GasesGases

Dr. Ron RusayDr. Ron Rusay

© Copyright 2007-2010 R.J. Rusay© Copyright 2007-2010 R.J. Rusay

GasesGases

Uniformly fill any container.Uniformly fill any container. Exert pressure on its surroundings. Exert pressure on its surroundings. Mix completely with other gasesMix completely with other gases

Gases: Gases: Pressure, Mass, Volume, TemperaturePressure, Mass, Volume, Temperature

PressurePressure

is equal to force/unit areais equal to force/unit area SI units = Newton/meterSI units = Newton/meter22 = 1 Pascal = 1 Pascal

(Pa)(Pa) 1 standard atmosphere = 101,325 Pa1 standard atmosphere = 101,325 Pa 1 standard atmosphere = 1 atm =1 standard atmosphere = 1 atm =

760 mm Hg = 760 torr760 mm Hg = 760 torr

QUESTIONQUESTIONFour bicycle tires are inflated to the following pressures. Which one has the highest pressure? Tire A 3.42 atm; Tire B 48 lbs/sq in; Tire C 305 kPa; Tire D 1520 mmHg. (Recall; 1.00 atm = 760 mmHg = 14.7 lb/sq in = 101.3 kPa)

A. Tire AB. Tire BC. Tire CD. Tire D

Toricellian BarometerToricellian Barometer05_47

h = 760mm Hgfor standardatmosphere Vacuum

Pressure & VolumePressure & VolumeBoyle’s LawBoyle’s Law**

Boyle’s LawBoyle’s Law**

Pressure Pressure Volume = Constant Volume = Constant (T = constant)(T = constant)

PP11VV11 = P = P22VV22 (T = constant) (T = constant) V V 1/ P (T = constant) 1/ P (T = constant)

((**Holds Holds preciselyprecisely only at very low only at very low pressures.)pressures.)

QUESTIONQUESTION

A) 4.0 LA) 4.0 LB) 0.57 LB) 0.57 LC) 5.7 LC) 5.7 LD) 0.4 LD) 0.4 L

An empty one gallon can An empty one gallon can is hooked to a vacuum is hooked to a vacuum

pump. pump.

What do you expect to What do you expect to happen? happen?

Explain why the Explain why the can collapsed. can collapsed.

Pressure vs. VolumePressure vs. Volume

05_50

0V(in3)20406050100PP2V2V(a)

00 1/P (in Hg)0.010.020.032040slope = k

(b)

Definition: A gas that Definition: A gas that strictly obeys strictly obeys Boyle’s Boyle’s Law is called an Law is called an ideal gasideal gas..

Ideal GasesIdeal GasesReal vs. “Ideal”Real vs. “Ideal”

Temperature & Volume Temperature & Volume

Temperature & VolumeTemperature & Volume

Charles’s LawCharles’s Law

The volume of a gas is directly The volume of a gas is directly proportional to temperature, and proportional to temperature, and extrapolates to zero at zero Kelvin.extrapolates to zero at zero Kelvin.

V = V = T (P = constant)T (P = constant)

= a proportionality constant= a proportionality constant

Temperature and VolumeTemperature and Volume (@ constant P)(@ constant P)

Charles’s LawCharles’s LawVTVTP1122==(constant)

Pressure vs. TemperaturePressure vs. Temperature (@ constant V)(@ constant V)

05_1542Pext Pext

Temperature is increasedChanging VolumeChanging Volume

The Meaning of TemperatureThe Meaning of Temperature

Kelvin temperature is an index of the random Kelvin temperature is an index of the random motions of gas particles (higher T means greater motions of gas particles (higher T means greater motion.)motion.)

(KE)32avg=RT

QUESTIONQUESTIONKinetic molecular theory helps explain the definition of temperature based on molecular motion. Which statement describes an important aspect of this connection?

A) Temperature is inversely related to the kinetic energy of thegas particles.

B) At the same temperature, more massive gas particles will bemoving faster than less massive gas particles.

C) As the temperature of a gas sample increases, the average velocity of the gas particles increases.

D) Kinetic energy is directly related to temperature. This is validfor any units of temperature.

Kinetic Molecular TheoryKinetic Molecular Theory

1.1. Volume of individual particles is Volume of individual particles is zero.zero.

2.2. Collisions of particles with container Collisions of particles with container walls cause pressure exerted by gas.walls cause pressure exerted by gas.

3.3. Particles exert no forces on each Particles exert no forces on each other.other.

4.4. Average kinetic energy Average kinetic energy Kelvin Kelvin temperature of a gas.temperature of a gas.

Molecular Motion / TheoryMolecular Motion / TheoryThe Meaning of TemperatureThe Meaning of Temperature

Temperature (Kelvin) is an index of the random motions of gas particles (higher T means greater motion.)(KE)32avg=RT

Velocity & TemperatureVelocity & Temperature

QUESTIONQUESTIONWhy is it critical that the temperature be held constant when applying Boyle’s law to changing the pressure of a trapped gas?

A) Gas molecules may expand at higher temperatures; this wouldchange the volume.

B) Changing the temperature causes the gas to behave in non-idealfashion.

C) Changing the temperature affects the average particle speed,which could affect the pressure.

D) Allowing the temperature to drop below 0°C would cause thetrapped gas to no longer follow Boyle’s Law.

QUESTIONQUESTIONAs the temperature of a gas increases, which statement best correlates to information about molecular velocity?

A) The average molecular velocity will increase, but thedistribution of molecular velocities will stay the same.

B) The average molecular velocity will stay the same, but themolecular velocity distribution will spread.

C) The average molecular velocity will increase, and thedistribution of the molecular velocities will spread.

D) The average molecular velocity will stay the same, and thedistribution of the molecular velocities will stay the same.

View Gas Molecules AppletView Gas Molecules Applethttp://chemconnections.org/Java/molecules/index.html

View Molecular Vibrations: IRGasTutorhttp://chemistry.beloit.edu/Warming/pages/infrared.html

Changes in Temperature Changes in Temperature ((PV&T)PV&T)05_1543

PextPext

Energy (heat) added

Pressure, Volume & TemperaturePressure, Volume & Temperature

V1

An Ideal gasin a cylinder

V2 = 1/ 2 V1

Something(s) happen(s)

What can be going on?

Provide 3 different sets of conditions Provide 3 different sets of conditions (Pressure and Temperature) which can (Pressure and Temperature) which can

account for the volume of the gas account for the volume of the gas decreasing by 1/2.decreasing by 1/2.

Cases 1-3 in the handout.Cases 1-3 in the handout.

Applying a Gas LawApplying a Gas Law

Avogadro’s LawAvogadro’s Law

For a gas at constant temperature For a gas at constant temperature and pressure, the volume is directly and pressure, the volume is directly proportional to the number of moles of proportional to the number of moles of gas (at low pressures).gas (at low pressures).

VV = = nn

= proportionality constant= proportionality constant V = volume of the gasV = volume of the gas n = number of moles of gasn = number of moles of gas

Volume vs. n (moles of a gas)Volume vs. n (moles of a gas)

QUESTIONQUESTIONEach of the balloons hold 1.0 L of different gases. All four are at 25°C and each contains the same number of molecules. Of the following which would also have to be the same for each balloon? (obviously not their color)

A) Their densityB) Their massC) Their atomic numbersD) Their pressure