•Copyright Sautter 2003

GASES & THEIR PROPERTIES

THE GAS LAWS

WHAT MAKES A GAS DIFFERENT FROM SOLIDS AND LIQUIDS ?

• (1) ENERGY STATE - GASES HAVE A HIGHER ENERGY CONTENT

• (2) DENSITY - GASES HAVE LOW DENSITY

• (3) GASES ARE FLUIDS - THEY FLOW AND TAKE THE SHAPE OF THEIR CONTAINER (SO DO LIQUIDS)

• (4) THEY RESPOND NOTABLY TO CHANGES IN TEMPERATURE AND PRESSURE

HOW ARE TEMPERATURE AND PRESSURE MEASURED ?

• TEMPERATURE IS MEASURED WITH A THERMOMETER (OF COURSE !)

• PRESSURE IS MEASURED WITH A BAROMETER. IT ALLOWS MERCURY TO BE PUSHED UP A VERTICAL GLASS TUBE BY GAS PRESSURE AND THEN THE HEIGHT OF THE MERCURY IN THE TUBE IS READ. PRESSURE IS THEREFORE OFTEN MEASURED IN LENGTH UNITS SUCH AS MILLIMETERS, CENTIMETERS OR INCHES.

• YOU HAVE CERTAINLY HEARD TEMPERATURE AND BAROMETER READING DURING WEATHER REPORTS !

TORRICELLI’S BAROMETER

• Hg LEVEL

• NORMAL AIR PRESSURE:

• 1 ATM

• 760 MM OF Hg

• 29.92 INCHES

• 14.7 PSI

• 101.3 KPa

HOW DO GASES RESPOND TO TEMPERATURE & PRESSURE ?

• AS THE TEMPERATURE OF A GAS INCREASES, ITS VOLUME INCREASES (DIRECT)

• AS THE PRESSURE ON A GAS INCREASES, ITS VOLUME DECREASES (INVERSE)

VARIABLES WHICH EFFECT GAS BEHAVIOR

• (1) VOLUME (LITERS)

• (2) PRESSURE (ATM OR MM OF Hg)

• (3) TEMPERATURE (DEGREES KELVIN)

• (4) QUANTITY OF GAS (MOLES)

HOW DO CONFINED GASES AT A CONSTANT TEMPERATURE RESPOND TO PRESSURE CHANGES ?

• AS PRESSURE GOES UP, VOLUME GOES DOWN (INVERSE)

• BOYLE’S LAW SAYS:

• P1 x V1 = P2 x V2

• PRESSURE 1 AND 2 MUST BE IN THE SAME UNITS

• VOLUME 1 AND 2 MUST BE IN THE SAME UNITS

INVERSE!!

HOW DO CONFINED GASES AT A CONSTANT TEMPERATURE RESPOND TO PRESSURE

CHANGES ? (CONT’D) AN INVERSE RELATIONSHIP

HOW DOES TEMPERATURE EFFECT THE VOLUME OF A FIXED QUANTITY OF GAS AT A CONSTANT

PRESSURE ?

• AS TEMPERATURE GOES UP, VOLUME GOES UP (DIRECT)

• CHARLES LAW SAYS:

• V1 / T1 = V2 / T2

• VOLUMES 1 AND 2 MUST BE THE SAME UNITS• TEMPERATURES 1 AND 2 MUST BE IN KELVIN

KELVIN RULES

•CHARLES

Volume

T

E

M

P

E

R

A

T

U

R

E

- 273 0C

Absolute Zero

No Gas is Ideal

real gases liquefy before

absolute zero

An ideal gas would

shrink to zero volume

at absolute zero

WHAT IS KELVIN TEMPERATURE ?

• KELVIN TEMPERATURE STARTS AT ABSOLUTE ZERO (- 273 DEGREES CELSIUS)

• AT ABSOLUTE ZERO, NO HEAT IS PRESENT IN THE SYSTEM (A CONDITION WHICH CAN NEVER BE ACHIEVIED!)

• ALMOST ALL SCIENTIFIC CALCULATIONS INVOLVING TEMPERATURE USE THE KELVIN SCALE WHICH IS SOMETIMES CALLED THE ABSOLUTE TEMPERATURE SCALE.

• KELVIN = 273 + DEGREES CELSIUS

HOW DO PRESSURE CHANGES EFFECT A CONFINED GAS ?

• AS THE TEMPERATURE ON A CONFINED GAS GOES UP, ITS PRESSURE INCREASES (DIRECT)

• GAY-LUSSAC’S LAW SAYS:• P1 / T1 = P2 / T2

• PRESSURES 1 AND 2 MUST BE THE SAME UNITS• VOLUMES 1 AND 2 MUST BE THE SAME UNITS• TEMPERATURES 1 AND 2 MUST BE IN KELVIN

WHAT IS THE COMBINED GAS LAW ?

• THE COMBINED LAW IS BOYLE’S LAW, CHARLES LAW AND GAY-LUSSAC’S LAW TOGETHER.

• IT ALLOWS US TO CALCULATE THE EFFECTS OF TEMPERATURE, PRESSURE AND VOLUME ON A GAS AT THE SAME TIME.

• (P1 x V1 ) / T1 = (P2 x V2 ) / T2

• PRESSURES 1 AND 2 MUST BE THE SAME UNITS• TEMPERATURES 1 AND 2 MUST BE IN KELVIN

WHY ARE BOYLE’S, CHARLES AND GAY-LUSSAC’S LAWS CALLED “THE GAS LAWS” ?

• BECAUSE THEY APPLY TO ALL GAS• PROPERTIES THAT APPLY TO AN ENTIRE

CLASS OF SUBSTANCES AS A COLLECTION OF PARTICLES RATHER THAN INDIVIDUAL MATERIALS ARE CALLED “COLLIGATIVE PROPERTIES”. COLLIGATIVE MEANS “COLLECTIVE”

• PRESSURE, VOLUME, TEMPERATURE AND NUMBER OF MOLES ARE CALLED COLLIGATIVE PROPERTIES OF GASES !

AVOGADRO’S HYPOTHESIS

• THE UNDER LYING THEORY THAT ALLOWS THE GAS LAWS TO BE APPLIED EQUALLY WELL TO ALL GASES IS:

• “EQUAL VOLUMES OF DIFFERENT GASES, AT THE SAME TEMPERATURE AND PRESSURE, CONTAIN EQUAL MOLES”

ANY GAS - ONE MOLE

22.4 LITERS AT STPAVOGADRO

COMMONLY USED CONDITIONS OF TEMPERATURE AND PRESSURE ARE ZERO DEGREES CELSIUS AND

ONE ATOMSPHERE PRESSURE

• THESE CONDITIONS ARE CALLED STANDARD TEMPERATURE AND STANDARD PRESSURE (STP)

• ONE MOLE OF ANY GAS OCCUPIES 22.4 LITERS AT STP CONDITIONS.

• TO OBTAIN MOLES OF GAS PRESENT AT STP FROM THE NUMBER OF LITERS, DIVIDE LITERS BY 22.4

• TO OBTAIN LITERS OF GAS PRESENT AT STP FROM THE NUMBER OF MOLES, MULTIPLY BY 22.4

STP VOLUMES OF DIFFERENT GASSES

• DALTON’S LAW DEALS WITH THE PARTIAL PRESSURE OF GASES.

• PARTIAL PRESSURE REFERS TO THE FACT THAT EACH GAS IN A MIXTURE OF GASES CAUSES ITS OWN INDIVIDUAL PRESSURE AS IF IT WERE ALONE IN THE CONTAINER.

• IF ALL THE PRESSURES OF EACH OF THE GASES IN A MIXTURE ARE ADDED TOGETHER, WE GET THE TOTAL PRESSURE OF THE GASES IN THE CONTAINER.

DALTON’ LAWS

OF PARTIAL PRESSURES

DALTON’S LAWS (CONT’D)• TOTAL PRESSURE = PRESSURE OF GAS A + PRESSURE OF

GAS B + PRESSURE OF GAS C + ETC.

OR

P TOTAL = PGAS A + P GAS B + P GAS C + P …….

DALTON’S LAWS (CONT’D)

• DALTON’S LAW ALSO TELLS US THAT THE PARTIAL PRESSURE OF EACH GAS IS RELATED TO THE NUMBER OF MOLES OF THAT GAS PRESENT OUT OF THE TOTAL MOLES PRESENT

• PRESSURE OF GAS A = (MOLES OF GAS A / TOTAL MOLES OF GAS) x TOTAL PRESSURE

• PGAS A = (N GAS A / N TOTAL) x PTOTAL

THE “KING” OF THE GAS LAWS – THE UNIVERSAL GAS LAW OFTEN CALLED THE

IDEAL GAS LAW• PRESSURE x VOLUME = MOLES x A CONSTANT x TEMPERATURE

OR

P x V = N x R x T

P = PRESSURE (ATM) V = VOLUME (LITERS)N = MOLES T = TEMPERATURE (K)R = .0821 ATM x L/ MOLES x KELVIN (A CONSTANT)R IS CALLED “THE GAS CONSTANT”

SUMMARY OF EQUATIONS• BOYLE’S LAW:

P1 x V1 = P2 x V2

• CHARLES LAW:

V1 / T1 = V2 / T2

• GAY-LUSSAC’S LAW: P1 / T1 = P2 / T2• KELVIN = 273 + DEGREES CELSIUS

• COMBINED GAS LAW

(P1 x V1 ) / T1 = (P2 x V2 ) / T2

• DALTON’S LAW

P TOTAL = PGAS A + P GAS B + P GAS C + P …….

SUMMARY OF EQUATIONS (CONT’D)

• DALTON’S LAW (CONT’D)

• PGAS A = (N GAS A / N TOTAL) x PTOTAL

• AVOGADRO’S HYPOTHESIS “EQUAL VOLUMES OF DIFFERENT GASES, AT THE SAME

TEMPERATURE AND PRESSURE, CONTAIN EQUAL MOLES”

UNIVERSAL GAS LAW P x V = N x R x T

THINGS YET TO COME !!

• OUR NEXT PROGRAM WILL DEAL WILL SOLVING GAS LAW PROBLEMS.

• BRING YOUR CALCULATOR !!