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CHEMICAL REACTIONS

Whatisachemicalreaction?

Achemicalreactionisthechangeofasubstanceintoanewonethathasa
differentchemicalidentity.

HowcanItellifachemicalreactionisoccurring?

Achemicalreactionisusuallyaccompaniedbyeasilyobservedphysicaleffects,
suchastheemissionofheatandlight,theformationofaprecipitate,theevolution
ofgas,oracolorchange.Absoluteconfirmationofachemicalchangecanonlybe
validatedbychemicalanalysisoftheproducts!

Takealookatthefollowingimageanddescribewhatyouseegoingon?Whatare
thekeyindicatorsofachemicalchange?Thekeyisobservation!

ClickonSherlockHolmesasawarmtotestyourpowersofobservation!


Oh..dontforgettocheckoutthebarkingdog!

TypesofChemicalReactions

Therearemanydifferenttypesofchemicalreactions.Chemistshave
classifiedthemanydifferentreactionsintogeneralcategories.Thechemical
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reactionswewillexplorearearepresentationofthetypesofreactionsfoundin
eachgroup.Thereisageneraldescriptionofthemainreactiontypesandspecific
examplesprovidedintheselectionboxes.

SynthesisReaction(CombinationReaction)

Inasynthesisreaction,twoormoresubstancescombinetoformanew
compound.Thistypeof
reactionisrepresentedbythefollowingequation.

A+BAB

AandBrepresentthereactingelementsorcompoundswhileABrepresentsa
compoundastheproduct.
Thefollowingexamplesarerepresentativeofsynthesisreactions.

Aluminumand FormationofAluminumBromide:WhenAlisplacedonthe
Bromine surfaceofliquidBr2anexothermicreactionoccurs.TheAlis
oxidizedtoAl3+bytheBr2,whichisreducedtoBrions.Theionic
product,AlBr3,canbeobservedonthewatchglassafterthe
reaction.

Sodiumand FormationofSodiumChloride:Moltensodiumburnswhenitisput
Chlorine intoacontainerofchlorinegas.Inthereactionasodiumionlosesan
electrontoformasodiumcationandachlorineatomsimultaneously
gainsanelectrontoformachlorideanion.Theproductofthereaction
istheioniccompoundsodiumchloride,whichisthewhitesolid
observed.

Zincand FormationofZincOxide:Oxidationisalossofelectronsand
Oxygen reductionisagainofelectrons.TheoxidationofmetallicZnbyO2to
formZnO(s)isillustratedatthemolecularlevel.Thetransferof
electronsfromZntoO2isshown.Atomscanbeobservedtochange
astheyareoxidizedorreduced,respectivelytotheirionicforms.

Sodiumand FormationofSodiumHydroxideandPotassiumHydroxide:
Potassiumin WhenasmallpieceofNaisaddedtoasolutioncontainingan
Water indicator,evidenceofthereactioncanbeobservedbythechangein
thecolorofthesolutionasNaOHisformed,bythemeltingoftheNa
andbythemovementoftheNacausedbyformationofhydrogen
gas.KismorereactivethanNaasdemonstratedbyitsreactionwith

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water.ThisreactionproducesenoughheattoignitetheH2
produced.


SingleReplacementReaction

Inasinglereplacementreaction(displacementreaction)oneelementreplacesa
similarelement
inthecompound.Singlereplacementreactionscanberepresentedbythefollowing
equations.

AB+CAC+B


Iron(III)Oxide ThermiteReaction:Inthethermitereaction,AlreducesFe2O3to
andAluminum FeinanextremelyexothermicreactioninwhichAlisoxidizedto
Al2O3.Thereactionproducesenoughheattomelttheiron.Because
Reaction2
oftheextremeheatproducedinthethermitereaction,itisused
industriallytoweldiron.

Copper(II) ReductionofCuO:Whenblackcarbonandblackcopperoxideare
Oxideand heatedtogethertheCu2+ionsarereducedtometallicCuandagas
Carbon isevolved.WhenthegasiscollectedinCa(OH) awhiteprecipitate
2
ofCaCO3isformed.Thereactionwhichoccursinvolvesthe
reductionofCu2+ionsbycarbonwhichisoxidizedtoCO2.

SilverNitrate FormationofSilverCrystals:Whenacopperwireisplacedina
andCopper solutionofAgNO3,theCureducesAg+tometallicAg.Atthesame
time,CuisoxidizedtoCu2+.AsthereactionprogressesAgcrystals
canbeseentoformontheCuwireandthesolutionbecomesblueas
aresultoftheformationofCu2+ions.

Tin(II) FormationofTinCrystals:OxidationreductionchemistryofSnand
Chlorideand Zn.WhenacidifiedSn(II)Cl2isaddedtoabeakercontainingapiece
Zinc
ofZn,someoftheSn2+reactswithH+inthesolutiontoproduceH2
gas.Immediatechangescanalsobeobservedonthesurfaceofthe
ZnasitquicklybecomescoatedwithSncrystals.Afterthereaction
hasprogressedforatimeneedlesofSncanbeobservedonthe
surfaceoftheZn.

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DoubleReplacementReaction

Inadoublereplacementreaction,theionsoftwocompoundsexchangeplacesin
anaqueoussolution
toformtwonewcompounds.Adoublereplacementreactioncanberepresentedby
thefollowingequation.

AB+CDAC+BD


Calcium Thismarblestatuehasbeenerodedbyacidrain.Marbleisa
carbonateand materialhavingCaCO3asitsprimarycomponent.Acidsreactwith
SulfurousAcid anddissolvethemarble.Theacidcomesfromsulfurdioxideinthe
atmospherecombiningwithwatertoformsulfurousacid.

Lead(II)Nitrate AnaqueoussolutionofPotassiumIodideisaddedtoanaqueous
andPotassium solutionofLead(II)Nitrateforminglead(II)iodide.Theformationof
Iodide aprecipitateoccurswhenthecationsofonereactantcombineswith
theanionsoftheotherreactanttoformaninsolubleorslightly
insolublecompound.

Sodium AnaqueoussolutionofSodiumChlorideisaddedtoanaqueous
chlorideand solutionofSilverNitrateformingsilverchloride.
Silvernitrate




DecompositionReaction

Inadecompositionreaction,singlecompoundundergoesareactionthatproduces
twoormoresimpler
substances.Adecompositionreactioncanberepresentedbythefollowingequation.

ABA+B


Waterinto ElectrolysisofWater:Whenadirectcurrentispassedthrough
Hydrogenand wateritdecomposestoformoxygenandhydrogen.Thevolumeof
Oxygen hydrogengasproducedatthenegativeelectrodeistwicethevolume
oftheoxygengasformedatthepositiveelectrode.Thisindicates
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thatwatercontainstwiceasmanyhydrogenatomsasoxygenatoms,
whichisanillustrationofthelawofconstantcomposition.

Nitrogen DecompositionofNitrogenTriiodide: Nitrogentriiodideis
Triiodide extremelyunstablewhenitisdry.Touchingitwithafeathercausesit
todecomposeexplosively.Theexplosionoccursaschemicalenergy
isreleasedbythedecompositionofnitrogentriiodidetoN2andI2.
Violetiodinevaporcanbeobservedaftertheexplosion.


CombustionReaction

Inacombustionreaction,asubstancecombineswithoxygen,releasingalarge
amountofenergyintheform
oflightandheat.Fororganiccompounds,suchashydrocarbons,theproductsofthe
combustionreactionarecarbondioxideandwater.

CH4+2O2CO2+2H2O


Hydrogenand Thecombustionofhydrogenyieldswatervaporasareaction
Oxygen product.Threeballoonsofhydrogenandoneballoonmixedwith
hydrogenandoxygenformanexplosivemixture
ReactionII

Various ReactionswithOxygen.Magnesium,steelwool,whitephosphorous,
Substances andsulfurareburnedinoxygen.Theresultingreactionsare
withOxygen combinationreactionsinwhichtwosubstancesreacttoformone
product.TheproductsformedinthesereactionsareMgO,Fe2O3,
P4O10andSO2.Allofthesecombustionreactionsarevery
exothermic.


Phosphorous Thecombustionofyellowphosphorusoccursinanoxygen
andOxygen atmosphere.Themainproductofthisreactionisphosphorus
pentoxide.

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