Chemistry - an illustrated guide to science (794128), страница 10
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Inorder to change the pH of the titrationmixture over this pH range, asignificant volume of sodiumhydroxide solution must be added.The result would be an overestimateof the volume of sodium hydroxidesolution needed to neutralize the acid.© Diagram Visual Information Ltd.● At54CHANGES IN MATTERKey wordsacidalkaliend pointequivalence pointpHtitrationTitration of weak acids1 Titration of weak acid against strong alkali141 Titration of weak acidagainst strong alkali● Atthe en d poin t of a weakacid–strong alkali titration , the pHchange for the addition of one drop ofacid or alkali is significant.
However,the pH at equivalence (when there areequivalent amounts of acid and alkali)is greater than 7. A suitable indicatorshould change color above pH 7. Thusphenolphthalein would be a goodchoice because it changes colorbetween pH 8.2 and 10.0. Within thisrange, the pH of the titration mixturechanges significantly for a very smallchange in volume of sodiumhydroxide solution.● Conversely, bromothymol blue andmethyl orange would not be goodchoices because they change colorbetween pH 3.2 and 7.6, which isbefore the equ ivalen ce poin t of thetitration is reached.
The result wouldbe an underestimate of the volume ofsodium hydroxide solution needed toneutralize the acid.phenolphthaleinpH 7bromothymol bluemethyl orange0Volume of base2 Titration of weak acid against weak alkali142 Titration of weak acidagainst weak alkali© Diagram Visual Information Ltd.● ThepH changes too slowly around theequivalence point to give a colorchange with the addition of one dropof acid or alkali. The use of methylorange, bromothymol blue, orphenolphthalein would not giveaccurate results.● It is not usual to titrate weak acidswith weak alkali, but if it must bedone, a pH meter is necessary to findthe equivalence point accurately.There is no suitable indicator for thistype of titration.phenolphthaleinpH 7bromothymol bluemethyl orange0Volume of base55pH and soilCHANGES IN MATTER1 Soil classificationKey wordspHpHDescription< 5.5strongly acid5.5 – 5.9medium acid6.0 – 6.4slightly acid6.4 – 6.9very slightly acid1 Soil classificationneutral● Soil7.07.1 – 7.5very slightly alkaline7.6 – 8.0slightly alkaline8.1 – 8.5medium alkaline> 8.5strongly alkalinecan be classified according to itspH.● Soils naturally tend to become moreacidic due to organic acids beingreleased into the soil as a result of thedecay of organic material.● The acidity of soil can be reduced byspreading slaked lime (calciumhydroxide) or lime (calciumcarbonate).2 pH range of common fruit and vegetablesFruit or vegetableSoil pH rangecabbage6.0 – 7.5cauliflower6.5 – 7.52 pH range of commonfruits and vegetablescelery6.5 – 7.5● Mostcucumber5.5 – 7.0potato5.0 – 6.0peas6.0 – 7.5strawberry5.0 – 6.0tomato5.5 – 7.0plants grow best in soil that isslightly acidic, with a pH valuebetween 6.3 and 7.2.
Plants will growoutside this range but not as well. Thishas serious implications for foodcrops.● The soil pH is an importantconsideration in preparing soil to growcrops.3 Elements needed by plants3 Elements needed byplants10.0need a number of major andminor elements in order to grow well,and they obtain these from the soil.The minerals dissolve in soil water andare absorbed into the plant throughthe roots.● The pH of the soil determines howeasily minerals containing theseelements can be absorbed.
At soil pHvalues between 6.0 and 7.0, all majorelements and minor elements can beabsorbed, although some are absorbedmore easily than others. In very acidicor very alkaline soils, relatively fewplants prosper because they cannotabsorb all of the minerals needed forhealthy growth.9.0Difficult toabsorb8.07.06.0Minor elementsd enumcl ybZi nMoerCoppr onBoIr onManganeseurmMagn esi uMinor elementsSulfmCal ci uusPot assi umh orospPh4.0r ogen5.0Ni tpH© Diagram Visual Information Ltd.● PlantsEasy toabsorb56CHANGES IN MATTERThe water cycleKey wordsatmosphereconvectioncurrent1 EvaporationevaporationevaporationevaporationThe water cycle● Earth’swater is always moving in acycle called the hydrologic or watercycle.● The Sun provides the energy drivingthe cycle.1 Evaporation● Heatenergy causes water to evaporatefrom the surface of the oceans, leavingall dissolved substances behind. Therate of evaporation is greater in areasof Earth where the seas are warmer.● Water vapor rises into the atm osphere,where it eventually condenses to formclouds.
These are dispersed by winds,which carry them to the colder regionsof Earth.2 Transportationtransportationcondensationprecipitation2 Transportation● Whenclouds reach landmasses, theyare carried up on con vection cu rren ts.As they rise, the temperaturedecreases, and eventually the watervapor condenses, formingprecipitation, which falls to Earth.● Rainwater contains dissolved gases,which makes it slightly acidic.3 Deposition© Diagram Visual Information Ltd.● Thefresh water flows over rocks andthrough soils before gathering instreams and rivers. As the water flowsthrough the ground, solids dissolve init.● Water is removed from rivers for bothindustrial and domestic use. Much ofthis water is ultimately returned to therivers. Finally, the water flows out tosea, thus completing the cycle.
Anydissolved solids are carried in it andeventually deposited in the oceans.3 Depositionsurface runoffgroundwater flow57Treatment of water andsewageCHANGES IN MATTERKey wordsmethaneoxidizing agentsewage1 water treatmentabh1 Water treatment● Particlesare removed from water bypassing it through a series of sandfilter beds and sedimentation tanks.The filter beds also contain bacteria,which break down and destroy microorganisms in the water.● Chlorine is a powerful oxidizin g agen tthat is used to kill any remainingmicroorganisms in the water before isstored ready for distribution.
Storagetanks are covered to prevent the entryof foreign bodies.2 Sewage treatmentdgfb● Rawi2 Sewage treatmentjacblcnokabcdefghscreenpumpsedimentation tankwater incoarse sand filterfine sand filterchlorine addedcovered storage tankmi to homes and factoriesj sewage ink settling tankl digester aeration tankm sludge collectedn clean water to rivero methane outp digested sludge outsewage cannot be released intorivers because of the threat to healthand the effects on the environment.The waste materials it contains mustfirst be broken down by the action ofdecomposing bacteria.● Solids are removed from the sewageby a series of screens and settlingtanks. The remaining liquid passes intoa digester, where bacteria break downthe waste products.
Streams of air areblown into the tank in order toprovide the bacteria with the oxygenneeded to survive and to keep themixture circulating.● After settling, the clean water isallowed to pass into the river, whilethe sludge undergoes furtherdigestion during which m ethan e isreleased. The digested sludge containsnitrogenous compounds and is oftenused as a fertilizer.p© Diagram Visual Information Ltd.ce58CHANGES IN MATTERKey wordsbondcovalentcompoundhydrogenlone pairoxygenThe water molecule1 A covalent compoundH1 A covalent compoundis essentially a covalen tcom pou n d formed by two atoms ofhydrogen and one atom of oxygen .O2 The water moleculeO● Thesix outer-shell electronsAtoms presentFull shells of a water moleculeOOOH● Oxygen© Diagram Visual Information Ltd.H2 The water molecule3 The polar nature of themoleculeis more electronegative thanhydrogen and, therefore, has astronger attraction for the electrons inthe oxygen–hydrogen bon d.
The resultis that the electrons in the bond residecloser to the oxygen atom. Sinceelectrons are negatively charged, thisleaves the oxygen atom slightlynegative and the hydrogen atomslightly positive. This is shown using !notation; oxygen is !- and hydrogen is!+ .HH● Wateroxygen atom in a water moleculehas two pairs of bonding electrons(sometimes called shared pairs) andtwo pairs of non-bonding electrons(sometimes called lon e pairs).● These four pairs of electrons aredirected toward the corners of atetrahedron. However, the tetrahedralshape is distorted.
The non-bondingpairs of electrons repel each othermore strongly than the bonding pairsof electrons. Repulsion between theseand the bonding pairs of electronsreduces the angle between theoxygen–hydrogen bonds to 104.5 °.one outer-shellelectronone outer-shellelectron104.5°HHWater moleculeBonding pairsNon-bonding pairs(lone pairs)3 The polar nature of the moleculeδ–shared pairs nearer the oxygenHδ+OHδ+H59Water as a solvent ofionic saltsCHANGES IN MATTERKey wordsanionbody-centeredcubiccationface-centeredcubic1 Ionic latticeshydrationionic crystallattice1 Ionic lattices● The–+Cl ionCs ion+Cl ion–Na ion2 The effect of water on an ionic latticeHaHcOHOHHOHHHHOOHHHHOOHHHHHOOHHObHIonic lattice put into waterThe ions attract the water moleculesHHOHHOions in an ion ic crystal arearranged in a lattice.
Each ion issurrounded by a number of oppositelycharged ions. The lattice structure isdetermined by:- the ratio of the number of positivelycharged ions (cation s) to negativelycharged ions (an ion s)- the ratio of the radii of the ions(rcation / r anion )● The radius ratio in sodium chloride is0.57. The ions are arranged in a facecen tered cu bic structure in whicheach sodium ion is surrounded by sixchloride ions, and each chloride ion issurrounded by six sodium ions.● The radius ratio of cesium chloride is0.94 (due to the larger cesium ion).The ions are arranged in a bodycen tered cu bic structure in whicheach cesium ion is surrounded byeight chloride ions, and each chlorideion is surrounded by eight cesiumions.HHOOHOH2 The effect of water on anionic lattice● Whenan ionic compound is placed inwater, the water molecules collide withthe lattice. If the water moleculescollide with sufficient energy toovercome the forces of attractionbetween the oppositely charged ions,hydration occurs, and the compoundwill dissolve, forming a solution.HHHHOThe lattice starts to split upa latticeb water moleculesc charged ends of molecules attractedto ions of opposite charge© Diagram Visual Information Ltd.HO60CHANGES IN MATTERKey wordsanionbondcationionionic compoundlatticetransition metalsIonic solutions1 Stabilizing free ionsHHOHHO● Ionicsolutions are ion ic com pou n dsdissolved in water.HOHO-OHH+HHHHHHOIonic solutionsOHHOpositively chargedhydrogen atomnegatively chargedoxygen atom1 Stabilizing free ions● Dueto the uneven sharing ofelectrons in the oxygen–hydrogenbon ds of a water molecule, the oxygenatom is slightly negatively charged andthe hydrogen atoms are slightlypositively charged.● Water molecules surround andstabilize the ion s in the compound:positively charged cation s arestabilized by the negative oxygenatoms, and negatively charged an ion sare stabilized by the positive hydrogenatoms.2 Transition metals in solutionHHOOOHHHHMOOO2 Transition metals insolutionm etal ions form complexeswith water.● Transition metals have an incompleteouter shell and can fill this with theelectric charge on the water molecule.Non-bonding pairs of electrons fromthe water molecules are donated toform coordinate bonds.● The bonds between the metal ions andwater are so strong that they remainwhen solids are obtained from theirsolutions.HHHHHH● Tran sition3 Production of silverchlorideHAg+ (aq) + Cl - (aq) ➞ AgCl(s)● Thesilver ions and chloride ions aremore stable when bonded together inan ionic lattice than existing apartsurrounded by water molecules.OHHH HHOHNO–3HHH HH HOOOOHHHHOH+ OO AgHOHOHHHHO● When© Diagram Visual Information Ltd.silver nitrate solution andsodium chloride solution are mixed,insoluble silver chloride forms a whiteprecipitate.3 Production of silver chloride when a metal chloride isadded to silver nitrate solutionHHHHOO HH+OONaOHHOHOH HHOHHHHOHHHHH HOSilver nitrate solution and sodium chloridesolutionAg+ Cl – Ag+Cl – Ag+ Cl –HHH OCl –Cl – Ag+ Cl –OHHH HOOHOHHO O+NaOHHOHHHHO HHOHHNO–3HHOHHOAfter reaction, clusters of watermolecules have been forced away fromaqueous silver and chloride ions toleave solid silver chloride61SolubilityCHANGES IN MATTER1 Table of solubility of ionic compoundsSolubleKey wordsionic compoundinsolublesolubleInsolubleall salts of ammonium, potassium, andsodiumall nitrates1 Solubility of ioniccompoundslead and silver bromides, chlorides, andiodidesmost sulphates (calcium sulfate isslightly soluble)barium and lead sulfateammonium, potassium, and sodiumcarbonatemost other carbonatesammonium, potassium, and sodiumhydroxides (calcium hydroxide is slightlysoluble)most other hydroxides2 Table of the most abundant compounds in seawaterName of compound● Allion ic com pou n ds are solu ble inwater to some extent.
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