BCC - balancing chemical equations, stoichiometry

[c_d]

Hi,

I'm trying to write an equation to show what happens when mercury liquid (Hg) is placed on a block of aluminium (Al). But I'm having some problems. Here's my current line of thinking (I haven't yet worked out any subscripts so I will just use the atomic symbols):

Al has an aluminium oxide layer, which is denoted by AlO. Do I include this aluminium oxide in the left hand side of the equation? The way I see it is the mercury is being added to both the aluminium and the aluminium oxide. And do I add the oxygen in the air to the left hand side. The oxygen in the air will react with the aluminium.

Hg + O + Al + AlO -> Some result

As for the result I'm not sure what it should be. The mercury never bonds with the aluminium does it? I think that the mercury reacts with the aluminium oxide (is AlO a gas or a solid):

Hg(s) + AlO(?) -> Hg(s) + Al(s) + O(g)

And then the oxygen reacts with the aluminium to form aluminium oxide again:

Al(s) + O(g) -> AlO(?)

From my working out it the reaction never ends. So I've either made a great discovery or I've made a mistake . Can someone give me some pointers as to the final result?

Thanks.

 

[DrMark]

Simply look at the reduction potentials for the metal/metal oxides involved.
BTW it's Al₂O₃ and Hg can be either Hg⁺ or Hg ²⁺.

 

[Borek]

DrMark: it is probably about metallic Hg, so it is just Hg

c_d - AFAIK Al doesn't react with Hg. What is happening is that Al is dissolved in Hg and oxidized to Al2O3 on the mercury surface. Now, if the Al2O3 appears on the aluminum surface it protects metal from further oxidation. But when it appears on the mercury surface it is of no use and it doesn't prevent further reaction.

I have no idea whether Al2O3 reacts with mercury. IMHO no, but I was told recently that it does.


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[pack_rat2]

Hg will not react with Al2O3. Al is very electropositive and won't easily let go of any oxygen atoms its bonded to. What happens when Hg contacts Al (and many other metals, as well) is amalgamation, which is like forming a solution. It's a physical, not chemical, process.

 

[c_d]

Hg will not react with Al2O3. Al is very electropositive and won't easily let go of any oxygen atoms its bonded to. What happens when Hg contacts Al (and many other metals, as well) is amalgamation, which is like forming a solution. It's a physical, not chemical, process.

That's interesting. I did some more googling and found these 2 equations, which match what you say, but they don't mention the aluminium oxide:

Hg + Al -> AlHg (1)

The amalgam is weaker than Al and therefore water does it more damage.

2AlHg + 6H2O -> 2Al(OH)3 + 2Hg + 3H2 (2)

The Hg generated would circulate to attack Al again.

So what happens to the aluminium oxide in the amalgamation process?

The reason I'm asking this is because I have some mercury and I want to demonstrate its effect on aluminium. Some websites describe a violent reaction, but so far I don't believe this to be the case.

 

[Borek]

In water you may get (at first) Al(OH)3. On the air you will get Al2O3.

We did it many years ago - take two aluminium coins, put small mercury drop between them (1 mm diameter will be enough) and rub them. Reaction is not violent, but you should be able to see growing deposit of aluminum oxide. Not for long, as mercury is mechanically taken from the surface by growing oxide so the reaction eventually comes to an end.

On our coins reaction didn't start without rubbing - probably becasue aluminum was passvated with oxide which have to be removed mechanically, that's where the rubbing part comes into play.

Chemical calculators for labs and education

BATE - pH calculations, titration curves
CASC - concentration conversions, solution preparation

 

[r3dxP]

Hg will not react with Al2O3. Al is very electropositive and won't easily let go of any oxygen atoms its bonded to. What happens when Hg contacts Al (and many other metals, as well) is amalgamation, which is like forming a solution. It's a physical, not chemical, process.

check the activity series, if Hg is above Al, it would react as a single replacement, otherwise, like pack_rat2 said, it will not react.

 

[dextercioby]

That's interesting. I did some more googling and found these 2 equations, which match what you say, but they don't mention the aluminium oxide:



So what happens to the aluminium oxide in the amalgamation process?

The reason I'm asking this is because I have some mercury and I want to demonstrate its effect on aluminium. Some websites describe a violent reaction, but so far I don't believe this to be the case.

Al+Hg->AlHg

is NOT a chemical reaction.And

Al+3H_{2}O\rightarrow Al(OH)_{3}+3H_{2}\uparrow

is impossible at room temperature and normal pressure.

Daniel.

 

[Borek]

Al+3H_{2}O\rightarrow Al(OH)_{3}+3H_{2}\uparrow

is impossible at room temperature and normal pressure.

Can you elaborate? I can't believe Al will survive in water and will not get passivated.

Chemical calculators for labs and education

BATE - pH calculations, titration curves
CASC - concentration conversions, solution preparation