Chemistry - an illustrated guide to science (794128), страница 4
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Scientists can observe this byusing a glass smoke chamber.● Smoke consists of large particles thatcan be seen using a microscope.● In the smoke chamber, the smokeparticles move around randomly dueto collisions with air particles.3 Diffusionsabcdeftapecardboardfine stainless steel wiremagnifying glass1/ 2 mm scaleview through magnifyingglassg oil droph waxed sticksi wax-coated trayj lycopodium powderk oil patchl microscopem removable lidn windowo lampp glass rod for converginglightq glass smoke chamberr glass diffusion tubes liquid bromine capsulet rubber stopperu tapv bromine capsulew rubber tubex point at which pressure isapplied to break capsuleis the spreading out of onesubstance through another due to therandom motion of particles.● The diagram illustrates how scientistsuse a diffusion tube to observe this.Initially the color of the substance isstrongest at the bottom of the tube.● After a period of time, as a result ofdiffusion, the particles of thesubstance mix with air particles, andthe color becomes uniform down thelength of the tube.© Diagram Visual Information Ltd.● Diffu sion20ATOMIC STRUCTUREKey wordsanodeAvogadro’sconstantelectrolysisFaraday constantmoleDetermination ofAvogadro’s constantDetermination of Avogadro’s constantaDefining Avogadro’sconstantcon stan t is the number ofparticles in a m ole of a substance.
Itequals 6.023 x 1023 mol-1.● It is F, the Faraday con stan t—96,500coulombs per mole, the amount ofelectric charge of one mole ofelectrons—divided by 1.60 x 10-19coulomb—the charge on one electron(expressed as e).● Thus, the Avogadro constant, N , isgiven by: N = F–b+● Avogadro’sAeor:96,50 01.60 x 10 -19= 6.0 23 x 10 23 mol -1Determining the Constant● Thenumber of molecules in one moleof substance can be determined byusing electrochemistry.● During electrolysis, current (electronflow) over time is measured in anelectrolytic cell (see diagram).
Thenumber of atoms in a weighed sampleis then related to the current tocalculate Avogadro’s constant.cdIllustrating the Procedure© Diagram Visual Information Ltd.● Thediagram illustrates the electrolysisof copper sulfate. To calculateAvogadro’s constant, the researcherweighs the rod to be used as thean ode before submerging the twocopper rods in copper sulfate. Shethen connects the rods to a powersupply and an ammeter (aninstrument used to measure electriccurrent). She measures and recordsthe current at regular intervals andcalculates the average amperage (theunit of electric current).
Once sheturns off the current, she weighs theanode to see how much mass was lost.Using the figures for anode mass lost,average current, and duration of theelectrolysis, she calculates Avogadro’sconstant.efabcdefpower supply with ammeterrheostathardboard or wooden electrode holdercopper rod cathodecopper rod anodecopper sulfate solution21The moleATOMIC STRUCTUREKey words1 Defining a mole6.023 × 10 23 particles1 particle –x amuatomionmolaritymolemolecule1 Defining a molex grams● Because2 Moles of gas(71 g) Cl 2(44 g) CO2H2 (2 g)22.4 litersN2 (28 g)atom s, ion s, and m olecu leshave very small masses, it is impossibleto count or weigh them individually.
Asa result, scientists use m oles in achemical reaction.● A mole is the amount of substancethat contains as many elementaryentities (atoms, molecules, ions, anygroup of particles) as there are atomsin exactly 0.012 kilogram of carbon-12.This quantity is Avogadro’s constant(6.023 x 1023 mol-1).● The significance of this number is thatit scales the mass of a particle inatomic mass units (amu) exactly intograms (g).● Chemical equations usually imply thatthe quantities are in moles.2 Moles of gas● One(16 g) CH4O2 (32 g)3 Molaritymole of any gas occupies22.4 liters at standard temperature andpressure, (which is 0 º C andatmospheric pressure).● The diagram shows the mass in gramsof one mole of the following gases:chlorine ( Cl 2), carbon dioxide ( CO2),methane ( CH4 ), hydrogen ( H2),nitrogen ( N2), and oxygen ( O2).3 Molarity(127 g) FeCl 2(95 g) MgCl 2(233 g) BaSO4NaOH (40 g)1 literCa(NO3) 2 (164 g)KBr (119 g)is concerned with theconcentration of a solution.
Itindicates the number of particles in1 liter of solution.● A 1 molar solution contains 1 mole ofa substance dissolved in water or someother solvent to make 1 liter ofsolution.● The diagram shows the mass in gramsof one mole of the followingsubstances: iron(II) chloride ( FeCl 2),magnesium chloride ( MgCl 2), bariumsulfate ( BaSO4 ), sodium hydroxide( NaOH), calcium nitrate ( Ca(NO3) 2),and potassium bromide ( KBr ).© Diagram Visual Information Ltd.● Molarity22ATOMIC STRUCTUREKey wordsatomic emissionspectruminfraredspectrumultravioletwavelengthAtomic emissionspectrum: hydrogenEmission spectrum in the near ultra-violet and visibleBalmer seriesVioletAtomic spectrumRedH!H"H#H$H%7.30 96.9076.1674.568● Theatom ic em ission spectru m of anelement is the amount ofelectromagnetic radiation it emitswhen excited.
This pattern ofwavelengths is a discrete linespectru m , not a continuous spectrum.It is unique to each element.Investigating hydrogen● Towardthe end of the nineteenthcentury, scientists discovered thatwhen excited in its gaseous state, anelement produces a unique spectralpattern of brightly colored lines.Hydrogen is the simplest element and,therefore, was the most studied.Hydrogen has three distinctivelyobservable lines in the visiblespectrum—red, blue/cyan, and violet.Series1885 Swiss mathematician andphysicist Johannes Jakob Balmerproposed a mathematical relationshipfor lines in the visible part of thehydrogen emission spectrum that isnow known as the Balmer series.● The series in the u ltraviolet region ata shorter wavelen gth than the Balmerseries is known as the Lyman series.● The series in the in frared region atthe longer wavelength than the Balmerseries is known as the Paschen series.● The Brackett series and the Pfundseries are at the far infrared end of thehydrogen emission series.© Diagram Visual Information Ltd.● InaSchematic series306ba frequency (× 10 14 Hz)b Lyman seriesc Balmer seriesad Paschen seriese Bracket seriesf Pfund seriesc0.6def23Energy levels: hydrogenatomEnergy-level schematicn=?n=5n=4n=3ATOMIC STRUCTUREKey wordsground stateorbitalquantum numbershellultravioletEnergy levels● Electronsare arranged in definiteenergy levels (also called shells ororbitals), at a considerable distancefrom the nucleus.● Electrons jump between the orbits byemitting or absorbing energy.● The energy emitted or absorbed isequal to the difference in energybetween the orbits.Energy levels of hydrogen● TheEnergyn=2graph shows the energy levels forthe hydrogen atom.
Each level isdescribed by a qu an tu m n u m ber(labeled by the integer n).● The shell closest to the nucleus hasthe lowest energy level. It is generallytermed the grou n d state. The statesfarther from the nucleus havesuccessively more energy.Transition from n level toground state# ! "● Transitionfrom n=2 to the groundstate, n=1:Frequency =24.66 x 10 14 Hz● Transition from n=3 to the groundstate, n=1:Frequency =29.23 x 10 14 Hz● Transition from n=4 to the groundstate, n=1:Frequency =30 .83 x 10 14 HzGround staten=1Line spectrumradiation is in the u ltravioletregion of the electromagneticspectrum and cannot be seen by thehuman eye.Line spectrum#30.83!29.23Frequency / 10 14 Hz"24.66© Diagram Visual Information Ltd.● This24ATOMIC STRUCTUREKey wordsfluorescenceluminescencephosphorescenceLuminescence1 LuminescenceE3SecondexcitedstateE2FirstexcitedstateEnergylevelsn=31 Luminescenceis the emission of lightcaused by an effect other than heat.● Luminescence occurs when asubstance is stimulated by radiationand subsequently emits visible light.● The incident radiation exciteselectrons, and as the electrons returnto their ground state, they emit visiblelight.● If the electrons remain in their excitedstate and emit light over a period oftime, the phenomenon is calledphosphorescen ce.● If the electrons in a substance returnto the ground state immediately afterexcitation, the phenomenon is calledflu orescen ce.Energy (E)● Lu m in escen cePhotonPhotonGroundstateE1Electronabsorbsphoton2 Fluorescencethis diagram, a fluorescent lighttube contains mercury vapor at lowpressure.
Electrons are released fromhot filaments at each end of the tubeand collide with the mercury atoms,exciting the electrons in the mercuryatoms to higher energy levels. As theelectrons fall back to lower energystates, photons of ultraviolet light areemitted.● The ultraviolet photons collide withatoms of a fluorescent coating on theinside of the tube.
The electrons inthese atoms are excited and thenreturn to lower energy levels, emittingvisible light.n=2n=1Electronemitsphoton● In2 Fluorescencevisible lightfilamentultravioletphotonsare emittede–e–© Diagram Visual Information Ltd.filamentmercuryatomsfluorescentcoating25Organizing the elementsELEMENTS AND COMPOUNDS1 Antoine LavoisierKey wordsGroup 1Group 2heatGroup 3sulfurphosphoruslightoxygennitrogenatomic massatomic volumeelementGroup 4coppertinleadlimezincaluminabarytamagnesia1 Antoine Lavoisiersilica● In1789, French chemist AntoineLavoisier organized what he believedwere the elem en ts into four groups:Group 1 gases, Group 2 non-metals,Group 3 metals, and Group 4 earths.2 Johann DobereinerTriadRelative atomic massLiNaSClSeKTe732BrICaSrBa23397912835.5 8040881272 Johann Dobereiner● In1817, German chemist JohannDobereiner noticed that the atom icm ass of strontium was about half waybetween that of calcium and barium.After further study, he found that hecould organize other elements intosimilar groups based on the samerelationship to each other.
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