3.electrochemistry

8/9/2019 3.Electrochemistry

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Question 3.1:

How would you determine the standard electrode potential of the systemMg2+| Mg?

• Answer

The standard electrode potential of Mg2+| Mg can be measured with respect to the standardhydrogen electrode, represented by Pt(s, H2(g(! atm | H

+(aq(! M"

# cell, consisting of Mg | Mg$%&(aq ! M as the anode and the standard hydrogen electrode asthe cathode, is set up"

Then, the emf of the cell is measured and this measured emf is the standard electrode

potential of the magnesium electrode"

Here, for the standard hydrogen electrode is 'ero"

∴

Question 3.2:

an you store copper sulphate solutions in a 'inc pot?

• Answer

)inc is more reacti*e than copper" Therefore, 'inc can displace copper from its salt solution" fcopper sulphate solution is stored in a 'inc pot, then 'inc will displace copper from the coppersulphate solution"

Hence, copper sulphate solution cannot be stored in a 'inc pot"

Question 3.3:

onsult the table of standard electrode potentials and suggest three substances that can

oidise ferrous ions under suitable conditions"


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• Answer

$ubstances that are stronger oidising agents than ferrous ions can oidise ferrous ions"

- . /0"11

This implies that the substances ha*ing higher reduction potentials than+0"11 can oidise ferrous ions to ferric ions" Three substances that can do so are 32, l2, and%2"

Question 3.4:

alculate the potential of hydrogen electrode in contact with a solution whose pH is !0"

• Answer

3or hydrogen electrode, , it is gi*en that pH . !0

∴4H+5 . !0/!0M

6ow, using 6ernst e7uation8

.

./0"09:! log !0!0

./0"9:!

Question 3.5:

alculate the emf of the cell in which the following reaction ta;es place8


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• Answer

#pplying6ernst e7uation we ha*e8

. !"09 / 0"02:99 log & = !0&

. !"09 /0"02:99 (log !0000 + log &

. !"09 /0"02:99 (& + 0">02!

. 0":!&

Question 3.6:

The cell in which the following reactions occurs8

has . 0"2> at 2:@ A"

alculate the standard

./&99&!"@>& C mol/!


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./&9"9& ;C mol/!

#gain, /2"0DT log K c

. 1":@!

∴K c. #ntilog (1":@!

. :"91 = !01

Question 3.7:

Bhy does the conducti*ity of a solution decrease with dilution?

• Answer

The conducti*ity of a solution is the conductance of ions present in a unit *olume of thesolution" The number of ions (responsible for carrying current decreases when the solution is

diluted" #s a result, the conducti*ity of a solution decreases with dilution"

Question 3.8:

$uggest a way to determine the *alue of water"

• Answer

• #pplying AohlrauschEs law of independent migration of ions, the *alue of water can

be determined as follows8

Hence, by ;nowing the *alues of Hl, 6a%H, and 6al, the *alue of water can bedetermined"

Question 3.9:


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The molar conducti*ity of 0"029 mol F/! methanoic acid is

&>"! $ cm2 mol/!"

alculate its degree of dissociation and dissociation constant" $ cm2 mol

• Answer

C . 0"029 mol F/!

6ow, degree of dissociation8

Thus, dissociation constant8

Question 3.10:


6/27

f a current of 0"9 ampere flows through a metallic wire for 2 hours, then how many electrons

would flow through the wire?

• Answer

I . 0"9 #

t . 2 hours . 2 = >0 = >0 s . 1200 s

Thus, Q . It

. 0"9 # = 1200 s

. >00

Be ;now that number of electrons"

Then,

Hence, number of electrons will flow through the wire"

Question 3.11:

$uggest a list of metals that are etracted electrolytically"

• Answer

Metals that are on the top of the reacti*ity series such as sodium, potassium, calcium, lithium,magnesium, aluminium are etracted electrolytically"

Question 3.12:

Bhat is the 7uantity of electricity in coulombs needed to reduce ! mol of

? onsider the reaction8

• Answer


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The gi*en reaction is as follows8

Therefore, to reduce ! mole of , the re7uired 7uantity of electricity will be8

.> 3

. > = :>&@1

. 91@:22

Question 3.13:

Brite the chemistry of recharging the lead storage battery, highlighting all the materials that

are in*ol*ed during recharging"

• Answer

# lead storage battery consists of a lead anode, a grid of lead pac;ed with lead oide (Pb%2 asthe cathode, and a @I solution of sulphuric acid (H2$%& as an electrolyte"

Bhen the battery is in use, the following cell reactions ta;e place8

#t anode8

#t cathode8

The o*erall cell reaction is gi*en by,

Bhen a battery is charged, the re*erse of all these reactions ta;es place"

Hence, on charging, present at the anode and cathode is con*erted into and

respecti*ely"

Question 3.14:

$uggest two materials other than hydrogen that can be used as fuels in fuel cells"

• Answer


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Methane and methanol can be used as fuels in fuel cells"

Question 3.15:

Jplain how rusting of iron is en*isaged as setting up of an electrochemical cell"

• Answer

n the process of corrosion, due to the presence of air and moisture, oidation ta;es place at aparticular spot of an obKect made of iron" That spot beha*es as the anode" The reaction at theanode is gi*en by,

Jlectrons released at the anodic spot mo*e through the metallic obKect and go to another spotof the obKect"

There, in the presence of H+ ions, the electrons reduce oygen" This spot beha*es as thecathode" These H+ ions come either from H2%,which are formed due to the dissolution ofcarbon dioide from air into water or from the dissolution of other acidic oides from theatmosphere in water"

The reaction corresponding at the cathode is gi*en by,

The o*erall reaction is8

#lso, ferrous ions are further oidi'ed by atmospheric oygen to ferric ions" These ferric ionscombine with moisture, present in the surroundings, to form hydrated ferric oide

i"e", rust"

Hence, the rusting of iron is en*isaged as the setting up of an electrochemical cell"

Question 3.1:

#rrange the following metals in the order in which they displace each other from the solution

of their salts"

#l, u, 3e, Mg and )n


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• Answer

The following is the order in which the gi*en metals displace each other from the solution oftheir salts"

Mg, #l, )n, 3e, u

Question 3.2:


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(ii) ons are carriers of current in the cell and in the eternal circuit, current will flow fromsil*er to 'inc"

(iii) The reaction ta;ing place at the anode is gi*en by,

The reaction ta;ing place at the cathode is gi*en by,

Question 3.4:

alculate the standard cell potentials of gal*anic cells in which the following reactions ta;e

place8

(i) 2r(s + d2+(a7 → 2r+(a7 + d

(ii) 3e2+(a7 + #g+(a7 → 3e+(a7 + #g(s

alculate the OrG and e7uilibrium constant of the reactions"

• Answer

(i)

The gal*anic cell of the gi*en reaction is depicted as8

6ow, the standard cell potential is

n the gi*en e7uation,

n . >

http://cbse.meritnation.com/study-online/solution/Chemistry/jbeo07BxqqU$n$83EES3Gg!!#answer%23answerhttp://cbse.meritnation.com/study-online/solution/Chemistry/jbeo07BxqqU$n$83EES3Gg!!#answer%23answer


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3 . :>&@1 mol/!

. +0"&

Then, . /> = :>&@1 mol/!

= 0"&

. /!:>@"&@ mol/!

. /!:>@"&@ C mol/!

. /!:>"@ ;C mol/!

#gain,

. /DT ln K

. &"&:>

A . antilog (&"&:>

. "! = !0&

(ii)

The gal*anic cell of the gi*en reaction is depicted as8

6ow, the standard cell potential is

Here, n . !"


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Then,

. /! = :>&@1 mol/! = 0"0

. /2@:&">! C mol/!

. /2"@: ;C mol/!

#gain,

. 0"901

A . antilog (0"901

. "2 (approimately

Question 3.5:

Brite the 6ernst e7uation and emf of the following cells at 2:@ A8

(i) Mg(s | Mg2+(0"00!M || u2+(0"000! M | u(s

(ii) 3e(s | 3e2+(0"00!M || H+(!M|H2(g(!bar | Pt(s

(iii) $n(s | $n2+(0"090 M || H+(0"020 M | H2(g (! bar | Pt(s

(iv) Pt(s | Qr2(l | Qr/(0"0!0 M || H+(0"00 M | H2(g (! bar | Pt(s"

• Answer

(i) 3or the gi*en reaction, the 6ernst e7uation can be gi*en as8

http://cbse.meritnation.com/study-online/solution/Chemistry/hZaYLCNXAxHhYf1ZFPzNtw!!#answer%23answerhttp://cbse.meritnation.com/study-online/solution/Chemistry/hZaYLCNXAxHhYf1ZFPzNtw!!#answer%23answer


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. 2"1 / 0"02:99

. 2">1 (approimately

(ii) 3or the gi*en reaction, the 6ernst e7uation can be gi*en as8

. 0"92@>9

. 0"9 (approimately

(iii) 3or the gi*en reaction, the 6ernst e7uation can be gi*en as8

. 0"!& / 0"02:9 = log!29

. 0"!& / 0"0>2

. 0"01@

. 0"0@ (approimately

(iv) 3or the gi*en reaction, the 6ernst e7uation can be gi*en as8


14/27

Question 3.6:

n the button cells widely used in watches and other de*ices the following reaction ta;es

place8

)n(s + #g2%(s + H2%(l → )n2+(a7 + 2#g(s + 2%H/(a7

Netermine and for the reaction"

• Answer

. !"!0&

Be ;now that,

. /2 = :>&@1 = !"0&

. /2!0&"2:> C

. /2!"0& ;C

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Question 3.7:

Nefine conducti*ity and molar conducti*ity for the solution of an electrolyte" Niscuss their

*ariation with concentration"

•Answer

onducti*ity of a solution is defined as the conductance of a solution of ! cm in length andarea of crossRsection ! s7" cm" The in*erse of resisti*ity is called conducti*ity or specificconductance" t is represented by the symbolκ " f S is resisti*ity, then we can write8

The conducti*ity of a solution at any gi*en concentration is the conductance (G of one unit*olume of solution ;ept between two platinum electrodes with the unit area of crossRsectionand at a distance of unit length"

i"e",

($ince a . !, l . !

onducti*ity always decreases with a decrease in concentration, both for wea; and strongelectrolytes" This is because the number of ions per unit *olume that carry the current in asolution decreases with a decrease in concentration"

Molar conductivity:

Molar conducti*ity of a solution at a gi*en concentration is the conductance of *olume of asolution containing ! mole of the electrolyte ;ept between two electrodes with the area ofcrossRsection A and distance of unit length"

6ow, l . ! and A . (*olume containing ! mole of the electrolyte"

Molar conducti*ity increases with a decrease in concentration" This is because the total *olume of the solution containing one mole of the electrolyte increases on dilution"

The *ariation of with for strong and wea; electrolytes is shown in the following plot8

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Question 3.8:

The conducti*ity of 0"20 M solution of Al at 2:@ A is 0"02&@ $cm/!" alculate its molar

conducti*ity"

• Answer

= !0/ $ cm/!"

• Answer


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. 0"!&> = !0/ = !900

. 0"2!: cm/!

Question 3.10:

The conducti*ity of sodium chloride at 2:@ A has been determined at different concentrations

and the results are gi*en below8

oncentrationLM 0"00! 0"0!0 0"020 0"090 0"!00

!02 = UL$ m/! !"21 !!"@9 2"!9 99"9 !0>"1&

alculate for all concentrations and draw a plot between and cV" 3ind the *alue of

"

• Answer

M!L2

. !2"1 $ cm2 mol/!


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"1& = !0/& $ cm/!, c!L2 . 0"!>2 M!L2

. !0>"1& $ cm2 mol/!

6ow, we ha*e the following data8


19/27

0"0!> 0"! 0"!&!& 0"22> 0"!>2

!2"1 !!@"9 !!9"@ !!!"! !0>"1&

$ince the line interrupts at !2&"0 $ cm2 mol/!, . !2&"0 $ cm2 mol/!"

Question 3.11:

onducti*ity of 0"002&! M acetic acid is 1"@:> = !0/9 $ cm/!" alculate its molar conducti*ity

and if for acetic acid is :0"9 $ cm2 mol/!, what is its dissociation constant?

• Answer


20/27

#gain, . :0"9 $ cm2 mol/!

6ow,

. 0"0@&

Nissociation constant,

. !"@> = !0/9 mol F/!

Question 3.12:

How much charge is re7uired for the following reductions8

(i) ! mol of #l+ to #l"

(ii) ! mol of u2+ to u"

(iii) ! mol of to Mn2+"

• Answer

(i)

De7uired charge . 3

. = :>&@1

. 2@:&>!

(ii)

De7uired charge . 2 3

. 2 = :>&@1

. !:2:1&

http://cbse.meritnation.com/study-online/solution/Chemistry/EZt5UYc@uSO40Gxp2cgzjg!!#answer%23answerhttp://cbse.meritnation.com/study-online/solution/Chemistry/EZt5UYc@uSO40Gxp2cgzjg!!#answer%23answer


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(iii)

i"e",

De7uired charge . 9 3

. 9 = :>&@1

. &@2&9

Question 3.13:

How much electricity in terms of 3araday is re7uired to produce

(i) 20"0 g of a from molten al2"

(ii) &0"0 g of #l from molten #l2%"

• Answer

(i) #ccording to the 7uestion,

Jlectricity re7uired to produce &0 g of calcium . 2 3

Therefore, electricity re7uired to produce 20 g of calcium

. ! 3

(ii) #ccording to the 7uestion,

Jlectricity re7uired to produce 21 g of #l . 3

Therefore, electricity re7uired to produce &0 g of #l

. &"&& 3

http://cbse.meritnation.com/study-online/solution/Chemistry/TDDCBp@c7CH$G8hxHJT9Lg!!#answer%23answerhttp://cbse.meritnation.com/study-online/solution/Chemistry/TDDCBp@c7CH$G8hxHJT9Lg!!#answer%23answer


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Question 3.14:

How much electricity is re7uired in coulomb for the oidation of

(i) ! mol of H2% to %2"

(ii) ! mol of 3e% to 3e2%"

• Answer

(i) #ccording to the 7uestion,

6ow, we can write8

Jlectricity re7uired for the oidation of ! mol of H2% to %2 . 2 3

. 2 = :>&@1

. !:2:1&

(ii) #ccording to the 7uestion,

Jlectricity re7uired for the oidation of ! mol of 3e% to 3e2% . ! 3

. :>&@1

Question 3.15:

# solution of 6i(6%2 is electrolysed between platinum electrodes using a current of 9

amperes for 20 minutes" Bhat mass of 6i is deposited at the cathode?

• Answer

• Given,

urrent . 9#

Time . 20 = >0 . !200 s

harge . current = time

http://cbse.meritnation.com/study-online/solution/Chemistry/4F$VpqDQWMI1t7t8yWUTSA!!#answer%23answerhttp://cbse.meritnation.com/study-online/solution/Chemistry/o0hi@1QrXWq8C4Nh$u5YIg!!#answer%23answerhttp://cbse.meritnation.com/study-online/solution/Chemistry/4F$VpqDQWMI1t7t8yWUTSA!!#answer%23answerhttp://cbse.meritnation.com/study-online/solution/Chemistry/o0hi@1QrXWq8C4Nh$u5YIg!!#answer%23answer


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. 9 = !200

. >000

#ccording to the reaction,

6ic;el deposited by 2 = :>&@1 . 9@"1! g

Therefore, nic;el deposited by >000

. !"@29 g

Hence, !"@29 g of nic;el will be deposited

Question 3.16:

Three electrolytic cells #,Q, containing solutions of )n$%&, #g6% and u$%&, respecti*ely are

connected in series" # steady current of !"9 amperes was passed through them until !"&9 g of

sil*er deposited at the cathode of cell Q" How long did the current flow? Bhat mass of copper

and 'inc were deposited?

• Answer

#ccording to the reaction8

i"e", !0@ g of #g is deposited by :>&@1 "

Therefore, !"&9 g of #g is deposited by .

. !2:9"&


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. @>"> s

. @>& s

. !&"&0 min

#gain,

i"e", 2 = :>&@1 of charge deposit . >"9 g of u

Therefore, !2:9"& of charge will deposit

. 0"&2> g of u

i"e", 2 = :>&@1 of charge deposit . >9"& g of )n

Therefore, !2:9"& of charge will deposit

. 0"&: g of )n

Question 3.17:

Wsing the standard electrode potentials gi*en in Table "!, predict if the reaction between the

following is feasible8

(i) 3e+(a7 and /(a7

(ii) #g+ (a7 and u(s

(iii) 3e+ (a7 and Qr/ (a7

(iv) #g(s and 3e+ (a7

(v) Qr2 (a7 and 3e2+ (a7"

• Answer

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$ince for the o*erall reaction is positi*e, the reaction between 3e+(aq and /

(aq is feasible"

$ince for the o*erall reaction is positi*e, the reaction between #g+ (aq and u(s is feasible"

$ince for the o*erall reaction is negati*e, the reaction between 3e+(aq and Qr/

(aq is notfeasible"

$ince J for the o*erall reaction is negati*e, the reaction between #g (s and 3e+

(aq is notfeasible"

$ince for the o*erall reaction is positi*e, the reaction between Qr2(aq and 3e2+

(aq is feasible"

Question 3.18:


26/27

Predict the products of electrolysis in each of the following8

(i) #n a7ueous solution of #g6% with sil*er electrodes"

(ii) #n a7ueous solution of #g6%with platinum electrodes"

(iii) # dilute solution of H2$%&with platinum electrodes"

(iv) #n a7ueous solution of ul2 with platinum electrodes"

• Answer

(i) #t cathode8

The following reduction reactions compete to ta;e place at the cathode"

The reaction with a higher *alue of ta;es place at the cathode" Therefore, deposition ofsil*er will ta;e place at the cathode"

#t anode8

The #g anode is attac;ed by ions" Therefore, the sil*er electrode at the anode dissol*esin the solution to form #g+"

(ii) #t cathode8


The reaction with a higher *alue of ta;es place at the cathode" Therefore, deposition ofsil*er will ta;e place at the cathode"

#t anode8

$ince Pt electrodes are inert, the anode is not attac;ed by ions" Therefore, %H/ or

ions can be oidi'ed at the anode" Qut %H/ ions ha*ing a lower discharge potential and getpreference and decompose to liberate %2"

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(iii) #t the cathode, the following reduction reaction occurs to produce H2 gas"

#t the anode, the following processes are possible"

3or dilute sulphuric acid, reaction (i is preferred to produce %2 gas" Qut for concentratedsulphuric acid, reaction (ii occurs"

(iv) #t cathode8


The reaction with a higher *alue of ta;es place at the cathode" Therefore, deposition ofcopper will ta;e place at the cathode"

#t anode8The following oidation reactions are possible at the anode"

#t the anode, the reaction with a lower *alue of is preferred" Qut due to the o*erRpotentialof oygen, l/ gets oidi'ed at the anode to produce l2 gas"

3.electrochemistry

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