In the equation:
2Al+6HCl-->2AlCl3+3H2
Why is the hydrogen by itself instead of the chlorine?
Al is above hydrogen in the activity series (sometimes called the electromotive force series); therefore, it replaces H. This is single replacement type reaction.
Isn't Al above Cl too?
Yes, it is. And for exactly that reason, it is easier to oxidize Al than it is to oxidize Cl^-. So Al is oxidized, H^+ is reduced (to H2 gas) and the Cl^- is merely a spectator ion.
2Al + 6H+ ==> 2Al+++ + 3H2
That's why Cl shouldn't be alone?
Yes. That's why Cl^- stays as is.
Why is Al easier to oxidize?
I suppose the easy answer is "that's the nature of the beast." But that isn't really an answer. The question is much like "Why do we call a car a car or a horse a horse or a cow a cow?" Because someone named them that. The best answer I can give as to why Al is easier to oxidize is that the three outside electrons on the aluminum atom can be stripped MUCH easier (leaving the Al+++) than it is to strip electrons from a chloride ion (especially with the ion already bearing a -1 charge). Another way of saying it is that it is MUCH easier for Cl2 (gas) to act as an oxidizing agent (thereby being reduced) than it is for the reaction to proceed in the other direction (from Cl- to Cl2. We can ask "Why" for that too, and the answer is "that's an exerimental fact."
thanks