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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:

  1. A magnesium atom reacts with a $\ce{H+}$ ion: $\ce{Mg + H+ -> Mg+ + H}$

  2. The $\ce{Mg+}$ intermediate produced reacts with another $\ce{H+}$ ion: $\ce{Mg+ + H+ -> Mg^2+ + H}$

  3. 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).

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  • $\begingroup$ Please visit this page, this page and this ‎one on how to format your posts better.‎ Alternatively, visit this chatroom for further formatting guidance. $\endgroup$ – M.A.R. ಠ_ಠ Jan 10 '16 at 13:19
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    $\begingroup$ I suspect the reaction mechanism is much more complicated than what you have proposed. Have tried searching any publications? If so it would be helpful if you could include this into your post. For example, protons in aqueous environment are hydrated and even $\ce{H3+O}$ is already an approximation. I also doubt the existence of hydrogen atoms in liquid phase. From there on in it probably gets even more complicated. $\endgroup$ – Martin - マーチン♦ Jan 10 '16 at 13:32
  • $\begingroup$ In a similar question on this site it is proposed that the HCL dissociates in water to form H3O+ ions which then oxidize the magnesium. $\endgroup$ – Technetium Jan 10 '16 at 14:30
  • $\begingroup$ What rate law do you expect from your proposed mechanism(s)? (Remember to replace the concentration of any intermediates with terms based on their generation, so the final expression only contains substrate concentration terms.) $\endgroup$ – R.M. Jan 10 '16 at 20:54
  • $\begingroup$ Martin: I can't find any official publications on the mechanism unfortunately :( I've tried to think of a mechanism involving H3O+, however, this just ends up with the same general mechanism as in the opening post. It also leaves 2H2O on the right hand side of the equation when cancelled down (is having H2O when cancelled down allowed?). Joel: HCl is already dissociated in solution so is not in the mechanism. I believe the Mg does not get oxidised straight away and instead is done via multiple/compicated steps e.g. intermediates. (Although it would conform to my mechanism, I think(!)) $\endgroup$ – Henry Jan 12 '16 at 17:49

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