Cie a2 Electrochemistry
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Transcript of Cie a2 Electrochemistry
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A2 CHEMISTRY
SAHAN SATHARASINGHE
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Electrochemistry encompasses chemical and physical processes that involve the transfer
of charge. Some examples are electroplating iron oxidation !r"sting# solar$energy
conversion energy storage !%atteries# photosynthesis and respiration.
Reduction and oxidation
Redox reactions involve oxidation and red"ction occ"rring sim"ltaneo"sly.
Rules for assigning oxidation number:
& ' (xidation n"m%er for an "n$com%ined element is ).
2 ' The s"m of the (xidation n"m%ers in a compo"nd is e*"al to the overall charge.
+ ' Gro"p & compo"nds are ,& - gro"p 2 are ,2.
' / is al0ays $& in a compo"nd. And 1l is "s"ally $& except 0hen its 0ith ( or /.
3 ' H is ,& !except metal hydrides# and ( is $2 !except 4eroxides#.
Redox is any reaction 0hich involves one species %eing oxidised !loss of electrons#and
another %eing red"ced !gain in electrons#.
Half E*"ationsare "sed to sho0 0hich species is oxidised and 0hich is red"ced.
changes in oxidation number to reaction stoichiometr
The n"m%er of electrons transferred in a reaction m"st al0ays %e the same on each side
so %y "sing half e*"ations yo" can split it "p eg the reaction %et0een chloride ions and
chromi"m !III# ions5
1l$!a*#61l2!g# , 2e$x3 to make the electrons balance
1r+,!a*# , +e$6 1r!s#x2 to make the electrons balance
So this gives the overall e*"ation5
21r
+,
!a*# , 71l
$
!a*#621r!s# , +1l2!g#
!xidation is the loss of electrons
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Reduction is the gain of electrons
As stated previo"sly these t0o reactions cannot occ"r in isolation.
The t0o e*"ations are added to provide the reaction e*"ation5
In terms of oxidation n"m%er5
/or the oxidation5 there has %een an increase of ,2
/or the red"ction5 there has %een an increase of $2
Displacement reactions
8isplacement reactions occ"r if for example yo" place a more reactive metal in a sol"tion
containing ions of a less reactive metal./or example5
If 0e no0 compare this 0ith the follo0ing reaction5
9e see that metals can %e %oth oxidised and red"ced 0e 0rite5
Metals in solutions of their ions
If a piece of metal is placed into a sol"tion of its o0n ions t0o occ"rrences may appear.
+
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Metal atoms ma lea"e the solid and become metal ions in solution:
!xidation
In this case the electrons that are released stay on the s"rface of the solid 0hich ca"ses
it to gain a negative charge.
(r metal ions in sol"tion may %ecome metal atoms on the s"rface of the solid.
Reduction
In this case electrons are attracted into the sol"tion.
Electrolysis
The electrolysis cell
The process of decomposing a compo"nd "sing electricity is called electrolsis.
2H2(!l#2H2!g#, (2!g#
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Electrolysis is a most important ind"strial process 0ith a 0ide application. Ho0ever the
largest application is that of the man"fact"re of chlorine and sodi"m hydroxide from
concentrated a*"eo"s sodi"m chloride in the 1hlor$al:ali ind"stry.
The cell consists of t0o cond"cting rods called electrodes dipped into a compo"nd in amolten state or in sol"tion that is a%le to cond"ct electricity called the electrolte.
Electrolysis only 0or:s 0ith direct c"rrent !the charge ;o0s in only one direction#. 8irect
c"rrent ca"ses one electrode to ta:e on a positive charge called the anode. The second
electrode ta:es on a negative charge called the cathode.
Note5 Anions!negative ions# are attracted to the anode. Cations!positive ions# are
attracted to the cathode.
An exam#le of electrolsis:
If 0e place moltensodi"m chloride into the electrolysis cell and allo0 c"rrent to ;o0 the
follo0ing reactions occ"r at the electrodes5
At the cathode:Red"ction
Na,!l#, e$Na!l#
At the anode:(xidation
21l$!l#1l2!g#, 2e$
These t$o half e%uations gi"e the o"erall reaction:
2Na,!l#, 21l$!l#Na!l#, 1l2!g#
A redox reaction has occured&
Predicting products in electrolysis
In the example a%ove of molten sodi"m chloride there 0as one cation Na ,and one
anion 1l$
. Therefore the prod"cts 0ere easily calc"lated.
Ho0ever if a compo"nd is in an a%ueous sol"tion then 0e have t0o more ions to deal
0ith (H$and H,.
So how do we decide with ions will appear at the electrodes and which remain in
solution?
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In the case of the cathode 0e m"st decide 0hich of the t0o cations H,or Na,is red"ced
most easily %ac: to their atoms. Hydrogen ions are most easily red"ced so the follo0ing
cathode reaction occ"rs5
2H
,
!a*#, 2e
$
H2!g#
In the case of the anode 0e m"st decide 0hich of the t0o anions (H $or 1l$0ill %e most
easily oxidised %ac: to atoms. 1hloride ions are most easily oxidised hence the follo0ing
anode reaction5
21l$!a*#1l2, 2e$
This leaves Na,!a*#and (H$!a*#in sol"tion i.e. sodi"m hydroxide.
9e can refer to the electrochemical series redox series or reactivity series for this
information.
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ichael /araday ded"ced that5
The %uantit of electricit #assed is #ro#ortional to the amount of substance
discharged at the electrode&
'uantit of electricit (charge) * current x time
The "nits "sed are coulombsfor charge am#sfor c"rrent and secondsfor time.
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Note 1:(ne mole of electrons has a charge of ?73))1. This *"antity of charge is called
the +arada constant+. Th"s the /araday constant is related to A"agadro,s
constant- and the charge on the electron e.
/ @ x e
Note 2:The n"m%er of moles of electrons re*"ired to discharge & mole of ions is e*"al to
the charge on the ion.
/or example ions 0ith a do"%le charge s"ch as 1"2,$ it 0ill ta:e t0o moles of electrons
to deposit one mole of copper.
Chlor-alali industry
This ind"stry involves the prod"ction of chlorine and the al:ali sodi"m hydroxide from the
electrolysis of concentrated a*"eo"s sodi"m chloride !%rine#.
The electrolysis cell "sed for this reaction is called thedia#hram cell&
As previo"sly stated fo"r ions are present Na, 1l$ (H$ H,.
At the cathode:
2H,!a*#, 2e$H2!g#
At the anode:
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21l$!a*#1l2!g#, 2e$
The ions remaining in solution are !H.and /a0&
In the diaphragm cell a poro"s as%estos partition is placed %et0een the electrodes to
prevent the sodi"m hydroxide ma:ing contact 0ith the chlorine !sodi"m hydroxide tends
to concentrate near the cathode#.
4"riBed %rine sol"tion is fed into the anode side and the level :ept a%ove that of the
cathode. This allo0s the sodi"m chloride sol"tion to seep into the cathode compartment
and also prevents (H$ions migrating to the anode.
!tandard hydrogen electrode
Ho0 do 0e Bnd the potential of any half$cell individ"allyC The ans0er is that 0e "se
a standard hdrogen electrode (s&h&e)and give it a potential of Dero.
Therefore 0hen it is connected to another half$cell the electromagnetic
Beld (e&m&f)%et0een the s.h.e and the second half$cell is e*"al to the potential of the
second half$cell.
The diagram %elo0 o"tlines the main characteristics and conditions re*"ired to esta%lish
the s.h.e5
The reaction that ta:es place is5
2H,!a*#, 2e$2H2 !g#
Standard Electrode 4otential
?
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&)
De"nition$ 9e can place redox systems in order of their oxidisingred"cing a%ility if 0emeas"re their electrode potential against the standard hydrogen electrode 0hich has a
potential of ).))F.
/ote: Standard conditions m"st %e adhered to so for example if "sing copper rods they
m"st %e immersed into a sol"tion of & mol dm$+copper ions.
1 con"ention $e de3ne the e&m&f of an electrochemical cell as follo$s:
E4* E4right.hand half.cell. E4 left.hand half.cell
The s.h.e is al0ays placed on the left$hand side.
The standard electrode potential of a metal is the potantial ac*"ired 0hen the metal is
immersed in a & mol dm$+sol"tion of its ions at a temperat"re of 23o1.
It has the sym%ol E and the "nits F.
The electrochemical series
As stated previo"sly 0e can ta%"late the order of oxidisingred"cing a%ility of a systemthis 0e call the electrochemical series.
The most positive E val"e is at the top i.e. the greatest oxidising agent. The most
negative val"e E at the %ottom i.e. the greatest red"cing agent.
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&&
Reactio
n
Eo(2567)89
+2(g) ,2
e$-r"ldhar 2/$!a*# ,2.=
anganese5
S#ecies !xidatio
n no&
Colour S#ecies !xidatio
n no&
Colour
F2, ,2 4"rple >n2, ,2 4ale 4in:
F+, ,+ Green >n(2 , lac:
F(2, , l"e >n($ ,< 4"rple
F(2, ,3 Mello0
understanding of the #rocedures of the redox titrations belo$ ( and 2) and
carr out a redox titration $ith one:
& $ 4otassi"mmanganate!FII# eg the estimation of iron in iron ta%lets
>n($!a*#is p"rple and /e2,!a*#is colo"rless !"t 0hen concentrated >n($!a*# is pale pin:
and /e2,!a*# is pale green#. At the end point the sol"tion 0ill t"rn pale pin: %y itself. So this
is a self$indicating reaction.
2 $ Sodi"m thios"lfate and iodine eg estimation of percentage of copper in an alloy
Thro"gho"t the titration I2slo0ly t"rns from ro0n to colo"rless I$. To clearly see the
change starch is added near the end 0hen the sol"tion is pale yello0 !other0ise Iodine is
adsor%ed onto it and red"ces acc"racy# and 0ill t"rn from %l"e$%lac: to colo"rless.
A third example5 4otassi"m manganate !FII# 0ith ethanedioic acid
This reaction is an example of a"tocatalysis. The reaction is catalysed %y manganese !II#
ions. So at the start there are none so it goes slo0 and needs heating. "t then once a
fe0 have %een prod"ced they 0ill %egin to catalyse it and the reaction goes faster and
faster. !This is sho0n in the graph#.
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the uncertaint of measurements
and their im#lications for the "alidit of the 3nal results
evel of acc"racy in a %"rette is ).)3 cm+
9hen 0or:ing o"t percentage of "ncertainty in the reading it has to %e m"ltiplied %y t0o
since yo" read the %"rette once at the start and once at the end of the titration5
O "ncertainty in a %"rette2
0.05
VolumeMeasured 100
Pncertainty in the readings can ma:e the res"lts less relia%le.
Electrochemical 1ells
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