In class I learnt that one can dissolve many substances using acid and that this can be predicted using standard electrode potentials. For example,

$$\begin{align} \ce{Zn &-> Zn^{2+} + 2e^-} & E_0 &= 0.76~\mathrm{V} \\ \ce{2H^+ + 2e^- &-> H2} &+ E_0 &= 0~\mathrm{V} \\\hline \ce{Zn + 2H^+ &-> H2 + Zn^{2+}} & E_0 &= 0.76~\mathrm{V} \end{align} $$

And so Zinc will dissolve in acidic solutions.

I wondered what bases do and so added up the standard electrode potentials of bases and silver.

$$\begin{align} \ce{Ag^+ + e^- &-> Ag} & E_0 &= 0.80~\mathrm{V} \\ \ce{4OH^- &-> O2 + 2H2O + 4e^-} &+ E_0 &= -0.40~\mathrm{V} \\\hline \ce{4Ag^+ + 4OH^- &-> 4Ag + O2 + 2H2O} & E_0 &= 0.40~\mathrm{V} \end{align} $$

This suggests to me that basic solutions will typically precipitate a silver solid from silver ions and produce oxygen gas.

Does this make sense?

  • $\begingroup$ What you have written is the precipitation of silver atoms, not ions. $\endgroup$ – SendersReagent Apr 4 '16 at 23:40
  • $\begingroup$ Also such reaction doesn't happen. $\endgroup$ – Mithoron Apr 4 '16 at 23:42
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    $\begingroup$ I think the precipitation of $\ce{Ag2O}$ would be a faster reaction: $$\ce{2Ag^+ + 2OH^- -> Ag2O v + H2O}$$ This reaction causes $\ce{Ag^+}$ to crash out of solution before the reduction can occur. The silver mirror (Tollens) test makes use of ammonia to bring it back into solution, but it doesn't seem to be reduced by NaOH in this case, either. Maybe not enough is used to make a large amount of precipitation? Or maybe the diamminesilver(I) is has a different reduction potential. $\endgroup$ – SendersReagent Apr 4 '16 at 23:51
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    $\begingroup$ Those two things are ways of saying the same thing. I think it's all reaction speed. Acid/base reactions are very fast, typically, so a silver ion would have to simply collide with hydroxide, and then another hydroxide would have to pull off a proton and then you have silver(I) oxide. Precipitation reactions are also quite fast. It doesn't take much energy for two atoms of opposite charge to collide. $\endgroup$ – SendersReagent Apr 5 '16 at 0:07
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    $\begingroup$ I guess you didn't hear about how difficult is to oxidate water. Big overpotential being serious obstacle. $\endgroup$ – Mithoron Apr 5 '16 at 0:34

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