each individual gasindividual gas

PPtotaltotal = P = P11 + P + P22 + P + P33 … …

or or xtotaltotal

x PPxmoles

moles=

)()(

Molecular View of Part. Press.Molecular View of Part. Press.

Single Gas Molecule – Each collision contributes to total pressureSingle Gas Molecule – Each collision contributes to total pressure

Molecular View of Part. Press.Molecular View of Part. Press.

Many molecules – each collision also contributes to total gas pressure; however, there Many molecules – each collision also contributes to total gas pressure; however, there are more collisions with more gas in the same volumeare more collisions with more gas in the same volume

Gases Collected by Water Gases Collected by Water DisplacementDisplacement

Collected gas will contain water vaporCollected gas will contain water vapor

PPtotaltotal = P = Patmatm = P = Pgasgas + P + Pwaterwater

PPatm atm = from barometer = from barometer

PPwaterwater = table of vapor pressures = table of vapor pressures

PPgasgas = must calculate = must calculate

Molecular ViewMolecular View

Water molecules are in constant motion. At the surface:Water molecules are in constant motion. At the surface: Some molecules have enough KE to evaporateSome molecules have enough KE to evaporate Some molecules lose enough KE to condenseSome molecules lose enough KE to condense A mixture of gases above water will always contain some water vaporA mixture of gases above water will always contain some water vapor

Water Displacement ExampleWater Displacement Example

Oxygen was collected by water Oxygen was collected by water displacement at 20.0 displacement at 20.0 ºC and 731.0 mm ºC and 731.0 mm Hg. What is the pressure of oxygen?Hg. What is the pressure of oxygen?

PPatmatm = P = Pgasgas + P + Pwaterwater

PPoxygenoxygen = 731.0 mm Hg – 17.5 mm Hg = 731.0 mm Hg – 17.5 mm Hg

= 713.5 mm Hg= 713.5 mm Hg

Vapor PressureVapor Pressure

Gas molecules above solution will achieve Gas molecules above solution will achieve some temperature dependent pressure some temperature dependent pressure (equilibrium)(equilibrium)

Water Vapor Pressure at 25 Water Vapor Pressure at 25 ºC is 23.8 mmHgºC is 23.8 mmHg

Water Vapor Pressure at 100 ºC is 760 mm Hg Water Vapor Pressure at 100 ºC is 760 mm Hg

Ever Wonder?Ever Wonder?

Why will a puddle of water evaporate on a Why will a puddle of water evaporate on a sunny day? Doesn’t water evaporate when sunny day? Doesn’t water evaporate when it boils (100 it boils (100 ºC)ºC)??

Why won’t the water in a half-filled water Why won’t the water in a half-filled water bottle that is sealed evaporate?bottle that is sealed evaporate?

Forming the Ideal Gas LawForming the Ideal Gas Law

Recall that the following equation can be Recall that the following equation can be used to describe a 2 state gas system used to describe a 2 state gas system (initial state = 1 and changed state = 2)(initial state = 1 and changed state = 2)

Since PSince P11VV11/T/T11nn11 = k and P = k and P22VV22/T/T22nn22 = k = k

22

22

11

11

nT

VP

nT

VP=

Forming the Ideal Gas LawForming the Ideal Gas Law

Using a single state:Using a single state:

Substitute R for k:Substitute R for k:

Rearranged:Rearranged:

kTn

VP=

RTn

VP=

nRTPV =

R… what the world is R?R… what the world is R?

R = Ideal Gas ConstantR = Ideal Gas Constant

R is always the same for any ideal gasR is always the same for any ideal gas

R depends upon units used to describe a R depends upon units used to describe a gasgas

R Values R Values (not for insulation)(not for insulation)

Common:Common:0.0821 L*atm/mol*K0.0821 L*atm/mol*K very commonvery common8.314 L*kPa/mol*K8.314 L*kPa/mol*K SI unitsSI units

Less Common:Less Common:62.4 L*mmHg/mol*K62.4 L*mmHg/mol*K8.314 J/mol*K8.314 J/mol*K (1L*atm = 101.325 J)(1L*atm = 101.325 J)OthersOthers

Ideal Gas LawIdeal Gas Law

PV = nRTPV = nRT

Used to solve for an unknown variable in a Used to solve for an unknown variable in a single state gaseous systemsingle state gaseous system

Must know 3 of 4 variables to solveMust know 3 of 4 variables to solve

Ideal Gas Law ExampleIdeal Gas Law Example

What is the pressure in atm exerted by a 0.500 mol sample of NWhat is the pressure in atm exerted by a 0.500 mol sample of N22 in in

a 10.0L container at 298K?a 10.0L container at 298K?

Solve for Pressure:Solve for Pressure:

atmPL

KKmolatmLmolP

V

nRTPornRTPV

22.10.10

)298)(*/*0821.0)(500.0(

=

=

==