Chemistry - an illustrated guide to science (794128), страница 21
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All othermetal carbonates decompose onheating, forming the metal oxide andcarbon dioxide gas:gReduction of oxidesa hydrogenb combustion tube clamped toslope downwardc metallic oxided porcelain vessele moisture collects heref hydrogen ignitedg heathLi 2CO3(s) ➞ Li 2O(s) + CO2(g)MgCO3(s) ➞ MgO(s) + CO2(g)CuCO3(s) ➞ CuO(s) + CO2(g)dioxide gas is more dense thanair and can be poured from one testtube into another.
Carbon dioxideturns lim ewater milky.ln● WhenZn(s) ➞ Zn2+ (aq) + 2e-Effect of heat on carbonateh metallic carbonatei carbon dioxidej limewater2 Generation of electric current by mechanical reaction2 Generating electriccurrentrods of zinc and copper areplaced in dilute su lfu ric acid, a simpleelectrical cell is formed, and there is apotential voltage difference betweenthe two metals. If the two metals areconnected externally, electric currentflows.● The zinc rod becomes the positiveelectrode (an ode) of the cell.
Zincatoms are oxidized to form zinc ions:jig● CarbonemokMethod 1: simple cellk zinc rodl electric bulbm electron transfern connecting wireo copper rodp beakerq dilute sulfuric acidpq● Thecopper rod becomes the negativeelectrode (cathode) of the cell.Hydrogen ions are reduced tohydrogen gas:sr2H+ (aq) + 2e- ➞ H2(g)● If© Diagram Visual Information Ltd.the copper rod is surrounded by aporous vessel containing copper(II)sulfate solution, a different reactionoccurs at the cathode:tCu2+ (aq) + 2e- ➞ Cu(s)● Zincatoms are oxidized to zinc ions,while copper ions are reduced tocopper atoms.uMethod 2r copper rods zinc rodt porous vesselu dilute sulfuric acidv copper sulfate solutionv121ElectrolysisCHEMICAL REACTIONS1 Electrolysis: schematicKey words2 Electrolysis of saltsolutionsaanionanodecathodecationelectrodebelectrolysiselectrolyte1 Electrolysis● Electrolysisichhggedfis the process by which anelectrolyte (a substance that conductselectricity) is decomposed when adirect current is passed through itbetween electrodes.
Positive cation smove to the cathode to gain electrons;negative an ion s move to the an ode tolose electrons.● Substances are either deposited orliberated at the electrodes dependingon the nature of the electrodes andelectrolyte.2 Salt solutions● Two3 Electrolysis of waterelectrolytes undergo electrolysisat the same time when they areconnected in a circuit by a salt bridge.● The platinum electrode in the lefthand beaker is the anode and attractsnegative ions, which are oxidized.● The platinum electrode in the righthand beaker is the cathode andattracts positive ions, which arereduced.4 U tube+–ml3 Water● Thenqonelectrolysis of water yieldshydrogen at the cathode and oxygenat the anode.
Hydrogen and oxygenare formed in the ratio of 2:1.4 U tube● Thek–+pabcdefghibatteryelectric bulbliquid under testpoly(ethene) supportcopper platesglass vesselplatinum electrodeselectrolyte solution in beakerssalt bridgej platinum cathodek platinum anodel hydrogenm oxygenn water acidified with dilute sulfuric acido dilute sulfuric acidp agar jelly colored pink by phenolphthaleinand alkaliq sodium sulfate solutionions present in dilute sulfuric acidare H+ , OH- , and SO4 2- . Hydroxide ionsare discharged at the anode, leaving asurplus of hydrogen ions, so theelectrolyte in the left side of the Utube becomes increasingly acidic.● The ions present in sodium sulfatesolution are H+ , Na+ , OH- , and SO4 2- .Hydrogen ions are discharged at thecathode, leaving a surplus ofhydroxide ions, so the electrolyte inthe right side of the U tube becomesincreasingly alkaline.© Diagram Visual Information Ltd.j122CHEMICAL REACTIONSKey wordsanodecathodeelectrodeelectrolysisinertElectrode activity andconcentrationElectrolysis: electrodeactivity and concentration1 Dilute solutionsodium chloride+2 Concentrated solutionsodium chloride+–results of electrolysis differdepending on the concentration of thesolution and type of electrodes used.● In ert electrodes take no part in thereaction; active electrodes take part inthe reaction.–● Thecarbonelectrodescarbonelectrodes1 Dilute solution● Reactionat the an ode: oxygenproduced● Reaction at the cathode: hydrogenproduced2 Concentrated solutionat the anode: chlorineproduced● Reaction at the cathode: hydrogenproducedconcentrated sodiumchloride solutiondilute sodiumchloride solution3 Inert electrodes● Reaction+–3 Inert electrodes+–carbonelectrodes● Thefollowing reactions occur at theelectrodes when copper(II) sulfateundergoes electrolysis using carbon(inert) electrodes.● Reaction at the anode: oxygen isproduced● Reaction at the cathode: copper metalis deposited on the cathode.carbonelectrodescopperdepositedcopper (II) sulfatesolutioncopper (II) sulfatesolution4 Active electrodes© Diagram Visual Information Ltd.● Thefollowing reactions occur at theelectrodes when copper(II) sulfateundergoes electrolysis using copper(active) electrodes.● Reaction at the anode: copper goesinto solution as copper ions, and theanode grows smaller.● Reaction at the cathode: copper metalis deposited, and the cathode growsbigger.4 Active electrodes+–+copperelectrodescopper (II) sulfatesolution–copperelectrodescopper (II) sulfatesolution123Acids: reactionsCHEMICAL REACTIONSKey words1 Main reactions of an acidsalt +CO2+ H2 Ocarbonateacidacidbasecarbonatecatalysthydroxideoxidationoxidesalt1 Main reactions of an acidmetal● Dilutebasesalt+ H2 Osalt+ H22 Examples of reaction typeAcid with carbonateNa2CO3(s) + 2HNO3(aq) ➞2NaNO3(aq) + CO2(g) + H2O(l)Acid with baseHCl(aq) + NaOH(s) ➞ NaCl(s) + H2O(l)Acid with metalZn(s) + 2HCl(aq) ➞ ZnCl 2(aq) + H2(g)Acid neutralizedby oxideCuO(s) + H2SO4 (aq) ➞ CuSO4 (aq) + H2O(l)acids react with all metalcarbon ates to give a metal salt, carbondioxide, and water.● Dilute acids react with bases to givesalts plus water.● Dilute acids react with most metals togive a metal salt and hydrogen.● Dilute acids are neutralized by metaloxides and metal hydroxides to form ametal salt and water.2 Example of reaction type● Sodiumcarbonate reacts with dilutenitric acid to give sodium nitrate,carbon dioxide, and water.● Hydrochloric acid reacts with sodiumhydroxide to form a salt and water.● Zinc reacts with dilute hydrochloricacid to give zinc chloride andhydrogen.● Copper(II) oxide reacts with dilutesulfuric acid to give copper(II) sulfateand water.3 Sulfur trioxide3 Laboratory preparation of sulfur trioxidecdabgfabcdefgoxygendry SO2plantinized asbestos as a catalystcombustion tubecrushed ice and saltwhite smoke of SO3heatetrioxide is a white crystallinesolid obtained by oxidation of sulfurdioxide.
It dissolves in water with ahissing noise and the production ofheat, forming sulfuric acid. Sulfurtrioxide is employed as a dehydratingagent.● Sulfur trioxide is made in thelaboratory by passing a mixture of drysulfur dioxide and dry oxygen over aheated platinum catalyst. Sulfurtrioxide melts at 17°C and condensesas a solid in a suitably cooled beaker.● Industrially it is made using thecontact process (see pages 75 & 76).© Diagram Visual Information Ltd.● Sulfur124CHEMICAL REACTIONSKey wordshydrochloric acidhydrogenchloridenitric acidsolublesulfuric acidPreparation of acids1 Preparation of hydrogenchloride (gas)2 Preparation ofhydrochloric acidecb1 Preparing HCl gas● Hydrogenchloride gas is made by thereaction of sodium chloride andconcentrated sulfuric acid:f2NaCl(s) + H2SO4 (aq) ➞Na2SO4 (aq) + 2HCl(g)The gas is more dense than air and iscollected by downward delivery.ag2 Preparing HCl acidchloride is extremelysolu ble in water, forming hydrochloricacid.
It cannot be dissolved simply byplacing a delivery tube carrying the gasdirectly into water because the waterwould be sucked back into thereaction vessel.● The gas is dissolved in water bypassing it into an inverted funnelpositioned so the lip is just under thesurface of the water. The funnelprevents suck back.d● Hydrogenabcdrock saltconcentrated sulfuric acidHCl gas collectedheate HCl filterf filter funnelg water (to become diluteHCl acid)3 Laboratory preparation of nitric acidj3 Preparing nitric acid● Nitricacid can be made by thereaction of solid sodium or potassiumnitrate with concentrated su lfu ric acid:KNO3(s) + H2SO4 (aq) ➞KHSO4 (aq) + HNO3● Theproduct of this reaction isnormally yellow due to the presenceof nitrogen dioxide, formed by thethermal decomposition of the acid:4HNO3(l) ➞4NO2(g) + 2H2O(g) + O2(g)ihk4 Industrial preparation of nitric acidm4 Industrial preparationof HNO3no● Nitric© Diagram Visual Information Ltd.acid is made industrially by theoxidation of ammonia in a processinvolving three stages (see page 76):production of nitrogen oxide gas,oxidation of nitrogen oxide to nitrogendioxide gas, reaction of nitrogendioxide and water.● This process can be modeled in thelaboratory by passing ammonia vaporover a heated platinum catalyst.h heati solid sodium nitrateplus concentratedsulfuric acidj water jacketk pure nitric acidhplql concentratedammonia dilutedwith water (50 %)m combustion tuben platinized asbestoso pump sucks gasesthrough apparatusp brown gasq litmus goes red125Bases: reactionsCHEMICAL REACTIONSKey words1 General reactions of a base with an acid+acid+metaloxidesalt+acidwater+metalhydroxidesaltmetalcarbonatesalt+acidacidbasecarbonatehydroxideoxideBaseswater+● A baseis a compound that reacts withan acid to form a salt.
Common basesare metal oxides, metal hydroxides,and metal carbon ates.+watersaltuniversalindicatorCO21 General reactions withacids● Metaloxides react with acids to formsalts and water.● Metal hydroxides also react with acidsto form salts and water.● Metal carbonates react with acids toform salts, water, and carbon dioxide.2 Metal oxide and acid2 Metal oxide and acid● Thereaction of magnesium oxide( MgO) with hydrochloric acid ( HCl )can be followed by adding a few dropsof u n iversal in dicator to the acid.● Initially the indicator is red. Whenmagnesium oxide is added to thereaction, the following reaction occurs:MgO(s) + 2HCl(aq) ➞MgCl 2(aq) + H2O(l)● Whenheat is appliedMagnesium oxide is added to hydrochloricacid and indicatorNeutral solution, indicator is greenthere are equivalent amounts ofmagnesium oxide and hydrochloricacid, the indicator turns green,signifying all of the acid has reactedand the mixture is neutral.3 Carbonate and acid3 Carbonate and acid● Thereaction of magnesium carbonate( MgCO3) with hydrochloric acid ( HCl )can be followed by observing thecarbon dioxide gas evolved.● Initially bubbles of gas are evolved asthe following reaction occurs:MgCO3(s) + 2HCl(aq) ➞MgCl 2(aq) + H2O(l) + CO2(g)all of the hydrochloric acid hasreacted, no gas is produced, andexcess insoluble magnesium carbonateremains in the beaker.Cabonate is added to hydrochloricacid and indicatorNeutral solution, indicator is green© Diagram Visual Information Ltd.● Whengasbubble126CHEMICAL REACTIONSKey wordsacidbaseindicatorinsolubleneutralsaltsolubletitrationBases: forming pure salts1 From a soluble base (alkali)Example: sodium chloride from sodium hydroxide and hydrochloric acidabd005510101515202025251 From a soluble base● Titrationis used to make salts fromacids and solu ble bases, e.g., sodiumchloride from hydrochloric acid andsodium hydroxide.● The burette is filled with hydrochloricacid, and a known volume of sodiumhydroxide solution is placed in aconical flask.
A few drops of a suitablein dicator are added to the sodiumhydroxide solution. Hydrochloric acidis run into the flask until the color ofthe indicator changes, showing thatthe reaction mixture is n eu tral. Thevolume of hydrochloric acid in theburette is noted before and afteraddition so the volume of acid neededcan be calculated.● The flask contains a solution ofsodium chloride, which is impure dueto the presence of the indicator. Theprocedure must be repeated usingexactly the same volumes ofhydrochloric acid and sodiumhydroxide solution but no indicator.● Sodium chloride crystals are obtainedby boiling off some of the water fromthe sodium chloride solution andallowing the remaining solution tocool.fSet up theapparatusas shownceMeasure the volumeof acid needed forneutralization (e–d)Repeat the procedure,but without using theindicator, adding theamount of acidmeasured above(i.e., e–d)ghEvaporate offexcess water2 From an insoluble baseExample: copper oxide and sulfuric acidi2 From an insoluble base© Diagram Visual Information Ltd.● Saltsare made from in solu ble bases byadding an excess of the base to anacid.
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