Chemistry - an illustrated guide to science (794128), страница 9
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When a m ixtu re of sugarand salt is mixed with ethanol, thesugar dissolves while the salt does not.● When the mixture is filtered, theundissolved salt remains as the residu ein the filter. The filtrate, sugarsolution, passes through the filter.● If the filtrate is left open to the air, theethanol evaporates, and solid sugarremains.2 Separating two solutes insolutiondissolves in water but not incarbon tetrachloride.● Iodine is slightly soluble in water but isfar more soluble in carbontetrachloride.● When a mixture of salt and iodine isshaken in a mixture of water andcarbon tetrachloride, the salt dissolvesin the water and the iodine in carbontetrachloride.● Water and carbon tetrachloride areim m iscible, they do not mix, and formtwo layers in a separating funnel.Carbon tetrachloride is more densethan water and forms the lower layer.● When the layers are run into separateevaporating basins and left, thesolven ts—carbon tetrachloride andwater—evaporate, leaving salt andiodine respectively.abSalt and sugarmixtureEthanol is added tomixture — sugar dissolvesbut not saltThe solution isfiltered● Salta salt left on filter paperb sugar solution2 Separating two solutes in solutionccddBrown solution ofsalt and iodine© Diagram Visual Information Ltd.The ethanol evaporatesleaving solid sugarCarbon tetrachlorideis addedThe two immiscible solutionsare separated using aseparating funnelEach of the solvents are evaporatedoff to obtain the two solutesc salt solution in waterd purple solution of iodine in carbon tetrachloride49Paper chromatographyCHANGES IN MATTERKey words1 Paper chromatographysupportchromatographyRf valuemixturesolutesolutionsolventchromatography paper1 Paper chromatography● Chrom atographysolvent frontsamplepencil linesolvent2 Rf valueis a technique forseparating and identifying m ixtu res ofsolu tes in solu tion s.● In paper chromatography, absorbentpaper is suspended on a support sothat only the bottom rests in thesolven t.● A base line is drawn in pencil abovethe level of the solvent.
(If ink wereused, the dyes in the ink wouldseparate during the process and mixwith the sample.)● A concentrated solution of the samplemixture is made by dissolving as muchas possible in a very small volume ofsolvent.● A small amount of the concentratedsolution is spotted onto the base line.The chromatography paper issuspended over the solvent.● The solvent rises up thechromatography paper.solvent front2 Rf valueRf = 0.710 cmRf = 0.47cm4cmRf = 0.11cmbase lineR f valu e is the ratio of thedistance moved by a substance in achromatographic separation to thedistance moved by the solvent.
Thegreater the attraction between asubstance and the solvent molecules,the greater the Rf value.● The molecules of each substance in amixture are attracted both to thechromatography paper and to thesolvent molecules.● The greater the attraction between asubstance and the solvent molecules,the quicker it will be carried up thechromatography paper.
Dyes that arevery soluble in the solvent are carriedup to the top of the paper, while thosethat are less soluble remain lowerdown.● The Rf value is independent of theheight of the solvent front but isdependent on the solvent used.© Diagram Visual Information Ltd.● The50CHANGES IN MATTERKey wordsalkanechromatographygas-liquidchromatographymassspectrometrymobile phasestationary phase1 Gas-liquidchromatographychromatography, substances arepartitioned between a station aryphase and a m obile phase. Thestationary phase is the substance thatretards the components of the sample.The mobile phase is the componentsof the sample.● In gas-liqu id chrom atography, thestationary phase, packed into thecolumn, consists of a high-boilingpoint liquid, such as a long-chainalkan e, supported by a porous inertsolid, such as charcoal or silica.● The mobile phase consists of a carriergas—usually nitrogen, hydrogen,helium, or argon.● A sample mixture is injected into thechamber where it vaporizes and iscarried through the column by thecarrier gas.
Various compounds in thesample pass through the column atdifferent rates due to their attractionto the stationary phase.● The separated compounds pass to adetector or directly into a massspectrometer.Gas-liquidchromatography andmass spectrometry1 Gas-liquid chromatography● Ininjection ofsampleovenrecordercolumncarriergasdetectorflow meter2 Mass spectrometryreservoirionisation chambernegatively chargedplatessampleinlet2 Mass spectrometry© Diagram Visual Information Ltd.● Massspectrom etry is a technique usedto identify the chemical constitution ofa substance by means of analyzing itsions.● The sample passes into the ionizationchamber, where it is bombarded byelectrons and forms a series of positiveions.● The ions are accelerated by an electricfield and deflected along a circularpath by a magnetic field.
The lighterthe ions, the greater the deflection.● The intensity of the ion beam isdetected electrically, amplified, andfinally recorded.● Each compound gives a characteristicspectrum from which it can beidentified.electrongunvariablemagneticfieldrecorderlighterparticlesamplifierheavierparticlesvacuum51The pH scaleCHANGES IN MATTERKey words1 pH scaleacidityalkalinitypHpH meter[ H+ ] / mole dm –31010 –1 10 –2 10 –312310 –4410 –510 –6 10 –756710 –8810 –9 10 –10 10 –11 10 –12 10 –14 10 –149101112131 pH scale14● pHis a measure of the acidity oralkalin ity of a solution. The term pHwas originally introduced by theDanish biochemist Søren Sørensen in1909 while working on methods ofimproving the quality control of beer.The letters pH stand for “potential ofhydrogen.”● Acidic solutions always have a pH ofless than 7, and alkaline solutionsalways have a pH of more than 7.
Thelower the pH value, the more acidicthe solution; conversely, the higherthe pH value, the more alkaline thesolution.● The pH of a solution is the logarithmto base 10 of the reciprocal of thenumerical value of the hydrogen ionconcentration:pHIncreasing acidityIncreasing alkalinityNeutralLower pH/ sronger acidHigher pH/ sronger alkalipH = lg(1/ [ H+ ] 0 = -lg [ H+ ]2 Schematic of a pH meter● ThepH of a neutral solution can becalculated directly from the ionicproduct ( Kw) of water:bKw = [ H+ ] [ OH- ] = 10 -14 mol 2 dm -6For a neutral solution:[ H+ ] = [ OH- ] = 10 -7 mol dm -3catherefore the pH of a neutral solution= 7.● The pH scale is logarithmic, sohydrogen ion concentration increasesor decreases by a power of 10 for eachstep down or up the scale.2 pH meterhegfa platinum wireb sensitive voltmeterc silver wire coated with silverchloride (AgCl)d saturated potassium chloride (KCl)efghcapillary opening with porous plugsolution of unknown pHThin glass membrane through which H+ ions can passsolution of fixed acid pH● A pHm eter is an electrochemical cellconsisting of an electrode, such as aglass electrode, which is sensitive tohydrogen ion concentration, and areference electrode.● The emf (electromotive force) of thecell can be measured using a highresistance voltmeter.
A pH meter is ahigh-resistance voltmeter calibratedwith the pH scale.© Diagram Visual Information Ltd.d52CHANGES IN MATTER1 Table of common indicatorsKey wordsacidacid-baseindicatoralkaliend pointIndicatorsequilibriumtitrationuniversalindicatorIndicatorColor in acidColor in alkalilitmusredbluemethyl orangeredyellowphenolphthaleincolorlesspink1 Common indicators● Acid-basein dicators are substancesthat are different colors in acids andalkalis so they “indicate” whether asolution is an acid or alkali.2 Changing equilibrium (phenol phthalein)OH2 Changing equilibriumindicators are usually weakacids that disassociate to give an ionthat is a different color than the acid.A change in pH causes a change in theposition of the equilibrium of thereaction and, therefore, the color ofthe solution.● Phenolphthalein is such an indicator. Itis a colorless, weak acid thatdissociates in water, forming pinkanions.
Under acidic conditions, theequ ilibriu m of the reaction is to theleft, and the concentration of theanions is too low for the color to bevisible. Under alkaline conditions, theequilibrium is to the right, and theconcentration of anions is highenough for the pink to be seen.OH● Acid-base3 Universal indicator● Incontrast to an indicator such asphenolphthalein, which is able toshow whether a substance is an acid orbase only in the broadest terms, au n iversal in dicator has a range ofcolors that indicate how acidic or howalkaline a solution is.OHC© Diagram Visual Information Ltd.indicators do not change colorwhen the pH of a solution is exactly 7.This means that the en d poin t of thetitration , the point at which theindicator undergoes the maximumcolor change, occurs at a differenttime to the equivalence point of thetitration, the point at which there areequivalent amounts of acid and alkali.● The suitability of an indicator for usein a titration depends on whatcombination of strong and weak acidand alkali is to be used.+ H2O+C+ H2 OO–OCCOOcolorless(acid)pink(base)3 Universal indicatorpH0123red4567orangeyellowgreen89101112blue13purpleColor4 Table of pH range over which acid–base indicatorschange color4 pH range of indicators● MostOHColorIndicatorAcidAlkalipH range overwhich colorchange occursbromocresol greenyellowblue3.8–5.4bromothymol blueyellowblue6.0–7.6methyl orangeredyellow3.2–4.4methyl redyellowred4.8–6.0phenolphthaleincolorlesspink8.2–10.0phenol redyellowred6.8–8.41453Titration of strong acidsCHANGES IN MATTERKey words1 Titration of strong acid against strong alkaliacidalkalibaseend pointpH(pH changes during the titration of 50 cm 3 of 0.1M HClwith 0.1M NaOH)121 Titration of strong acidagainst strong alkali10● Atthe en d poin t of strong acid–strongbase alkali titration , the pH changesby 5 or 6 pH units when only 1 drop ofacid or alkali is added.● Methyl orange, bromothymol blue,and phenolphthalein are all suitableindicators for this titration becausethey all change color within a verysmall change in volume of sodiumhydroxide solution.phenolphthalein8pH numbertitrationbromothymol blue64methyl orange2 Titration of strong acidagainst weak alkali200102030405060708090100Volume of 0.1 M NaOH added (cm3)2 Titration of strong acid against weak alkali(pH changes during the titration of 50 cm 3 of 0.1 M HClwith 0.1 M NH3)1210phenolphthaleinpH number8bromothymol blue64methyl orange200102030405060Volume of 0.1 M NH3 added70(cm3)8090100the end point of a strong acid–weakalkali titration, the pH change for theaddition of one drop of acid or alkali issignificant.
However, the pH atequivalence (when there areequivalent amounts of acid and alkali)is less than 7. A suitable indicatorshould change color below or aroundpH 7. Thus both methyl orange andbromothymol blue would be suitableindicators because they change colorbetween pH 3.2 and 7.6. Within thisrange, the pH of the titration mixturechanges significantly for a very smallchange in volume of sodiumhydroxide solution.● Phenolphthalein would not be a goodchoice of indicator because it changescolor between pH 8.2 and 10.0.
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