How is the reaction between Aluminium and Mercury Chloride possible?

Aluminium is coated with a thin layer of Aluminium Oxide. Why then do we write the reaction as



> 
> $$\ce{2Al + 3HgCl2 → 2AlCl3 + 3Hg}$$
> 
> 
> 


Shouldn't we first write an equation showing how $\ce{HgCl2}$ reacts with $\ce{Al2O3}$ ?




The equation is correct assuming the aluminium is pure. In reality, $\ce{HgCl2}$ won't readily react with aluminium sheet since both are solids and $\ce{HgCl2}$ is a *weakly-dissociated* salt meaning that even in aqueous environment there won't be much dissociation into $\ce{Hg^2+}$ and $\ce{Cl^-}$. The molecule would simply go into solution as is.


Maybe there will be reaction if you mix aluminium and mercury(II) chloride powders and heat the mixture - still there will be formation of mercury(II) oxide rather than metallic mercury:


$$\ce{Al2O3 + 3 HgCl2 -> 2 AlCl3 + 3 HgO}$$


I think the reaction really goes off in aqueous solution or at least moist environment, where some mercury(II) chloride gives acidic reaction and we get some hydrogen chloride that will readily dissolve the oxides, allowing for the displacement reaction.


Note that aluminium amalgam is commonly prepared by first scratching the metal surface under water or *under mercury* (as otherwise it would immediately re-oxidize to alumina), grinding the two metals together or first etching the metal with dilute $\ce{HCl}$ or $\ce{NaOH}$. Under water, the aluminium is protected from air and will form hydroxide rather than oxide, which reacts much faster with aqueous $\ce{HgCl2}$.


Indeed aluminium with thick oxide layer reacts very slowly with $\ce{HgCl2}$. It eats away the oxide layer but at the cost of some dissolved mercury(II) turning into insoluble $\ce{HgO}$. There is a neutralization going on rather than displacement plus other side reactions (like formation of insoluble calomel - $\ce{Hg2Cl2}$, formation of oxychlorides etc.).


The typical procedure for preparing aluminium amalgam is therefore properly degreasing and etching aluminium with solvents and dilute base first, then washing off excess solvent, hydroxides and base (the metal is still wet) and *then* getting it into contact with dilute mercury(II) solution (which can now be very dilute as we don't waste $\ce{HgCl2}$ for the etching).


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