reactivity series ('o' levels)

donkhead333 SG

so..i had this question (will try to describe as clearly as possible, cant find diagram)

2 beakers, A and B, contains a bar of zinc and iron each along with indicators phenolphthalein(detects alkaline solution) and potassium hexacyanoferrate (detects presence of Fe2+: will turn blue, otherwise colorless). the metals are linked by a wire and a voltage is observed. a salt bridge connects the 2 beakers together, with the salt bridge being partially submerged in both beakers' indicators.

the question was "would any color change be observed at the iron electrode?EYA"

so, i reasoned out this way

zinc is more reactive than iron.

zinc will reduce iron, so iron will gain electrons.

Fe + 2e- -> Fe2- (wtf?)

the answer is that color change will be detected (meaning there is Fe2+ present), but im quite lost as to how Fe2+ is actually obtained......

pls feel free to clear any misconceptions, i'm not very adept with this topic

SBS n SMRT SG

Zn at one electrode and Fe at the other electrode. Zn will lose electrons to the circuit to form Zn2+ ions. Zn is more reactive, so it will react with the salt (let say FeCl2) (This question had a problem , the salt is not given, if the cation in the salt is more reactive than Zn, nothing will occur) If it is FeCl2, Zn will displace Fe out of the salt to form ZnCl2, and Fe will be "kicked out", and since the salt is aqueous, H+ and OH- will be present also, therefore, the kicked out Fe will react with the OH- ions to form Fe(OH)2, O.S of Fe in Fe(OH)2 is +2, so Fe2+ will be present, and the H+ ions will gain electrons at the Fe electrodes to form hydrogen gas

Equation at the Zn electrode

Zn (s) ---> Zn 2+ + 2 e

At the electrolyte

FeCl2 + Zn 2+ --> ZnCl2 + Fe2+

Fe2+ + OH- -->Fe(OH)2 + H+

At the iron electrode

H+ + e- --> H2

I hope this is correct

I had assume the salt to be FeCl2 . I think this question can only be done if the salt is a Iron (II) salt. or lower in reactivity series.

If it is other salt lower than iron

this will happen

all the same for zinc, instead of the kicked out iron reacting with the OH ions, the kick out cation from the salt compund will remain a solid while the iron electrode will dissolve and react with the chloride.

I hope i am correct. Ultima or others can correct me...thxs

dkcx SG

The question is indeed flawed with no electrolyte given unless we assume its the indicator in water solution as electrolyte.

WIthout enough info and we just base on redox, Zn being more reactive will be oxidised to Zn2+ and a reduction reaction will happen at the Fe electrode. If thats the case, no Fe2+ ions should be formed at all and no colour change should be observed at the iron electrode

UltimaOnline SG

'O' & 'A' Level Qn.

so..i had this question (will try to describe as clearly as possible, cant find diagram)

the question was "would any color change be observed at the iron electrode?EYA"

so, i reasoned out this way

zinc is more reactive than iron.

zinc will reduce iron, so iron will gain electrons.

Fe + 2e- -> Fe2- (wtf?)

the answer is that color change will be detected (meaning there is Fe2+ present), but im quite lost as to how Fe2+ is actually obtained......

pls feel free to clear any misconceptions, i'm not very adept with this topic

UltimaOnline's comments :

The question is obviously incomplete (thus the answer is open to possibilities) because the electrolytes in both compartments (connected by the salt bridge) are not specified.

If instead, we modify the question to :

2 beakers, A and B, contain a bar of zinc and iron each along with indicators phenolphthalein (detects alkaline solution) and potassium hexacyanoferrate (detects presence of Fe2+: will turn blue, otherwise colorless). The metals are linked by a wire and a voltage is observed. A salt bridge connects the 2 beakers together, with the salt bridge being partially submerged in both beakers' indicators. A colour change was observed at the cathodic electrolyte. Suggest and explain two possible scenarios (including identities of electrolytes) that would allow this.

Solution :

One possible scenario, would have the cathodic electrolyte as an aqueous Fe3+ solution.

Assuming unimolarities (ie. standard conditions), the standard reduction potential of the cathodic half equation Fe3+ + e- --> Fe2+ is +0.77V; the standard oxidation potential of the anodic half equation Zn --> Zn2+ + 2e- is +0.76V; hence a current would certainly flow and the standard cell potential would be +1.53V.

3[Fe(CN)₆]^3- + 4Fe²⁺ --> Fe₄[Fe(CN)₆]₃

Prussian Red --> Prussian Blue

In such a scenario, the colour change at the cathodic electrolyte would be due to Prussian Red reacting with Fe2+(aq) to generate Prussian Blue.

Another possible scenario would have the cathodic electrolyte as an aqueous acidic solution.

Assuming unimolarities (ie. standard conditions), the standard reduction potential of the cathodic half equation 2H+ + 2e- --> H2 is +0.00V; the standard oxidation potential of the anodic half equation Zn --> Zn2+ + 2e- is +0.76V; hence a current would certainly flow and the standard cell potential would be +0.76V.

Notice that H+ ions came from the solvent water, generating OH- ions as a consequence.

In such a scenario, the colour change at the cathodic electrolyte would be due to phenolphthalein reacting with the OH- ions; specifically, di-deprotonation and structural change from a non-conjugated, tetrahedral center and absorbing UV frequencies (hence colourless) to a highly conjugated, trigonal planar center and absorbing blue-green frequencies (hence pink/fuchsia).

Further reading on the fascinating chemistry of

PrussianRed / PrussianBlue :

http://en.wikipedia.org/wiki/Prussian_Red

http://en.wikipedia.org/wiki/Prussian_blue

Phenolphthalein :

http://en.wikipedia.org/wiki/Phenolphthalein

http://www.science.ca/askascientist/viewquestion.php?qID=534

http://antoine.frostburg.edu/chem/senese/101/features/water2wine.shtml

dkcx SG

Oh yeah forgot abt the phenolphthalein which will turn pink with the OH after reduction of H+ to H2

donkhead333 SG

opps, sorry forgot to specify, but the electrolyte is just water with the indicators. the salt is just described as "salt bridge", no specification as well.

er..ultima..sorry but i don't understand what you are trying to explain..a little too cheem for me

i assumed that the zinc and iron bar would just be Zn and Fe, without their charges, because that is how the question labelled those 2 metals as.

so if Fe undergoes reduction..which should happen because zinc is more reactive, Fe will never be able to attain Fe2+

UltimaOnline SG

opps, sorry forgot to specify, but the electrolyte is just water with the indicators. the salt is just described as "salt bridge", no specification as well.

er..ultima..sorry but i don't understand what you are trying to explain..a little too cheem for me

i assumed that the zinc and iron bar would just be Zn and Fe, without their charges, because that is how the question labelled those 2 metals as.

so if Fe undergoes reduction..which should happen because zinc is more reactive, Fe will never be able to attain Fe2+

Solid iron electrode is in reduced form, hence cannot be reduced further, as you yourself said. Hence, H+ ions from water is reduced at the cathode, generating OH- ions, which causes the phenolphthalein to change coloour.