Cie a2 Electrochemistry

7/24/2019 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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Cie a2 Electrochemistry

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