Rusting of Iron Nail

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[Patrick Chai Teck Khiong]

May someone please clarify my doubt..
Experiment to investigate different electropositivity of metal on rusting
Test tube A : Iron nail only
Test tube B : Iron nail coiled with magnesium
Test tube C : Iron nail coiled with copper
According to the reference Book Focus Pelangi, pink colouration is present for all the test tube.
My doubt is "Why pink colouration is present in test tube A"?
As i know, when oxygen and water is reduced to form hydroxide ion, it will immediately combine with iron(II) ion to form rust. Thus, pink colouration is absent.
Am i correct?

[Wong Hee Ting]

I would like to share my opinion:

The pink colouration is present but is very low.
Reason:

O2 + 2H2O +4e --> 4 OH-

The OH- then combine with Fe2+ to form Fe(OH)2.

So the concentration of OH- becomes low. the low intensity of pink colouration.

I think the best way is we do the experiment and see the result ourselves.

[Patrick Chai Teck Khiong]

Yes, ur explanation is correct. But the quantity of OH- ion formed in this test tube is vry low coz the process of rusting here is slow. In other hand, OH- ion formed is immediately combine with Fe²⁺. tHUS THE PINK COLOURATION is not clear seen.
To conclude, ya we btter do experiment ourselves to see the result..Hahahaha

[Ling Teck Ping]

from my point of view,pink color still can be observed is due to jelly solution.the jelly slow down the diffusion.

[Wong Hee Ting]

I'd tried that day.

The intensity of pink colour is very-very low and almost can be ignored.

[Patrick Chai Teck Khiong]

Thanks Mr.wong hee ting..Then is it we can tell the student to write  intensity of pink colour is absent?

[Jong Kah Yin]

There are 2 ways to explain the results of the experiment:
1st explanation:  For Mg/Fe
At the anode: Magnesium . The oxidation of magnesium to Mg2+ ions
    Mg(s)  ->   Mg²⁺(aq)   +  2e                                           
Thus, magnesium are corroded instead of iron.
   At the cathode: Iron nail  Water molecules dissociate to form hydrogen ions (H+) and hydroxide ions (OH-).
           H₂O (l)   ->     H⁺(aq)   +       OH-(aq) 
Hydrogen ions accept electrons and are reduced to hydrogen gas.
      2 H⁺ (aq)  +  2e   ->   H₂(g)   Reduction
When hydrogen ions are discharged to form hydrogen gas, relatively the concentration of hydroxide ions in water increases. Consequently, the area around the iron nail becomes alkaline and causes the colour of phenolphthalein to change from colourIess to pink     
This method can explain the presence of pink colour and also the gas produced

2nd explanation : At the anode: Magnesium . The oxidation of magnesium to Mg²⁺ ions
    Mg(s)  ->   Mg²⁺(aq)   +  2e                                           
   At the cathode: Iron nail 
      2H₂O  +   O₂  +  4e  ->  4OH^-   } alkaline
This method can only explain why pink color is present but not able to explain the presence of gas. This is the current explanation used in the text book

The pink color intensity differs in each of the test-tube because of the difference in the electropositivity of metals. Magnesium being more electropositive, will release more electrons, hence producing more OH^-ions as compared to Fe (lower pink colour intensity)

Please feel free to comment as it would encourage us to explore further and exchange opinions, there maybe others who can enlighten us on this matter.
Thank you

[Patrick Chai Teck Khiong]

I use Mr.Jong 1st explanation when i deliver to my student.
But how about the control apparatus which contain iron nail only? The pink colouration is present or absent?
I told to my student is absent because when OH- formed from the reduction of water molecule will immediately combine with Fe²⁺ ion to form iron(II) hydroxide. Thus the pink colouration is absent. Thanks.

[Jong Kah Yin]

Hi,
Personally I would like to state that all test tubes will have pink colour, just that the intensities of pink colour differs. By the same argument as in test tube A (Fe only), then in test-tubes containing Mg/Fe , Zn/Fe and Sn/Fe and others, then they should not form pink colour as they will form metal hydroxides too.
That is my opinion

Any comments on that?

[Patrick Chai Teck Khiong]

Hi, Just want to share my opinion:)
For test tube (Fe/Cu) : When iron coiled by copper, electrochemical cell is build whereby Fe is the anode and copper is the cathode. Iron loses electron to form iron(II) ion and electrons flow to the copper. Electrons are received by hydrogen ion from the water leaving behind a lot of hydroxide ions. At the same time too, water and oxygen in the jelly solution also reduced to form hydroxide ions. Then most of the hydroxide ions combine with iron(II) ions to form Fe(OH)₂. Hence, intensity pink colour shown is low.

For test tube (Fe only) : The rusting of iron nail is not speeded up. This is a natural process whereby iron loses its electron to form iron(II) ions , and O₂ and H₂O are reduced to form hydroxide ions. Once hydroxide ions are formed, it immediately combine with Fe²⁺ to form iron(II)hydroxide. Hence, the presence of pink colour can be ignored.

Thank you.

[Wong Hee Ting]

Thank you for the explanation, Mr Jong & Mr Patrick.

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