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Chapter 20: Problem 89

(a) What is electrolysis? (b) Are electrolysis reactions thermodynamically spontaneous? (c) What process occurs at the anode in the electrolysis of molten \(\mathrm{NaCl}\) ? (d) Why is sodium metal not obtained when an aqueous solution of \(\mathrm{NaCl}\) undergoes electrolysis?

Short Answer

Expert verified
(a) Electrolysis is a process that uses an electric current to drive a non-spontaneous chemical reaction, forcing a redox reaction to occur in the direction that it would not spontaneously occur. (b) Electrolysis reactions are not thermodynamically spontaneous and require an external source of electrical energy. (c) During the electrolysis of molten NaCl, oxidation occurs at the anode, with chloride ions (Cl⁻) being oxidized to form chlorine gas (Cl₂) and electrons: \[ 2\mathrm{Cl}^- \rightarrow \mathrm{Cl}_2 \ +\ 2e^- \] (d) Sodium metal is not obtained during the electrolysis of an aqueous NaCl solution because water gets reduced at the cathode instead of sodium ions, as its reduction potential is more favorable than that of sodium ions: \[ 2\mathrm{H}_2\mathrm{O} + 2e^- \rightarrow \mathrm{H}_2 \ +\ 2\mathrm{OH}^- \] Hence, hydrogen gas is produced at the cathode instead of sodium metal.

Step by step solution

01

a. Definition of Electrolysis

Electrolysis is a process that uses an electric current to drive a non-spontaneous chemical reaction. In electrolysis, electrical energy is used to force a redox reaction to occur in the direction that it would not spontaneously occur.
02

b. Thermodynamic spontaneity of electrolysis reactions

Electrolysis reactions are not thermodynamically spontaneous. They require an external source of electrical energy to cause the redox reaction to proceed in a non-spontaneous direction.
03

c. Process occurring at the anode during electrolysis of molten NaCl

During the electrolysis of molten NaCl, oxidation occurs at the anode. In this reaction, chloride ions (Cl⁻) are oxidized to form chlorine gas (Cl₂) and electrons. The half-reaction representing this process is: \[ 2\mathrm{Cl}^- \rightarrow \mathrm{Cl}_2 \ +\ 2e^- \]
04

d. Reason for not obtaining sodium metal during the electrolysis of aqueous NaCl solution

When an aqueous solution of NaCl undergoes electrolysis, sodium metal is not obtained because water gets reduced at the cathode instead of sodium ions. In an aqueous environment, water is present in large quantities, and its reduction potential is more favorable than that of sodium ions. The half-reaction representing the reduction of water at the cathode is: \[ 2\mathrm{H}_2\mathrm{O} + 2e^- \rightarrow \mathrm{H}_2 \ +\ 2\mathrm{OH}^- \] Therefore, hydrogen gas is produced at the cathode instead of sodium metal during the electrolysis of an aqueous NaCl solution.

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Most popular questions from this chapter

Disulfides are compounds that have \(S-S\) bonds, like peroxides have \(\mathrm{O}-\mathrm{O}\) bonds. Thiols are organic compounds that have the general formula \(\mathrm{R}-\mathrm{SH}\), where \(\mathrm{R}\) is a generic hydrocarbon. The \(\mathrm{SH}^{-}\) ion is the sulfur counterpart of hydroxide, \(\mathrm{OH}^{-}\). Two thiols can react to make a disulfide, \(\mathrm{R}-\mathrm{S}-\mathrm{S}-\mathrm{R} .\) (a) What is the oxidation state of sulfur in a thiol? (b) What is the oxidation state of sulfur in a disulfide? (c) If you react two thiols to make a disulfide, are you oxidizing or reducing the thiols? (d) If you wanted to convert a disulfide to two thiols, should you add a reducing agent or oxidizing agent to the solution? (e) Suggest what happens to the H's in the thiols when they form disulfides.

A voltaic cell utilizes the following reaction: $4 \mathrm{Fe}^{2+}(a q)+\mathrm{O}_{2}(g)+4 \mathrm{H}^{+}(a q) \longrightarrow 4 \mathrm{Fe}^{3+}(a q)+2 \mathrm{H}_{2} \mathrm{O}(l)$ (a) What is the emf of this cell under standard conditions? (b) What is the emf of this cell when $\left[\mathrm{Fe}^{2+}\right]=1.3 \mathrm{M},\left[\mathrm{Fe}^{3+}\right]=\( \)0.010 \mathrm{M}, P_{\mathrm{O}_{2}}=50.7 \mathrm{kPa},\( and the \)\mathrm{pH}$ of the solution in the cathode half-cell is \(3.50 ?\)

Indicate whether each of the following statements is true or false: (a) If something is reduced, it is formally losing electrons. (b) A reducing agent gets oxidized as it reacts. (c) An oxidizing agent is needed to convert \(\mathrm{CO}\) into \(\mathrm{CO}_{2}\).

Predict whether the following reactions will be spontaneous in acidic solution under standard conditions: (a) oxidation of \(\mathrm{Cu}\) to \(\mathrm{Cu}^{2+}\) by \(\mathrm{I}_{2}\) (to form \(\mathrm{I}^{-}\) ), \((\mathbf{b})\) reduction of \(\mathrm{Fe}^{2+}\) to \(\mathrm{Fe}\) by \(\mathrm{H}_{2}\) (to form \(\mathrm{H}^{+}\) ), \(\left(\mathbf{c}\right.\) ) reduction of \(\mathrm{I}_{2}\) to \(\mathrm{I}^{-}\) by $\mathrm{H}_{2} \mathrm{O}_{2},(\mathbf{d})\( reduction of \)\mathrm{Ni}^{2+}\( to \)\mathrm{Ni}$ by \(\mathrm{Sn}^{2+}\left(\right.\) to form \(\left.\mathrm{Sn}^{4+}\right)\).

A voltaic cell consists of a strip of cadmium metal in a solution of \(\mathrm{Cd}\left(\mathrm{NO}_{3}\right)_{2}\) in one beaker, and in the other beaker a platinum electrode is immersed in a NaCl solution, with \(\mathrm{Cl}_{2}\) gas bubbled around the electrode. A salt bridge connects the two beakers. (a) Which electrode serves as the anode, and which as the cathode? (b) Does the Cd electrode gain or lose mass as the cell reaction proceeds? (c) Write the equation for the overall cell reaction. (d) What is the emf generated by the cell under standard conditions?
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