Properties of a Gas

Edward A. Mottel

Department of Chemistry

Rose-Hulman Institute of Technology

Properties of a Gas

Reading Assignment:• Zumdahl, Chapter 5.1-5.3

This lecture introduces the interrelated dependence of moles, temperature, pressure and volume of a confined gas.

Based on assumptions of ideal gas behavior, these parameters when combined form the ideal gas equation, PV=nRT.

Gases

Large intermolecular distances,compared to molecular size

No long or short rangestructural order

What happens when the stopcock is opened?

GasesMix together in any proportions

Intermolecular collisions are elastic --energy is transferred through collisions

Important Physical Properties of a Gas

Temperature Pressure Volume Moles

Temperature

Temperature is a measure of molecular motion: vibration, rotation and translation

Gases move at high velocity• (> 1000 m·sec–1)

Common units:• Celsius (°C), Fahrenheit (°F)

Absolute units: Kelvin (K), Rankine (°R)

Temperature

Absolute units: Kelvin (K), Rankine (°R) 0.00 °C = 273.15 K

= 32.00 °F = 491.67 °R 100.00 °C = 373.15 K

= 212.00 °F = 671.67 °R Isothermal - constant temperature

Pressure

Due to moleculeshitting the surface

Force per unit area

Pressure

What do you knowabout

the pressurein a balloon?

Pressure

Within a closed containerwith a flexible ormoveable barrier,

the external forcespushing on thesurface of the

containercontaining

the gas

are equalto the pressure of the trapped gas pushing out.

Barometer

mercury

vacuumWhat forces determine

the heightof the mercury

in the glass tube?

Barometer

atmosphericpressure

What forces determinethe height

of the mercuryin the glass tube?

Barometer

760 mm 30 inHow much pressuredoes the atmosphereexert?

Hg = 13.6 g·cm–3

= 0.490 lb·in–31.03 kg·cm–2

= 14.7 lb·in–2

Barometer

How high would awater barometerhave to be?

H2O= 1.0 g·cm–3

33 feet

Pressure

Common units:• lbs·in–2, atmospheres (atm), mm Hg, torr,

pascals (N·m–2, Pa) 1.00 atm = 14.7 lbs·in–2

= 760 mm Hg = 760 torr

= 1.013 x105 Pa

Pressure

The actual atmospheric pressure is a function of the weather (high or low pressure) and altitude.• Typical Terre Haute pressure is 730-750

mm Hg. Isobaric - constant pressure

Volume

Gases occupy the complete volume to which they are constrained

Liquids and solids occupy a volume corresponding to their mass and density

Isochoric - constant volume

Moles

A counting unit for the number of molecules being considered

Physical Laws of Gases

Boyle's Law Charles' Law

Avogadro's Law Ideal Gas Law

Boyle's Law

P·V = constant

isothermalconstant amount of gas

volu

me

pressure

What does a plot of thisrelationship look like?

Charles' Law

VT

isobaricconstant amount of gas

volu

me

temperature

What does a plot of thisrelationship look like?

What does the y-interceptindicate?

Avogadro's Law

Vn

isobaricisothermal

volu

me

moles of gas

What does a plot of thisrelationship look like?

Gas Lawsvo

lum

e

pressure

Boyle

volu

me

temperature

Charles

volu

me

moles of gas

Avogadro

V 1/P V T V n

V nT

P

Ideal Gas Law

P·V n·T PV = nRT

• where R = ideal gas constant= 0.08205 L·atm·mol–1·K–1

Boyle's Law

P·V = constant

How will this plot changeif the same number of moles

of gas are at ahigher temperature?

isothermalconstant amount of gas

pre

ssu

re

volume

Common GasesRoom Temperature, One Atmosphere Pressure

Monatomic: He, Ne, Ar, Kr, Xe, Rn Polyatomic

• Elements: N2, O2, O3, F2, Cl2• Compounds: CO, CO2, NO, N2O, NO2,

N2O4, N2O5, CH4, C2H6, C2F4, etc.

Atmospheric Composition

Dry air

78% N2, 21% O2, 1%Ar Normal air

N2, O2, Ar, 0.03% CO2, 1.6% H2O