The Air Around UsThe Air Around Us

Air is more than one Air is more than one removed from nothingremoved from nothing

Learning objectivesLearning objectives

Describe what is meant by pressure Describe what is meant by pressure Identify units of gas pressureIdentify units of gas pressure Describe the basic gas lawsDescribe the basic gas laws Apply gas laws to simple problemsApply gas laws to simple problems Describe composition of the atmosphereDescribe composition of the atmosphere Name important pollutants and their effectsName important pollutants and their effects Describe origin of the ozone holeDescribe origin of the ozone hole

Gas laws and crash safetyGas laws and crash safety

The airbag is chemistry applied in a very The airbag is chemistry applied in a very practical areapractical area

Airbags have reduced serious injuries and Airbags have reduced serious injuries and fatalities by significant marginfatalities by significant margin

Chemistry plays a crucial role in airbag Chemistry plays a crucial role in airbag performanceperformance

Timing is everythingTiming is everything

The airbag must deploy within about 40 ms The airbag must deploy within about 40 ms of the impactof the impact

The airbag must not deploy unless there is The airbag must not deploy unless there is an impactan impact

Inflation depends upon a rapid chemical Inflation depends upon a rapid chemical reaction generating a quantity of gasreaction generating a quantity of gas

The bag, once inflated, must then deflate at The bag, once inflated, must then deflate at the point of impact with the driver to prevent the point of impact with the driver to prevent injuryinjury

Gas: no interactionsGas: no interactions

Not rigidNot rigid Completely fills containerCompletely fills container CompressibleCompressible Low densityLow density

Atom motion and temperatureAtom motion and temperature

Atoms in molecules have three types of Atoms in molecules have three types of motionmotion Rotation – moving about the centre of massRotation – moving about the centre of mass Vibration – vibrating about the centre of massVibration – vibrating about the centre of mass Translation – movement of the centre of massTranslation – movement of the centre of mass

As temperature increases, the energies of As temperature increases, the energies of all types of motion increaseall types of motion increase

In gases molecular energies exceeds all In gases molecular energies exceeds all interactionsinteractions

Kinetic molecular theory and Kinetic molecular theory and pressure – a case for atomspressure – a case for atoms

Pressure is caused by the energetic Pressure is caused by the energetic molecules striking the tire wallmolecules striking the tire wall

Pumping up a tire increases the number of Pumping up a tire increases the number of moleculesmolecules

More molecules – higher pressureMore molecules – higher pressure Higher temperature – higher pressureHigher temperature – higher pressure

Under pressureUnder pressure

Gases exert pressure by virtue of motionGases exert pressure by virtue of motion Gravity makes the air density higher near Gravity makes the air density higher near

the earth’s surfacethe earth’s surface Pressure decreases with elevationPressure decreases with elevation

Atmospheric pressureAtmospheric pressure

BarometerBarometer is used for measuring is used for measuring atmospheric pressureatmospheric pressure

Atmospheric pressureAtmospheric pressure

The weight of the air supports a column of The weight of the air supports a column of mercury 760 mm highmercury 760 mm high

Nominally atmospheric pressure is 760 mm Nominally atmospheric pressure is 760 mm HgHg

Atmospheric pressure changes with the Atmospheric pressure changes with the weatherweather

Units of pressureUnits of pressure

Gas laws: experience in math formGas laws: experience in math form

The properties of gases can be described by a The properties of gases can be described by a number of simple lawsnumber of simple laws

The laws establish quantitative relationships The laws establish quantitative relationships between different variablesbetween different variables

They are largely intuitively obvious and familiarThey are largely intuitively obvious and familiar

The four variablesThe four variables

Pressure (P)Pressure (P) Volume (V)Volume (V) Temperature (T in Kelvin)Temperature (T in Kelvin) Number of molecules (n in moles)Number of molecules (n in moles)

Variables and constantsVariables and constants

In the elementary gas laws two of the four In the elementary gas laws two of the four variables are kept constantvariables are kept constant

Each law describes how one variable reacts Each law describes how one variable reacts to changes in another variableto changes in another variable

All the simple laws can be integrated into All the simple laws can be integrated into one combined gas lawone combined gas law

Boyle’s lawBoyle’s law

The first experimental gas lawThe first experimental gas law Pressure increases, volume decreases (T, n Pressure increases, volume decreases (T, n

constant)constant)1

PV

Charles’ LawCharles’ Law

As temperature increases, volume As temperature increases, volume increases (P, n constant)increases (P, n constant) Temperature must be measured in KelvinTemperature must be measured in Kelvin

V T

Absolute zeroAbsolute zero

Gay-Lussac observed V changed by 1/273 Gay-Lussac observed V changed by 1/273 of value at 0of value at 0ºCºC

Plotted as V = Plotted as V = kkT (T = ºC + 273):T (T = ºC + 273): V = 0 at T = 0 KV = 0 at T = 0 K

Does the gas actually occupy zero volume?Does the gas actually occupy zero volume? No, at lower T the law is not followedNo, at lower T the law is not followed

Combined gas lawCombined gas law

Fold together Boyle and Charles:Fold together Boyle and Charles: PP11VV11/T/T11 = P = P22VV22/T/T22

Given five of the variables, find the sixthGiven five of the variables, find the sixth Units must be consistentUnits must be consistent Temperature in KelvinTemperature in Kelvin

Standard temperature and pressure Standard temperature and pressure (STP)(STP)

Standard conditions allow direct comparison Standard conditions allow direct comparison of properties of different substancesof properties of different substances Standard temperature is 273 K (0Standard temperature is 273 K (0ºC)ºC) Standard pressure is 760 mm Hg or 1 Standard pressure is 760 mm Hg or 1

atmosphereatmosphere

At STP, 1 mole of At STP, 1 mole of anyany ideal gas occupies ideal gas occupies 22.414 L22.414 L

What is the atmosphereWhat is the atmosphere

The atmosphere is layeredThe atmosphere is layered

TroposphereTroposphere Where the weather happensWhere the weather happens

StratosphereStratosphere Where the ozone isWhere the ozone is

MesosphereMesosphere IonosphereIonosphere

The brutal strength of solar radiation ionizes all the The brutal strength of solar radiation ionizes all the components – permits transmission of radio signals components – permits transmission of radio signals around the earth without need of mirrorsaround the earth without need of mirrors

Pollutants: things that shouldn’t be Pollutants: things that shouldn’t be therethere

SOSO22 power plant emissionspower plant emissions PM-10 PM-10 particulate matter from agriculture and particulate matter from agriculture and

constructionconstruction CO CO vehicle emissionsvehicle emissions OO33 sunlight and vehicle emissionssunlight and vehicle emissions NONO22 vehicle emissionsvehicle emissions Pb Pb smelters and battery plantssmelters and battery plants

Cleaning up our act: successesCleaning up our act: successes

Clean Air Act of 1970Clean Air Act of 1970 Amended in 1977Amended in 1977 Top six pollutants have all decreasedTop six pollutants have all decreased Problems remainProblems remain

Ozone: still a problemOzone: still a problem

Bad ozone, good ozoneBad ozone, good ozone

Stratospheric sunscreenStratospheric sunscreen OO33 + UV = O + UV = O22 + O + O

OO22 + O = O + O = O33 + heat + heat

Chlorofluorocarbons (CFCs): wonder chemicalsChlorofluorocarbons (CFCs): wonder chemicals

ApplicationsApplications PropertiesProperties

RefrigerationRefrigeration ColorlessColorless

Air-conditioningAir-conditioning NontoxicNontoxic

PropellantsPropellants InertInert

Foaming agentsFoaming agents OdorlessOdorless

Hidden dangersHidden dangers

CFCs are unstable in UVCFCs are unstable in UVCFCF22ClCl22 + UV = Cl + CF + UV = Cl + CF22ClCl

Cl radicals catalyze decomposition of ozoneCl radicals catalyze decomposition of ozoneCl + OCl + O33 = ClO = ClO + O+ O22

OO33 + UV = O + UV = O22 + O + O

ClO + O = Cl + OClO + O = Cl + O22

One Cl atom can react again and again destroying One Cl atom can react again and again destroying countless Ocountless O33 molecules molecules

The Antarctic ozone holeThe Antarctic ozone hole

Ozone depletion in the Antarctic is severeOzone depletion in the Antarctic is severe Stratosphere isolated by polar vortexStratosphere isolated by polar vortex Polar stratospheric clouds (PSCs) formPolar stratospheric clouds (PSCs) form Clouds produce chlorine gasClouds produce chlorine gas

Montreal Protocol and CFCsMontreal Protocol and CFCs

Phase-out of CFCs began 1978Phase-out of CFCs began 1978 Montreal Protocol (1987) called for 50 % decrease Montreal Protocol (1987) called for 50 % decrease

by 2000 by 2000 Amended to complete phaseoutAmended to complete phaseout Chemical ingenuity required for alternatives like Chemical ingenuity required for alternatives like

HCFCsHCFCs