Problem 12 - General Chemistry: Principles and Modern Applications

Question

For the reduction half-reaction \(\mathrm{Hg}_{2}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-}\) \(\longrightarrow 2 \mathrm{Hg}(1), E^{\circ}=0.797 \mathrm{V} .\) Will \(\mathrm{Hg}(\mathrm{l})\) react with and dissolve in HCl(aq)? in HNO3(aq)? Explain.

Step-by-Step Solution

Answer

Yes, Hg(l) will react with and dissolve in both HCl(aq) and HNO3(aq).

Step 1: Identify the possible reactions

HCl(aq) can be reduced to \(Cl_2(g)\) or \(H_2(g)\). Nitric acid (HNO3) can be reduced to \(NO_2(g)\), \(NO(g)\) and \(N_2O(g)\) or other forms. The half-cell reactions of these processes along with their standard reduction potentials need to be listed in order to compare them with the given reaction.

Step 2: Compare the standard reduction potentials

Standard reduction potentials can be found using a standard reduction potential table: \( 2H^+(aq) + 2e^- -> H_2(g); E^0= 0.0 V\), \( 2Cl^-(aq) + 2e^- -> Cl_2(g); E^0= 1.36 V\), \( 2HNO3(aq) + 2H^+ + 2e^- -> 2NO_2 + 2H2O ; E^0= 0.956 V\), \( 2HNO3(aq) + 2H^+ + e^- -> H_2O + 2NO ; E^0= 0.78 V\), \( 4HNO3(aq) + e^- -> 2H2O + 4NO_2 ; E^0= 1.29 V\). If the reduction potential of the possible reaction with HCl or HNO3 is less than the reduction potential of the given reaction (0.797 V), then Hg(l) will not react with the acid. Conversely, if the reduction potential is more positive than 0.797 V, Hg(l) will react with and dissolve in the acid.

Step 3: Conclude

The reduction potential of the reaction with HCl (1.36V) is more positive than 0.797V, meaning Hg(l) will react with and dissolve in HCl(aq). On the contrary, all possible reduction potentials of reactions with HNO3 (0.956V, 0.78V and 1.29V) are more positive, indicating that Hg(l) will also react and dissolve in HNO3(aq).

Key Concepts

Chemical Reactions

At the heart of chemistry lies the concept of chemical reactions, which are processes that involve the transformation of one set of chemical substances into another. These processes can be categorized into various types, such as synthesis, decomposition, single replacement, and double replacement reactions.

Understanding chemical reactions is crucial when predicting the behavior of substances under different conditions. For instance, whether mercury (I) will react and dissolve in hydrochloric acid (HCl) or nitric acid (HNO3) can be inferred from knowing the type of chemical reaction that could take place. In the case of mercury's interactions with these acids, a single replacement reaction might occur, where mercury would potentially replace another element in a compound.

Reduction Half-Reaction

Electrochemical reactions comprise two half-reactions: oxidation and reduction. The reduction half-reaction involves the gain of electrons. In order to understand whether a metal will react with an acid, chemists look at the standard reduction potentials.

For instance, the reduction half-reaction for mercury (I) ions, \(\mathrm{Hg}_{2}^{2+} + 2 \mathrm{e}^{-} \rightarrow 2 \mathrm{Hg}(\ell)\), indicates that mercury ions gain electrons to become liquid mercury. The standard reduction potential associated with this half-reaction provides insight into its likelihood to proceed when compared to other potential reduction reactions, such as those involving HCl or HNO3. The potential needs to be compared to those of other potential reactants to determine which substance gets reduced preferentially.

Electrochemistry

Electrochemistry is the branch of chemistry that deals with the relationship between electricity and chemical change. At the core of electrochemistry are the concepts of oxidation and reduction, where electrons transfer between reactants.

In the context of electrochemistry, the standard reduction potential is a measure of the tendency of a chemical species to be reduced, and it is quantified under standard conditions. A higher standard reduction potential indicates a stronger tendency to gain electrons and be reduced. Thus, by comparing the standard reduction potentials of the mercury (I) reduction half-reaction with those involving HCl and HNO3, we can predict the sequence of reactivity and determine whether mercury (I) will dissolve in those acids.