Balanced equation:
$$\ce{Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g)}$$
However, Cl- is a spectator ion so the actual reaction equation is:
$$\ce{Mg + 2H+ -> Mg^2+ + H2}$$
Here is my proposed reaction mechanism:
A magnesium atom reacts with a $\ce{H+}$ ion: $\ce{Mg + H+ -> Mg+ + H}$
The $\ce{Mg+}$ intermediate produced reacts with another $\ce{H+}$ ion: $\ce{Mg+ + H+ -> Mg^2+ + H}$
The two hydrogen atoms (from step 1 & 2) react together: $\ce{H + H -> H2}$
However, my problem arises when I compare this to my rate law that I found experimentally ($\ce{Mg}$ is not in the rate law as it is a solid and has no concentration):
$$\mathrm{rate} = k[\ce{H+}]^2$$
None of these mechanism steps above conform to my observed rate law (or do they?), could any of you enlighten me to any other mechanisms?
There are only two mechanisms I can think of: a one-step mechanism where it is literally $\ce{Mg + 2H+ -> Mg^2+ + H2}$ (though I've read that a one-step reaction is unlikely) and the other way is via an intermediate (as described above).