When you put $\ce{Mg}$ into water a few $\ce{H2}$ bubbles appear. But when you put $\ce{Mg}$ into a $\ce{NaCl}$ solution there is a vigorous release of $\ce{H2}$, why is this and what reactions are taking place?

  • 1
    $\begingroup$ My first thought is that the salt is providing nucleation sites for $\ce{H_{2}}$, but I wasn't able to find definitive support with modest searching. I could be totally wrong. Do you have a reference or two that you can share, or is it your observation you are sharing? $\endgroup$
    – Todd Minehardt
    Aug 6 '15 at 16:33
  • $\begingroup$ It might be wrong but my hypothesis is different from the actual answer. When magnesium hydroxide reacts it forms a protective layer above pure magnesium, as in passived aluminum. When NaCl is added it increases hydroxide solubility (because of activities and so on); as a consequence, makes the reaction faster... $\endgroup$
    – user43021
    Oct 13 '18 at 0:57

It would still be the same reaction.

\begin{align} \ce{Mg (s) &-> Mg^2+ + 2 e-} & E^\circ&=\pu{2.36 V}\\ \ce{2 e- + 2H2O (l) &-> H2 (g) + 2OH-} & E^\circ&=\pu{-0.828 V} \times 2 \end{align}

The combined reaction would be spontaneous ($E > 0$), but $\ce{Mg(OH)2 (s)}$ is highly insoluble, \begin{align} \ce{Mg(OH)2 (s) &<=> Mg^2+ (aq) + 2OH- (aq)}, & K_\mathrm{sp}=7.1 \times 10^{-12}, \end{align} leaving the solution insufficient ions to conduct the electrons necessary to fast complete the reaction (despite of the escaping hydrogen gas). $$\ce{Mg (s) + 2H2O (l) -> H2 (g) + Mg(OH)2 (s)}$$

This is like an electrochemical cell with no salt bridge (or bridge with only de-ionized water). Hence a little salt ions would accelerate the reaction forward.

In contrast, calcium solid, having comparable standard reduction potential, will undergo a similar redox reaction in water and needs no help for additional ions because $\ce{Ca(OH)2 (s)}$ is about a million times more soluble ($K_\mathrm{sp}=6.5 \times 10^{-6}$) than magnesium hydroxide, providing sufficient free ions in the solution to drive the vigorous release of hydrogen gas. (There are a couple of YouTube video demos.)

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    $\begingroup$ Calcium hydroxide is not a million times more soluble than magnesium hydroxide. The solubility product is a million times larger, yes, but with three ions forming this is proportional to the cube of the molarity. The real molar ratio is thus about 100 to 1, but that is still enough to make a big difference in reactivity. $\endgroup$ Oct 12 '18 at 11:32
  • $\begingroup$ Where in the original question is there mention of calcium? You have not answered the question. $\endgroup$
    – Waylander
    Oct 12 '18 at 16:35

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