Question

electrochemistry question! someone help me pls.

Answer 1

@ANISH95

Answer 2

=D

Answer 3

11 a)

Answer 4

you need help with 11-a-i part?

Answer 5

all parts of 11 a

it might take time...

a dauting question..

Answer 6

daunting...

Answer 7

last years A level question T.T. yea its all parts of 11 a). im alrdy stuck at part 1... . its ok, take your time =)

Answer 8

out of chlorine and iodine, which one according to you , is better oxidising agent?

Answer 9

iodine

Answer 10

wait!

Answer 11

chlorine!!

Answer 12

justify...
i mean reason// for your answer.

Answer 13

chlorine because it has a higher electronegativity or you can say that it has a very small atomic size, hence it has a strong tendency to attract electrons. Thus it makes another species lose its electron

Answer 14

it has the smallest atomic size in period 2 because it has the larges nuclear charge with similar electron shielding and quantum shell with the other elements in the same period.

Answer 15

well, here you are not looking at Cl atom, but chlorine molecule Cl2

here you need to consider oxidising ability of Cl2 and I2
not simply of Cl and I

Answer 16

I2 can easily loose electrons. for the electrons that are to be lost, belongs to 5th shell i guess.

these loosely held electrons can be lost easily
that means iodine has a strong tendency to oxidise

Answer 17

following?

Answer 18

yea

Answer 19

that means I2 will be at anode for it oxidises
and Cl2 will be at cathode for it reduces...

Answer 20

yes

Answer 21

electrons flow to cl2

Answer 22

yes..
i think you are almost done with the first part of the question.

you just now need to write the equations. for the electrode reactions

Answer 23

thats why im confused... cuz the equilibrium eqn they showed me does not have any electron produced at all

Answer 24

in a half cell, the element and its ions exist in equillibrium and the eqn usually includes electrons which is a "ion-electron eqn"

Answer 25

how does the equilibrium they showed me produce any potential at all in an electrode?

Answer 26

:-)
the equilibrium reaction they have written need not involve an electron.
it is just and equilibrium reaction.... it has nothing to do with your electrochemical cell.
this equill. just shows the formation of I3- from I2 ( I2 here, can't be used for it is insoluble in water, that's why we use I3-)

you use I3- instead of I2 in your equations.

Answer 27

what happened?

Answer 28

ok so in the half cell i have I- and I3-, how does these two ions interact?

Answer 29

potential is created only when electrons is removed or added into an electrode

Answer 30

I3- can loose electron i guess...

Answer 31

look
what i earlier said is that I2 will loose electron for it is easy for I2 to loose electron
but here,
we can not use I2 because it is insoluble in water

now, istead of using I2 to loose electron you can use I3- to loose the lectron

the question says that I3- is equivalent to aquesous solution of I2

Answer 32

but they showed us an equilibrium reaction in which there is no loss of electron by I3- =( omg i dun uds. is there another reaction involved?

Answer 33

so the equilibrium they showed us only shows the formation of I3-? so there must be another reaction involving I- and I3-? @.@

Answer 34

yes .. the actual reaction which shows the loss of electron by I3-

Answer 35

=( ok... so how do i create a standard condition in this case? i must have 1M of i- and 1M of I3-

Answer 36

http://web.missouri.edu/~puckettj/ChemistryTablesfolder/StandardReductionPotentials298K.pdf

look at this pdf

on the second page of the pdf, you can find the electrode of I3-

Answer 37

us ethat reaction

Answer 38

I-

Answer 39

I3- forming I-

Answer 40

what does it mean by freely soluble??

Answer 41

it almost means like completely soluble

Answer 42

yes i think it means completely soluble only..

Answer 43

then why did they show me that equilibrium? =/

Answer 44

because the statement says
I2 is freely soluble in KI and not soluble in water.

if you carefully see the equilibrium, you can see that I2 reacts with I-........( from KI)

Answer 45

but why does an equilibrium form if its completely soluble? shouldnt it just be I2 +I- --> I3-?

Answer 46

like one way reaction

Answer 47

i just find it hard to believe or uds that I3- and I- and undergo reaction in two different reaction..

Answer 48

I2 + I- <---> I3-

and

I3- <--->3I- + 2e

Answer 49

wait a sec.

Answer 50

so, you have a problem understanding as to how this complete reaction sequence works..

Answer 51

yes =( . because usually half cells only have an element establishing one equilibrium with its ion to create a potential at the electrode

Answer 52

in this case... its like the product of the solubility equilibrium, I3- is reacting with its reactant..

Answer 53

well that is not an equilibrium reaction...

i mean the ion-electron eqn. is not an equilibrium equation,

it is one way equation only,

Answer 54

in the ion-electron equation, one element either gains or looses electrons..

i can try to make you understand this sequence of reaction . with the help of an example.

Answer 55

http://www.chemguide.co.uk/physical/redoxeqia/ecs.html

Answer 56

in this site it says equilibrium =( my notes too...

Answer 57

i mean when the half cell is by itself

Answer 58

well..
do you know, for a reaction to be spontaneous, the delta G ( Gibbs free energy ) should be negarive?

Answer 59

yea

Answer 60

and the relation between E and G

Answer 61

delta G = - nFE

Answer 62

this inplies, for a spontaneous reaction, delta E = positive, isn't it??

so in these reactions, which you say equilibrium reaction, one side is a spontaneous reaction and a non- spontaneous reaction is the reverse reaction..

but still you may call it an equilibrium reaction. but keep in mind.
out of the forward and backward reaction there, one is spontaneous and the other is non- spontaneous

Answer 63

yea, but what i mean is, electrode potential is created when you dip an element into its ion. (i.e a zinc bar into an aqueous solution of zn 2+ . and equilibrium will be establish between zn and zn 2+ . thus depending on the side in which the equilibrium lies, it determines the potential created at the electrode)

Answer 64

ah... so can i just ignore the equilibrium they showed me and just say that the half cell of iodine is, 1M of I3- and 1M of I-?

Answer 65

yes you certainly can

Answer 66

because I3- --->3I- + 2 e ?

Answer 67

yes. this is the equation

Answer 68

ahh haha, ok, i think i have a slightly clearer picture =) THX anish!

Answer 69

is it??

:-) but all my concepts are shaken, by this question of yours ,, :-P

Answer 70

agreed! this question broke all my foundation of electrochemistry down to the point that i have to reread electrochemistry into more detail.. that how i realise about the equilibrium thing @.@

Answer 71

but still.. it doesnt flow.. T.T

Answer 72

i would try to help.. tomorow,
for its late now. it's just half and hour to midnight.

i must go to bed rite now,
and i will get back to you on this question tomorow .. OK??

Answer 73

OK! =)

Answer 74

i spent 2 days on this question alrdy XD

Answer 75

2 days..?/

i think,,, 1 more day you need to invest ,,, to get on with the ques.

Answer 76

to get done with the ques,

Answer 77

yea haha! i want to end it by tmr!!! ahhh

Answer 78

2.08 am here @.@ i need to get some sleep too haha

Answer 79

cya tmr! goodnight

Answer 80

2??

its too late ....

Answer 81

okay then ezizio.. gnyt .., this is all i can say as of now.
sleep well
and we will do the question tomorow.

Answer 82

yeap =)

Answer 83

at anode (oxidation reaction)
we know that I2 will react at anode. but we don't use I2 (solid) in the electrochemical cell, but use a solution. So, in order to get a solution of I2, we use KI. in which the Iodine is soluble as per the equation given in the question.

you must note here, that the solution is a solution of KI and, the electrode dipping in the solution is a solution of I2 in KI... i.e. we may say I3- is the potent species that can undergo oxidation here.

and this electrode of I3- dipping in the solution of I-, will have some electrode potential associated... the value can be seen from the pdf.

Answer 84

the equilibrium reaction in the question does not involve loss or gain of electrons.
it has no potential involved. This equilibrium just shows the formation of a potent species, which can loose electron easily and which is soluble too.

this solubility can and can not be an equilibrium reaction.
you would never hear from anyone that, a reaction showing solubility of something is a one way reaction ALWAYS.
i can give you an example of solubility of salts, which are not one - way reactions, but are equilibrium reactions.
SOLUBILITY PRODUCT ... you might have heard of it..

Answer 85

to summarize my point.

firstly, at anode, a more potent and soluble species is formed, which ultimately establishes a potential with the solution ( KI solution)

and i think you can work out the process for Cl2. that won't involve all these nuisance which I2 involved.

then calculate the emf of the resulting cell.
if it comes out to be positive, then you can conclude that I2 has more oxidizing power than Cl2.

Answer 86

pls explain the anode =/ how does the formation of I3- by dissolving I2 in I- create a potential?

Answer 87

it's not the formation of I3-, but when I3- is in the solutiion of I- (i.e. KI) this creatsa a potential difference.

Answer 88

is this because I3- + 2e <----> 3I- ??

Answer 89

yes yes.. exactly because of this equill. between I3- and I-

Answer 90

hey, sorry i havent got the time to thank you, been very busy revising this couple of days. Anyways, thank you so much anish! i've checked with my chemistry tutor, he said that the A level paper did not show the equill of I3- and I- because they wanted to "simplify" the question haha..