2005 HSC electrochem question (1 Viewer)

[Looking Glass]

Question 29
(b) (ii) "A student set up a laboratory experiment in which a beaker contained a strip of magnesium, an iron nail, and a solution of iron(II) sulfate and magnesium sulfate. This was left for several days.

Describe what would happen in this experiment. Include relevant equations in your answer." (2005 HSC)

"[...]The better responses gave detailed observations of the displacement reaction occuring on the magnesium strip and the corrosion processes occuring on the iron nail[...]" (2005 Notes from the Marking Centre)

Is this incorrect or am I missing something? I was of the opinion that the Mg strip will corrode on account of having a higher oxidation potential.

If it's not too much trouble, perhaps somebody could write up a sample answer to this.(?) I'm assuming there's a flaw in my thinking and not in the marker's comments.

Thanks for any help.

 

[brenton1987]

Yes the marking guide is wrong.

The magnesium strip will dissolve and iron would deposit on the nail. The green solution colour will fade as the Fe²⁺ ions are reduced.

The iron is being displaced from the solution by magnesium.
Mg_(s) --> Mg²⁺_(aq) + 2e^-
Fe²⁺_(aq) + 2e^- --> Fe_(s)
---
Mg_(s) + Fe²⁺_(aq) --> Mg²⁺_(aq) + Fe_(s)

 

[Looking Glass]

Thanks for the speedy reply. Dammnit, though. The topic is hard enough without paid professionals failing to run any kind of quality control on their marking comments!

One further question, though.

How do the electrons get from the Mg to the nail? They don't just travel through the water, right? The ions in the electrolyte carry them. I don't see how the MgSO4/FeSO4 would facilitate what a salt bridge normally would. How is the charge balanced out in this cell?

Thanks again.

 

[wrxsti]

electrochem is fun,