Chemistry - an illustrated guide to science (794128), страница 25
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If onereactant is reduced, then another mustbe oxidized. Such reactions aredescribed as redox reaction s.© Diagram Visual Information Ltd.● Reactions144CHEMICAL REACTIONSKey wordsdisplacementreactionoxidationredox reactionreductionRedox reactions 11 Redox reactions: oxidation and reduction2Mg + O2→2Mg2+ + 2O2–4Na + O2→2(Na2) 2O2–Redox reactionsWhen metals react with oxygen they form oxides● Redu ction2Mg→2Mg2+ + 4e–O2 + 4e–→2O2–and oxidation reactionsalways occur together and arecollectively referred to as redoxreaction s.1 Oxidation and reduction● Whenmagnesium is heated in air, itforms magnesium oxide:2Mg(s) + O2(g) ➞ 2MgO(s)● Magnesiumatoms are oxidized tomagnesium ions by losing twoelectrons.● Oxygen atoms are reduced to oxideions by gaining two electrons.● This is true of all metals when they areconverted to metal oxides.2 Electron transfer● A morereactive metal displaces theions of a less reactive metal from asolution of its salts.
This type ofreaction is called a displacem en treaction :The metal is oxidized and the metal is reduced. The oxygen takes the electrons givenup by the metal2 Electron transfer in redox reactionsZn(s) + Cu2+ (aq)→Zn2+ (aq) + Cu(s)When powered zinc is added to copper sulfate (II) solution, an exothermic reaction occursZn(s)→Zn2+ (aq) + 2e–Cu2+ (aq) + 2e–→Cu(s)→2Br –→Fe3+ + e–2Fe2+→2Fe3+ + 2e–Br 2 + 2e–→2Br –→2Fe3+ + 2Br –Redox equations for the reaction© Diagram Visual Information Ltd.Zn(s) + Cu2+ (aq) ➞ Zn2+ (aq) + Cu(s)Zinc atoms are oxidized to zinc ions bylosing two electrons.● Copper ions are reduced to copperatoms by gaining two electrons.3 Balancing redox reactions3 Balancing redoxreactions(a) The oxidizing agent is Br2● InFe2+balancing redox reactions, theelectrons lost must equal the electronsgained.● In the example at right, bromine (a) isgaining two electrons and iron (b) islosing 1 electron.● In order to balance the equation, theentire reaction has to be multiplied by2 (c).● The result is a balanced equation (d).Br 2 + 2e–(b) The reducing agent is Fe2+(c) Redox reaction2Fe2+ + Br 2(d) Balance equation145Redox reactions 2CHEMICAL REACTIONSKey words1 The reaction of metals with non-metalsFe + Sredox reaction→Fe2+ S2–→Fe2+ + 2e–→S2–Iron and sulfurFeS+2e–1 Metals with non-metals● Ironundergoes redox reaction s withnon-metals.Iron and sulfur:Fe(s) + S(s) ➞ Fe2+ S2- (s)Redox equation2Fe + 3Cl 2→2FeCl 32Fe→2Fe3+3Cl 2 + 6e–→6Cl –Iron atoms are oxidized to iron(II)ions by losing two electrons.Sulfur is reduced to sulfide ions bygaining two electrons.Iron and chlorine:Iron and chlorine+2Fe(s) + 3Cl 2(s) ➞ 2Fe3+ Cl - 3(s)6e–Iron atoms are oxidized to iron(III)ions by losing three electrons.Chlorine is reduced to chloride ions bygaining one electron.Redox equation2 Metals with water● Metalsare oxidized when they reactwith water.2 The reaction of metals with water→Ca2+ (OH–) 2 + H2Ca→Ca2+2H2O + 2e–→2OH– + H2Ca + 2H2OCalcium and water+Metal + water ➞metal hydroxide + hydrogenCa(s) + 2H2O(l) ➞Ca(OH) 2(aq) + H2(g)H2O(l) H+ (aq) + OH- (aq)Ca(s) + 2H+ (aq) ➞ Ca2+ (aq) + H2(g)In the example at left, calcium atomsare oxidized to calcium ions by theloss of two electrons.
Hydrogen ionsare reduced to hydrogen atoms bygaining one electron.2e–3 Metals with acidsRedox equation● Metalsare oxidized when they reactwith acids.→Zn2+ (aq) + H2Zn→Zn2+ + 2e–2H+ + 2e–→H2The reaction of zincRedox equationMetal + acid ➞ metal salt + hydrogenZn(s) + 2HCl(aq) ➞ ZnCl 2(aq) + H2(g)Zn(s) + 2H+ (aq) ➞ Zn2+ (aq) + H2(g)In the example at left, zinc atoms areoxidized to zinc ions by the loss of twoelectrons. Hydrogen ions are reducedto hydrogen atoms by gaining oneelectron.© Diagram Visual Information Ltd.Zn(s) + 2H+ (aq)→3 The reaction of metals with acids146CHEMICAL REACTIONSKey wordsreactivity seriesredox reactionDemonstrating redoxreactions1 Electron transfer in redox reactionsgExperimental set-up1 Electron transfer in redoxreactionsmovement of electrons duringredox reaction s can be demonstratedusing a simple cell consisting of twometals rods, suspended in solutions oftheir salts, connected by a wire and asalt bridge.● At the zinc rod, zinc atoms lose twoelectrons to become zinc ions.
Theelectrons pass along the wire andthrough the bulb to the copper rod.The zinc ions pass into solution.● At the copper rod, copper ions gaintwo electrons to become copperatoms. The copper ions come out ofsolution and are deposited as coppermetal on the copper rod.● Electric current passes through thewire in the external circuit as a flow ofnegatively charged electrons.● Ions flow through the salt bridge:positive ions from the zinc sulfatesolution to the copper sulfate, andnegative ions in the opposite directionfrom the copper sulfate solution to thezinc sulfate solution.● Electric current passes through ionicsolutions as a flow of positivelycharged and negatively charged ions.cdae● The2 Reaction equations© Diagram Visual Information Ltd.● Zincis higher than copper in thereactivity series.
Zinc atoms areoxidized to zinc ions, while copperions are reduced to copper atoms.● When any two metals are placed in acell, the direction of electrons in theexternal circuit depends on theirreactivities. The metal that is higher inthe reactivity series will be oxidized,while the ions of the metal that islower in the reactivity series will bereduced.fbMovement of chargearound the circuithi2e–2e–K+Zn2+SO42–NO–3abcdzinc rodzinc sulfate solutionelectron flowfilter paper soaked in potassiumnitrate as a salt bridgee copper rodCU2+f copper sulfate solutiong small light bulbh movement of negative charge (electronsand anions)i movement of positive charge (cations)2 Reaction equationsZn(s) + Cu 2+ → Zn 2+ (aq) + Cu(s)Zn(s) → Zn 2+ (aq) + 2e–Cu 2+ (aq) + 2e– → Cu(s)Copper ions are reduced to a deposit of red-brown147Assigning oxidation state1 Oxidation stateCHEMICAL REACTIONSKey wordsComponentOxidation stateoxidation statetransition metals0uncombined elementsGroup 2 metals in compounds+2Group 1 metals in compounds+1combined hydrogen except in metal hydrides+11 Oxidation statecombined hydrogen in metal hydrides–1● Thecombined halogens–1combined oxygen–222 Tetrachlorocuprate ion2–4CuClthe tetrachlorocuprate ion contains the transition metal copperability of tran sition m etals toexhibit different oxidation states indifferent compounds is central to thebehavior of these elements.● The oxidation state of simple ions isgiven by the charge they carry, e.g.:Na+ has an oxidation state of + 1O2- has an oxidation state of -2● The situation is more complicated in acomplex ion.
The oxidation state ofthe central atom in a complex ion isthe charge that the ion would have if itwere a simple ion. This is found byadding the oxidation states of thevarious components in the complexion.2 Tetrachlorocuprate ion● Total33 Manganate ion–4MnOthe manganate ion contains the transition metal manganeseoxidation number due tochlorine = 4 x -1 = -4.● Overall charge on the ion = -2.● Oxidation state of the central copperatom = -2 – (-4) = + 2.● This complex ion is more correctlycalled the tetrachlorocuprate(II) ion.3 Manganate ion● Totaloxidation number due to oxygen= 4 x -2 = -8.● Overall charge on the ion = -1.● Oxidation state of the centralmanganese atom = -1 – (-8) = + 7.● This complex ion is more correctlycalled the manganate(VII) ion.4 Dichromate ion2–7Cr 2Othe dichromate ion contains the transition metal chromium● Totaloxidation number due to oxygen= 7 x -2 = -14.● Overall charge on the ion = -2.● Total oxidation state of the two centralchromium atoms = -2 – (-14) = + 12.● Oxidation state of each chromiumatom = + 12 / 2 = + 6.● This complex ion is more correctlycalled the dichromate(VI) ion.© Diagram Visual Information Ltd.44 Dichromate ion148CHEMISTRY OF CARBONKey wordsallotropecarbondiamondfullerenesgraphiteThe allotropes of carbon:diamond and graphiteDiamondBond angle 10 9.5°Bond length 0.154 nmCarbon allotropes●Carbon exists in three allotropes:diam on d, graphite, and fu lleren es.1 DiamondIn diamond, each carbon atom iscovalently bonded to four othercarbon atoms.● The four bonds are directed towardthe corners of a pyramid ortetrahedron, and all bonds are thesame length, 0.154 nm.
The anglebetween any two bonds is 109.5°.● All four of the outer electrons on thecarbon atom form bonds with othercarbon atoms so there are no mobileelectrons. Diamond does not,therefore, conduct electricity.● Diamond has a rigid structure and isvery hard.●2 GraphiteThe carbon atoms in graphite arearranged in layers consisting ofinterlocking hexagons in which eachcarbon atom is covalently bonded tothree other carbon atoms. The lengthof the bond is 0.141 nm, and the anglebetween bonds is 120°.● The fourth outer electron on eachcarbon atom forms bonds withadjacent layers.
The bond length ismuch greater than between carbonatoms within a layer.● The electrons between the layers aremobile; therefore, graphite conductselectricity. Also, the layers are able toslide over each other relatively easily.● Graphite is soft.© Diagram Visual Information Ltd.●GraphiteBond angle 120 °Bond lengths– in layers 0.141 nm– between layers0.335 nm149The allotropes of carbon:fullerenesCHEMISTRY OF CARBONKey wordsallotropebuckyballcarbonBuckyballfullerenesgraphitenanotubeFullerenesFu lleren es are allotropes of carbon inthe form of a hollow sphere or tube.Spherical fullerenes are sometimescalled bu ckyballs, and cylindricalfullerenes are called n an otu bes.● Because the allotrope was onlydiscovered in the late twentiethcentury, its physical and chemicalproperties are still being studied.● Fullerenes are not very reactive andare only slightly soluble in manysolvents.
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