Zinc in a single displacement reaction with hydrochloric acid 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?

Thanks
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: $$\ce{Zn(s) + 2H^+(aq) -&gt; Zn^2+ + H2(g)}$$ 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: $$\ce{H2 -&gt; 2H^+ + 2e^-}$$ $$\ce{Cl2 + 2e^- -&gt; 2Cl^-}$$ $$\ce{2H^+ + Cl^- -&gt; 2HCl}$$  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:$$\ce{H2O + HCl -&gt; H+ + Cl-}$$ 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: $$\ce{H2O -&gt; H^+ + OH^-}$$
This reaction happens infrequently in water, so concentrations of $\ce{H^+}$ are very low. Therefore, zinc barely replaces hydrogen in water.