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Key concepts and summary

Electrochemical cells typically consist of two half-cells. The half-cells separate the oxidation half-reaction from the reduction half-reaction and make it possible for current to flow through an external wire. One half-cell, normally depicted on the left side in a figure, contains the anode. Oxidation occurs at the anode. The anode is connected to the cathode in the other half-cell, often shown on the right side in a figure. Reduction occurs at the cathode. Adding a salt bridge completes the circuit allowing current to flow. Anions in the salt bridge flow toward the anode and cations in the salt bridge flow toward the cathode. The movement of these ions completes the circuit and keeps each half-cell electrically neutral. Electrochemical cells can be described using cell notation. In this notation, information about the reaction at the anode appears on the left and information about the reaction at the cathode on the right. The salt bridge is represented by a double line, ‖. The solid, liquid, or aqueous phases within a half-cell are separated by a single line, │. The phase and concentration of the various species is included after the species name. Electrodes that participate in the oxidation-reduction reaction are called active electrodes. Electrodes that do not participate in the oxidation-reduction reaction but are there to allow current to flow are inert electrodes. Inert electrodes are often made from platinum or gold, which are unchanged by many chemical reactions.

Chemistry end of chapter exercises

Write the following balanced reactions using cell notation. Use platinum as an inert electrode, if needed.

(a) Mg ( s ) + Ni 2+ ( a q ) Mg 2+ ( a q ) + Ni ( s )

(b) 2 Ag + ( a q ) + Cu ( s ) Cu 2+ ( a q ) + 2Ag ( s )

(c) Mn ( s ) + Sn(NO 3 ) 2 ( a q ) Mn(NO 3 ) 2 ( a q ) + Au ( s )

(d) 3 CuNO 3 ( a q ) + Au(NO 3 ) 3 ( a q ) 3Cu(NO 3 ) 2 ( a q ) + Au ( s )

(a) Mg ( s ) Mg 2+ ( a q ) Ni 2+ ( a q ) Ni ( s ) ; (b) Cu ( s ) Cu 2+ ( a q ) Ag + ( a q ) Ag ( s ) ; (c) Mn ( s ) Mn 2+ ( a q ) Sn 2+ ( a q ) Sn ( s ) ; (d) Pt ( s ) Cu + ( a q ), Cu 2+ ( a q ) Au 3+ ( a q ) Au ( s )

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Given the following cell notations, determine the species oxidized, species reduced, and the oxidizing agent and reducing agent, without writing the balanced reactions.

(a) Mg ( s ) Mg 2+ ( a q ) Cu 2+ ( a q ) Cu ( s )

(b) Ni ( s ) Ni 2+ ( a q ) Ag + ( a q ) Ag ( s )

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For the cell notations in the previous problem, write the corresponding balanced reactions.

(a) Mg ( s ) + Cu 2+ ( a q ) Mg 2+ ( a q ) + Cu ( s ) ; (b) 2 Ag + ( a q ) + Ni ( s ) Ni 2+ ( a q ) + 2Ag ( s )

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Balance the following reactions and write the reactions using cell notation. Ignore any inert electrodes, as they are never part of the half-reactions.

(a) Al ( s ) + Zr 4+ ( a q ) Al 3+ ( a q ) + Zr ( s )

(b) Ag + ( a q ) + NO ( g ) Ag ( s ) + NO 3 ( a q ) ( acidic solution )

(c) SiO 3 2− ( a q ) + Mg ( s ) Si ( s ) + Mg ( OH ) 2 ( s ) (basic solution)

(d) ClO 3 ( a q ) + MnO 2 ( s ) Cl ( a q ) + MnO 4 ( a q ) (basic solution)

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Identify the species oxidized, species reduced, and the oxidizing agent and reducing agent for all the reactions in the previous problem.

Species oxidized = reducing agent: (a) Al( s ); (b) NO( g ); (c) Mg( s ); and (d) MnO 2 ( s ); Species reduced = oxidizing agent: (a) Zr 4+ ( aq ); (b) Ag + ( aq ); (c) SiO 3 2− ( a q ) ; and (d) ClO 3 ( a q )

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From the information provided, use cell notation to describe the following systems:

(a) In one half-cell, a solution of Pt(NO 3 ) 2 forms Pt metal, while in the other half-cell, Cu metal goes into a Cu(NO 3 ) 2 solution with all solute concentrations 1 M .

(b) The cathode consists of a gold electrode in a 0.55 M Au(NO 3 ) 3 solution and the anode is a magnesium electrode in 0.75 M Mg(NO 3 ) 2 solution.

(c) One half-cell consists of a silver electrode in a 1 M AgNO 3 solution, and in the other half-cell, a copper electrode in 1 M Cu(NO 3 ) 2 is oxidized.

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Why is a salt bridge necessary in galvanic cells like the one in [link] ?

Without the salt bridge, the circuit would be open (or broken) and no current could flow. With a salt bridge, each half-cell remains electrically neutral and current can flow through the circuit.

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An active (metal) electrode was found to gain mass as the oxidation-reduction reaction was allowed to proceed. Was the electrode part of the anode or cathode? Explain.

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An active (metal) electrode was found to lose mass as the oxidation-reduction reaction was allowed to proceed. Was the electrode part of the anode or cathode? Explain.

Active electrodes participate in the oxidation-reduction reaction. Since metals form cations, the electrode would lose mass if metal atoms in the electrode were to oxidize and go into solution. Oxidation occurs at the anode.

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The mass of three different metal electrodes, each from a different galvanic cell, were determined before and after the current generated by the oxidation-reduction reaction in each cell was allowed to flow for a few minutes. The first metal electrode, given the label A, was found to have increased in mass; the second metal electrode, given the label B, did not change in mass; and the third metal electrode, given the label C, was found to have lost mass. Make an educated guess as to which electrodes were active and which were inert electrodes, and which were anode(s) and which were the cathode(s).

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Practice Key Terms 8

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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