Sciencemadness Discussion Board
Not logged in [Login ]
Go To Bottom

Printable Version  
Author: Subject: Redox reaction explained incorrectly?
Eddygp
National Hazard
****




Posts: 858
Registered: 31-3-2012
Location: University of York, UK
Member Is Offline

Mood: Organometallic

[*] posted on 5-4-2015 at 03:52
Redox reaction explained incorrectly?


So I was in one of these boring chemistry demonstrations just after the students had learned how redox reactions work. The teacher explains that he is going to mix a solution of copper(II) sulphate with aluminium foil to yield aluminium(III) sulphate and copper metal, in the presence of chloride ions.

Up to this point, everything seems rather normal and straightforward. However, when he mixes the substances in the solution, I notice that, while copper is evidently being generated, gas bubbles are also formed, with that characteristic smell of SO2. I therefore realised that the reaction must not be

2Al + 3Cu2+ ---> 2Al3+ + 3Cu

but most likely is something similar to the following one:

8H+ + 2Al + 5Cu2+ + 2SO42+ ---> 2Al3+ + 5Cu + 2SO2 + 4H2O

(notice that the 8H+ imply that this is in an acidic solution)
(also, notice that the 3 "missing" sulphate ions are spectator ions)

This would imply that the aluminium is capable of reducing sulphur(VI) to sulphur(IV)... but I find no other possible explanation for the evolution of the sulphur dioxide gas.

Are there other common redox reactions in which the baccalaureate-level explanations and the practice have significant inconsistencies?

Edit: the formats do not seem to work :/

[Edited on 5-4-2015 by Eddygp]




there may be bugs in gfind

[ˌɛdidʒiˈpiː] IPA pronunciation for my Username
View user's profile View All Posts By User
blogfast25
International Hazard
*****




Posts: 10562
Registered: 3-2-2008
Location: Neverland
Member Is Offline

Mood: No Mood

[*] posted on 5-4-2015 at 06:37


The gas generated is almost certainly mainly hydrogen, not SO<sub>2</sub>. But hydrogen generated when metals dissolve often has a peculiar smell, that could easily be mistaken for a sulphurous smell.

'On paper', aluminium can indeed reduce sulphate ions to sulphite ions in acid conditions (reduction potential = 0.2 V). In reality that doesn't seem to happen, for instance Al metal with a sodium sulphate solution will not yield any reaction. In watery medium, sulphates are incredibly hard to reduce and have no oxidising properties at all. Hence also use of sulphuric acid as a non-oxidising acid.

The reduction of cupric sulphate by Al in the absence of chloride ions has also been discussed elsewhere on this forum and is quite slow, without any reaction of the Al with the sulphate anions.


[Edited on 5-4-2015 by blogfast25]




View user's profile View All Posts By User
Metacelsus
International Hazard
*****




Posts: 2539
Registered: 26-12-2012
Location: Boston, MA
Member Is Offline

Mood: Double, double, toil and trouble

[*] posted on 5-4-2015 at 06:47


Quote: Originally posted by Eddygp  

8H+ + 2Al + 5Cu2+ + 2SO42+ ---> 2Al3+ + 5Cu + 2SO2 + 4H2O

(notice that the 8H+ imply that this is in an acidic solution)
(also, notice that the 3 "missing" sulphate ions are spectator ions)


Sulfate is negatively charged, not positively. Thus, the second equation is balanced wrong.

The situation you proposed is equivalent to:
2 Al + 3 Cu2+ ---> 2 Al3+ + 3 Cu
and
2 Al + 12 H+ + 3 SO42- = 3 SO2 + 6 H2O + 2 Al3+
happening at the same time.

Let me find my reduction potential tables, and see if the reaction is spontaneous.

Edit: Ninja'd! Apparently the reaction is spontaneous, but the kinetics are unfavorable.

[Edited on 5-4-2015 by Cheddite Cheese]




As below, so above.

My blog: https://denovo.substack.com
View user's profile View All Posts By User
blogfast25
International Hazard
*****




Posts: 10562
Registered: 3-2-2008
Location: Neverland
Member Is Offline

Mood: No Mood

[*] posted on 5-4-2015 at 07:20


Quote: Originally posted by Cheddite Cheese  
Let me find my reduction potential tables, and see if the reaction is spontaneous.

Edit: Ninja'd! Apparently the reaction is spontaneous, but the kinetics are unfavorable.



'Spontaneous' is a very misleading term here. Aqueous sulphates are basically impossible to reduce, so the reduction reaction is hardly 'spontaneous'.

If the cell potential of a redox reaction Ecell = Eox + Ered > 0 that means that ΔGreaction < 0 and the reaction is thermodynamically favourable. But thermodynamics makes no pronouncements about kinetics. Here, the reduction of aqueous sulphates doesn't happen, depite the reaction being thermodynamically favourable.

Al can reduce sulphates in non-aqueous conditions, for instance stoichiometric mixtures of calcium sulphate and aluminium powder burn very hot, according:

CaSO4 + 8/3 Al === > CaS + 4/3 Al2O3 (S reduced from +6 to -2)

But the enthalpies and Free Energy involved are very different in that case.

[Edited on 5-4-2015 by blogfast25]




View user's profile View All Posts By User

  Go To Top