I understand that on the metal activity series, zinc can displace hydrogen from acids, but not from cold water. But is there a structural reason for this. For example, why can zinc displace hydrogen from hydrochloric acid, but not from water?



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  • $\begingroup$ Note that $\ce{Zn + H2O -> Zn(OH)2 + H2}$ when hot water is used. However, the $\ce{Zn(OH)2}$ layer formed doesn't allow the reaction to proceed. See here: socratic.org/questions/… $\endgroup$ – AbhigyanC Apr 5 '18 at 0:59

The zinc replaces the hydrogen in that is reduces (gives electrons to) $\ce{H^+}$, forming $\ce{H2(g)}$ and $\ce{Zn^2+(aq)}$ according to this reaction: $$\ce{Zn(s) + 2H^+(aq) -> Zn^2+ + H2(g)}$$ This reaction relies on the prescence of $\ce{H^+}$.

The hydrogen in acids exists as $\ce{H^+}$. Acids are also ionic compounds that involve hydrogen as the donor of electrons. For example, hydrogen gas and chlorine gas react to make hydrogen chloride: $$\ce{H2 -> 2H^+ + 2e^-}$$ $$\ce{Cl2 + 2e^- -> 2Cl^-}$$ $$\ce{2H^+ + Cl^- -> 2HCl}$$ The reverse of this happens when the hydrogen chloride is dissolved into water. this forms hydrochloric acid.The compound breaks apart into hydrogen and chlorine ions:$$\ce{H2O + HCl -> H+ + Cl-}$$ Hydrochloric acid completely dissociates, meaning that every $\ce{HCl}$ molecule yields one $\ce{H^+}$ ion.

Essentialy, acids increase the amount of $\ce{H^+}$ in solution. Normally, in pure water, concentrations of $\ce{H^+}$ are very low. The hydrogen in $\ce{H2O}$ is covalently bonded to the oxygen, so it is difficult for the hydrogen to react. In order to liberate a hydrogen ion from $\ce{H2O}$, this must occur: $$\ce{H2O -> H^+ + OH^-}$$ This reaction happens infrequently in water, so concentrations of $\ce{H^+}$ are very low. Therefore, zinc barely replaces hydrogen in water.


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