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According to this textbook, H₂O has lower reduction potential (-0.83 V) than Zn (-0.76 V). This mean zinc is unable to reduce water to hydroxide. If so, why this video demonstrate that by heating zinc in water, a reaction occurs producing hydrogen gas from the bubbles while also turning wet red litmus paper blue?

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    $\begingroup$ The given value of −0.83 V corresponds to pH = 14. You should also be able to find the value for pH = 0 in your list, and then estimate what happens with water at a roughly neutral pH $\endgroup$ – Loong Mar 10 at 20:49
  • $\begingroup$ Ok so i found out the answer, zinc cannot directly reduce water to form zinc hydroxide because it has lower reduction potential (-0.76V) than water (-0.83V). Zinc does not react with cold water, however it reacts with hot steam or boiling hot water. This is because high temperature increases water autoionization, thereby increasing the presence of H⁺ and OH⁻. So, zinc is actually reducing H⁺ ion to H₂ because H⁺ ion has higher reduction potential. The Zn²⁺ then reacts with OH⁻ to form Zn(OH)₂ $\endgroup$ – snowwatermelon Mar 11 at 6:53

I've seen similar demonstrations with magnesium, and in both cases the explanation is the same. The metal reacts to produce the hydroxide plus hydrogen; with zinc this would be

$\ce{Zn + 2 H2O -> Zn(OH)2 + H2}$

But, the zinc hydroxide is essentially insoluble in cold water and seals up the metal before the reaction gets very far.

Boiling the water speeds things up in two ways. First, the metal hydroxide, with either zinc or magnesium, is inherently more soluble. Second, the turbulent action created by the boiling (the bubbles you see are actually steam, not hydrogen) acts as a stirring mechanism, speeding up the mass transfer so the slight concentration of dissolved hydroxide gets to the litmus paper faster.

Magnesium differs from zinc because the former hydroxide is somewhat more soluble. Therefore, the basic nature of the product with magnesium can be demonstrated with phenolphthalein, which requires a slightly higher pH to react than litmus. Some demonstrations with magnesium show a rose coloring of the magnesium metal in cold water with the phenolphthalein showing that the magnesium hydroxide remains essentially on the surface.


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