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ChemistryChemistryTutorialTutorial

REDOX REACTIONSREDOX REACTIONS

byby

Dr John G WrightDr John G Wright

ress gDn or le!t "li"# !or the ne$t sli%e

The Wright Stu!!

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O$i%ation an% Re%u"tionO$i%ation an% Re%u"tion

O$i%ation Numbers 'this set o! 1( sli%es)O$i%ation Numbers 'this set o! 1( sli%es) Dis*ro*ortionationDis*ro*ortionation

+alan"ing ,al! E-uations+alan"ing ,al! E-uations

Ele"tro"hemi"al CellsEle"tro"hemi"al Cells

roblemsroblemsThe in%i.i%ual to*i"s "an be .ie/e% as se*arate sli%e sho/s0The in%i.i%ual to*i"s "an be .ie/e% as se*arate sli%e sho/s0

I! a sli%e sho/ is missing !rom the /eb *age it means it isI! a sli%e sho/ is missing !rom the /eb *age it means it is

being u*%ate% an% im*ro.e%0 'Or *erha*s I 2ust ha.en3tbeing u*%ate% an% im*ro.e%0 'Or *erha*s I 2ust ha.en3t

!inishe% /riting it yet4)!inishe% /riting it yet4)

Re%o$ Rea"tions

The tutorials are %i.i%e% into !i.e main se"tions ea"h

"o.ering a se*arate to*i" "on"erning re%o$ rea"tions *lus

a *roblems se"tion0

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O$i%ation an% Re%u"tion It is assume% that you ha.e rea% the !irst tutorial on O$i%ationIt is assume% that you ha.e rea% the !irst tutorial on O$i%ation

an% Re%u"tion an% are !amiliar /ith the meaning o! thesean% Re%u"tion an% are !amiliar /ith the meaning o! theseterms es*e"ially the ele"tron6base% %e!initions0terms es*e"ially the ele"tron6base% %e!initions0

Stu%ents on intro%u"tory le.el "ourses may only nee% to rea%Stu%ents on intro%u"tory le.el "ourses may only nee% to rea%

the *re.ious se"tion on O$i%ation an% Re%u"tion but those onthe *re.ious se"tion on O$i%ation an% Re%u"tion but those ona%.an"e% "ourses su"h as A6le.el an% abo.e /ill nee% toa%.an"e% "ourses su"h as A6le.el an% abo.e /ill nee% to

stu%y both tutorials /ith the em*hasis being *la"e% on thestu%y both tutorials /ith the em*hasis being *la"e% on the

mo%ern ele"tron6base% %e!inition o! o$i%ation an% re%u"tion0mo%ern ele"tron6base% %e!inition o! o$i%ation an% re%u"tion0

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O$i%ation NumbersThis se"tion is about "al"ulating the o$i%ation state orThis se"tion is about "al"ulating the o$i%ation state or

o$i%ation number o! an element in a "om*oun%0 It uses a seto$i%ation number o! an element in a "om*oun%0 It uses a seto! rules /hi"h re-uire little or no *re.ious #no/le%ge o! theo! rules /hi"h re-uire little or no *re.ious #no/le%ge o! the

"hemi"al being e$amine%0 'The o$i%ation number is similar to"hemi"al being e$amine%0 'The o$i%ation number is similar to

the .alen"y o! the element but has a 8 or 6 sign0)the .alen"y o! the element but has a 8 or 6 sign0)

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O$i%ation Numbers

The o$i%ation number o! an element is the "harge /hi"h theThe o$i%ation number o! an element is the "harge /hi"h the

atom /oul% ha.e i! the element /as a"ting as an ion in theatom /oul% ha.e i! the element /as a"ting as an ion in the

s*e"ies being stu%ie%0s*e"ies being stu%ie%0

This "an seem a bit strange at times es*e"ially /hen youThis "an seem a bit strange at times es*e"ially /hen you

#no/ that the "om*oun% you are e$amining is "o.alent0 +ut it#no/ that the "om*oun% you are e$amining is "o.alent0 +ut it

enables us to easily /or# out /hether an element is o$i%ise% orenables us to easily /or# out /hether an element is o$i%ise% or

re%u"e% in a rea"tion0 We 2ust "om*are the o$i%ation numberre%u"e% in a rea"tion0 We 2ust "om*are the o$i%ation number

be!ore an% a!ter the rea"tion an% /or# out /hether it hasbe!ore an% a!ter the rea"tion an% /or# out /hether it has

gaine% or lost ele"trons0gaine% or lost ele"trons0

There are a series o! sim*le rules /e use to "al"ulate theThere are a series o! sim*le rules /e use to "al"ulate the

o$i%ation number o! an element0o$i%ation number o! an element0

:irst the %e!inition0

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O$i%ation Numbers The rules are gi.en in or%er o! im*ortan"e0 I! you rea"h a ruleThe rules are gi.en in or%er o! im*ortan"e0 I! you rea"h a rule

/hi"h gi.es you %ata to /or# !rom an% a later rule a**ears to/hi"h gi.es you %ata to /or# !rom an% a later rule a**ears to"ontra%i"t the earlier result 2ust ignore it you use the earlier"ontra%i"t the earlier result 2ust ignore it you use the earlier

result0 I0e0 later rules %o not o.errule earlier %e"isions thatresult0 I0e0 later rules %o not o.errule earlier %e"isions that

you ha.e alrea%y ma%e0you ha.e alrea%y ma%e0

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7/19earn these rules0>earn these rules0

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O$i%ation Numbers

:luorine is al/ays A>WA?S 61 in its "om*oun%s0:luorine is al/ays A>WA?S 61 in its "om*oun%s0

Grou* I metals are al/ays A>WA?S 81 in "om*oun%s0Grou* I metals are al/ays A>WA?S 81 in "om*oun%s0

Grou* II metals are al/ays 8& in their "om*oun%s0Grou* II metals are al/ays 8& in their "om*oun%s0

O$ygen is almost al/ays 6& in "om*oun%s e$"e*t in *ero$i%esO$ygen is almost al/ays 6& in "om*oun%s e$"e*t in *ero$i%es

',',&&OO&& ROOR et"0) /here it is 610 'Or /hen o.errule% by one o! ROOR et"0) /here it is 610 'Or /hen o.errule% by one o!

the abo.e rules0)the abo.e rules0)

,ali%es are o!ten 61 but other numbers are o!ten *ossible0,ali%es are o!ten 61 but other numbers are o!ten *ossible0

,y%rogen "an be 81 or 610 'I! it is the !irst element gi.en in a,y%rogen "an be 81 or 610 'I! it is the !irst element gi.en in a

!ormula it is usually 81 i! gi.en se"on% or thir% it may be 610 I!!ormula it is usually 81 i! gi.en se"on% or thir% it may be 610 I!

in %oubt the most ele"tronegati.e element is 8.e an% the otherin %oubt the most ele"tronegati.e element is 8.e an% the other

element /ill be 6.e0)element /ill be 6.e0)

+ut /here %o you start@ We !in% that some atoms al/ays ha.e+ut /here %o you start@ We !in% that some atoms al/ays ha.e

the same o$i%ation number in their "om*oun%s or ions0 Asthe same o$i%ation number in their "om*oun%s or ions0 Asstate% earlier the !irst rules o.errule the later rules0state% earlier the !irst rules o.errule the later rules0

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O$i%ation Numbers

Cal"ulate the o$i%ation number o! Bn in BnOCal"ulate the o$i%ation number o! Bn in BnO7700

an% O are in our table o! #no/n o$i%ation numbers an% an% O are in our table o! #no/n o$i%ation numbers an%

BnOBnO77

is a "om*oun%0is a "om*oun%0 is 81 'Grou* I metal) an% o$ygen is almost al/ays 6&0 is 81 'Grou* I metal) an% o$ygen is almost al/ays 6&0

8 Bn 8 '7 $ O) 'that3s a =ero) 8 Bn 8 '7 $ O) 'that3s a =ero)

So 81 8 Bn 8 '7 $ 6&) So 81 8 Bn 8 '7 $ 6&)

i0e0 81 8 Bn 6 ( i0e0 81 8 Bn 6 (

i0e0 Bn 6 < i0e0 Bn 6 <

so Bn 8< 'We say 8< an% not

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O$i%ation Numbers

Cal"ulate the o$i%ation number o! "hlorine in the ion ClOCal"ulate the o$i%ation number o! "hlorine in the ion ClO5566

O$ygen "omes be!ore "hlorine in our #no/n table so it3s ruleO$ygen "omes be!ore "hlorine in our #no/n table so it3s rule

ta#es *re!eren"e an% it has a "harge o! 6&0ta#es *re!eren"e an% it has a "harge o! 6&0

Cl 8 '5 $ O) 61 ' the 61 is the "harge on this ion)Cl 8 '5 $ O) 61 ' the 61 is the "harge on this ion)Cl 8 '5 $ 6&) 61Cl 8 '5 $ 6&) 61

Cl 6 ; 61Cl 6 ; 61

Cl 89Cl 89

'Again it3s 89 an% not 9 as it3s the "harge on the Fion0)'Again it3s 89 an% not 9 as it3s the "harge on the Fion0)

,ere3s another e$am*le this time in.ol.ing an ion0

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O$i%ation Numbers Cal"ulate the o$i%ation number o! *hos*horus in OClCal"ulate the o$i%ation number o! *hos*horus in OCl&&::

O$ygen is almost al/ays 6& "hlorine is usually 61 an% !luorineO$ygen is almost al/ays 6& "hlorine is usually 61 an% !luorine

is al/ays A>WA?S 610 As is the un#no/n /e "an reasonis al/ays A>WA?S 610 As is the un#no/n /e "an reason

that although Cl "an ha.e other .alues then 61 the "ommonestthat although Cl "an ha.e other .alues then 61 the "ommonest

.alue is the one to use 'other/ise the *roblem is unsol.able)0.alue is the one to use 'other/ise the *roblem is unsol.able)0

8 O 8 '& $ Cl) 8 : 8 O 8 '& $ Cl) 8 :

6 & 6 & 61 6 & 6 & 61

6 9 6 9

89 89

+ut *hos*horus "an ha.e other .alues !or it3s o$i%ation+ut *hos*horus "an ha.e other .alues !or it3s o$i%ation

number %e*en%ing on the "om*oun% un%er in.estigation0number %e*en%ing on the "om*oun% un%er in.estigation0

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O$i%ation numbers

What is the o$i%ation number !or in Na,What is the o$i%ation number !or in Na,&&

OO55

@@Na 81 , 81 usually an% O 6& almost al/ays0Na 81 , 81 usually an% O 6& almost al/ays0

Na 8 &, 8 8 5O Na 8 &, 8 8 5O

8 1 8 & 8 6 ; 8 1 8 & 8 6 ;

6 5 so 85 6 5 so 85 What is "harge on the Cr in What is "harge on the Cr in &&CrCr&&OO

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O$i%ation Numbers

To a.oi% any "on!usion /hen an element "an ha.e se.eralTo a.oi% any "on!usion /hen an element "an ha.e se.eral

o$i%ation numbers the o$i%ation number is usually mentione%o$i%ation numbers the o$i%ation number is usually mentione%

in the "om*oun%3s name /ritten as Roman numerals in bra"#etsin the "om*oun%3s name /ritten as Roman numerals in bra"#ets

a!ter the element to /hi"h it re!ers0a!ter the element to /hi"h it re!ers0

In names li#e Felementate'X) the number re!ers to FelementIn names li#e Felementate'X) the number re!ers to Felement

an% not the asso"iate% o$ygens0an% not the asso"iate% o$ygens0 So i! /e loo# at the e$am*les /e3.e 2ust %one /e get theSo i! /e loo# at the e$am*les /e3.e 2ust %one /e get the

!ollo/ing namesH6!ollo/ing namesH6

BnOBnO77 *otassium manganate'II)*otassium manganate'II)

NaClONaClO55 so%ium "hlorate')so%ium "hlorate')

OClOCl&&:: *hos*horus') o$y%i"hloro!luori%e*hos*horus') o$y%i"hloro!luori%e

Na,Na,&&OO55 so%ium %ihy%rogen*hos*hate'III)so%ium %ihy%rogen*hos*hate'III)

&&CrCr&&OO

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O$i%ation numbers Che"# !or yoursel! that the numbers in the !ollo/ingChe"# !or yoursel! that the numbers in the !ollo/ing

"om*oun%s3 names are the same as the o$i%ation number o! the"om*oun%s3 names are the same as the o$i%ation number o! theelement /ith /hi"h they are asso"iate%element /ith /hi"h they are asso"iate%

>iNO>iNO55 lithium nitrate') tCl lithium nitrate') tCl77 *latinum'I) "hlori%e *latinum'I) "hlori%e

NaNONaNO&& so%ium nitrate'III) Na+rO so%ium nitrate'III) Na+rO77so%ium bromate'II)so%ium bromate'II)

OClOCl55 *hos*horus') o$y"hlori%e *hos*horus') o$y"hlori%e

CrClCrCl55 "hromium'III) "hlori%e TiO titanium'II) o$i%e "hromium'III) "hlori%e TiO titanium'II) o$i%e

TiOTiO&& titanium'I) o$i%e titanium'I) o$i%e

Di% you get them all right@Di% you get them all right@

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O$i%ation Numbers

Consi%er the e-uation belo/0Consi%er the e-uation belo/0

&:eCl&:eCl&& 8 Cl8 Cl&& &:eCl &:eCl55

The o$i%ation number o! the iron "hanges thusHThe o$i%ation number o! the iron "hanges thusH:e:e&8&8 :e :e5858 ' 8 e ' 8 e66o! "ourse)o! "ourse)

sometimes /ritten as :e'II)sometimes /ritten as :e'II) :e'III) 8 e :e'III) 8 e66

i0e0 :ei0e0 :e&8&8has been o$i%ise%0 This e$am*le is easy to s*ot as anhas been o$i%ise%0 This e$am*le is easy to s*ot as an

e$am*le o! o$i%ation but sometimes it3s a bit har%er0e$am*le o! o$i%ation but sometimes it3s a bit har%er0

An im*ortant use o! o$i%ation numbers is in the re"ognition o!

o$i%ation an% re%u"tion0 They let us -ui"#ly see /hen anelement has gaine% or lost ele"trons an% hen"e been o$i%ise%

or re%u"e%0 I! the o$i%ation number be"omes more *ositi.e

o$i%ation has o""urre% be"oming more negati.e means that

re%u"tion has o""urre%0

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O$i%ation Numbers

II66 IOIO5566

Is this o$i%ation or re%u"tion@Is this o$i%ation or re%u"tion@

Cal"ulating the o$i%ation numbers re.eals all0Cal"ulating the o$i%ation numbers re.eals all0

In IIn I66 the io%ine is 61 /hile in IO the io%ine is 61 /hile in IO5566it is 890 'Don3t ta#e myit is 890 'Don3t ta#e my

/or% !or it "al"ulate the o$i%ation numbers yoursel!0) So the/or% !or it "al"ulate the o$i%ation numbers yoursel!0) So the

io%i%e ion has lost ele"trons an% been o$i%ise% to 890io%i%e ion has lost ele"trons an% been o$i%ise% to 890II66 IOIO5566 8 ;e8 ;e66

/hi"h "oul% also be /ritten as/hi"h "oul% also be /ritten as

I'I)I'I) I') 8 ;e I') 8 ;e66

Consi%er a rea"tion /here the !ollo/ing "hange o""urs 'the

other reagents are not im*ortant at this stage)0

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