Why does the reaction between acid and aluminium start slow and then speed up?

[rahmatkat]

Can anyone help me with this homework question? it was like this:

The reaction between the acid and aluminium is slow in the beginning and then gets faster. Give an explanation for this observation.

Thanks.

 

[Borek]

Do you have ideas of your own?

 

[rahmatkat]

Well, my idea is that it has a high melting and boiling point and its resist corrosion, unfortunately my answer above is wrong and i don't have any other idea why it reacts slowly with acid at first and then faster.

 

[Borek]

It has nothing to do with melting points. What do you know about aluminum reacting with oxygen?

 

[rahmatkat]

Oh, I get it now, aluminium react with oxygen gas in the air and produce Aluminium Oxide forming in the outside part of an aluminium metal right? So when that aluminium metal is reacted with an acid, Aluminium Oxide reacts first with the acid and when it finished reacting with it, the acid then react with the aluminium which being coated by Aluminium Oxide.

 

[symbolipoint]

You started your answer (post #5) well and then you smashed your logic as you progressed.

 

[rahmatkat]

Oh,
So Aluminium Oxide did form?
As for the reaction, the oxygen present in the Alumium Oxide react first with the acid and when that Oxygen dissapear, the acid then react with Aluminium, is that rihgt?

 

[symbolipoint]

You must be on the correct track. Aluminum is a reactive metal, but Aluminum oxide is much less reactive. I am not myself fully in tune with the way the reaction with acid should progress; but the acid solution first meets the oxide, and then meets the mostly unreacted aluminum (which is then in contact to react).

 

[Borek]

You are homing on the target. Slow part - reaction with oxide, fast part - reaction with bare metal.

 

[rahmatkat]

Thanks guys, I think I get it now.