Question
Consider the following half-reactions:$$\begin{array}{rl}{\mathrm{Pt}^{2+}+2 \mathrm{e}^{-} \longrightarrow \mathrm{Pt}} & {\mathscr{E}^{\circ}=1.188 \mathrm{V}} \\ {\mathrm{PtCl}_{4}^{2-}+2 \mathrm{e}^{-} \longrightarrow \mathrm{Pt}+4 \mathrm{Cl}^{-}} & {\mathscr{E}^{\circ}=0.755 \mathrm{V}} \\ {\mathrm{NO}_{3}^{-}+4 \mathrm{H}^{+}+3 \mathrm{e}^{-} \longrightarrow \mathrm{NO}+2 \mathrm{H}_{2} \mathrm{O}} & {\mathscr{E}^{\circ}=0.96 \mathrm{V}}\end{array}$$Explain why platinum metal will dissolve in aqua regia (a mixture of hydrochloric and nitric acids) but not in either concentrated nitric or concentrated hydrochloric acid individually.
Step 1
The relevant half reactions are: $$\mathrm{Pt}^{2+}+2 \mathrm{e}^{-} \longrightarrow \mathrm{Pt} \quad \mathscr{E}^{\circ}=1.188 \mathrm{V}$$ $$\mathrm{PtCl}_{4}^{2-}+2 \mathrm{e}^{-} \longrightarrow \mathrm{Pt}+4 \mathrm{Cl}^{-} \quad \mathscr{E}^{\circ}=0.755 Show more…
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(a) Write the balanced reaction between aqueous hydrochloric acid (use H+ and ignore the Cl-) and solid copper to produce copper (II) chloride and hydrogen gas. Label the oxidation numbers. (b) If one uses the activity series to predict whether an acid (H+) should dissolve/react with copper, one would correctly predict that it should not. However, it is known that nitric acid can dissolve copper. The equation for that reaction is as follows: Cu(s) + 4HNO3(aq) ---> Cu(NO3)2(aq) + 2NO2(g) + 2H2O(l) Can you provide a possible explanation for why nitric acid might dissolve copper and hydrochloric acid would not?
Gold metal will dissolve only in aqua regia, a mixture of concentrated hydrochloric acid and concentrated nitric acid in a 3: 1 volume ratio. The products of the reaction between gold and the concentrated acids are $\mathrm{AuCl}_{4}^{-}(a q), \mathrm{NO}(g),$ and $\mathrm{H}_{2} \mathrm{O}$. The equation for this reaction where $\mathrm{HNO}_{3}$ and $\mathrm{HCl}$ are strong acids is $$ \begin{aligned} \mathrm{Au}(s)+4 \mathrm{Cl}^{-}(a q)+4 \mathrm{H}^{+}(a q)+\mathrm{NO}_{3}^{-}(a q) & \longrightarrow \\ \mathrm{AuCl}_{4}^{-}(a q)+\mathrm{NO}(g)+2 \mathrm{H}_{2} \mathrm{O} \end{aligned} $$ (a) What stoichiometric ratio of hydrochloric acid to nitric acid should be used? (b) What volumes of $12 \mathrm{M} \mathrm{HCl}$ and $16 \mathrm{M} \mathrm{HNO}_{3}$ are required to furnish the $\mathrm{Cl}^{-}$ and $\mathrm{NO}_{3}^{-}$ ions to react with $25.0 \mathrm{~g}$ of gold?
Nitric acid is a very strong acid, but is also a very strong oxidizing agent, and generally behaves as the latter. It will dissolve many metals. Balance the following oxidation–reduction reactions of nitric acid. $$ \begin{array}{l}{\text { a. } \mathrm{Cu}(s)+\mathrm{HNO}_{3}(a q) \rightarrow \mathrm{Cu}^{2+}(a q)+\mathrm{NO}_{2}(g)} \\ {\text { b. } \mathrm{Mg}(s)+\mathrm{HNO}_{3}(a q) \rightarrow \mathrm{Mg}^{2+}(a q)+\mathrm{H}_{2}(g)}\end{array} $$
Oxidation–Reduction Reactions and Electrochemistry
Balancing Oxidation–Reduction Reactions by the Half-Reaction Method
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