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CHEM 1050 Study Guide - Midterm Guide: Intensive And Extensive Properties, Sodium Chloride, Reaction Quotient

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blushrabbit835
1 Nov 2016
School
University of Guelph
Department
Chemistry
Course
CHEM 1050
Professor
Mario Monteiro

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Document Summary

Electrochemical cell a system consisting of electrodes that dip into an electrolyte and in which a chemical reaction either uses or generates an electric current. Voltaic/galvanic cell an electrochemical cell which a spontaneous reaction generates an electric current. Electrolytic cell an electrochemical cell in which an external energy source drives an otherwise nonspontaneous reaction. Electrons flow from one metal electrode to another through an external circuit. Ions flow from one half-cell to another through an internal cell connection. Half-cell the portion of an electrochemical cell in which a half-reaction takes place. Zn (s) > zn2+ (aq) + 2e- (oxidation half-reaction) Cu2+ (aq) + 2e- > cu (s) (reduction half-reaction) Anode the electrode at which oxidation occurs. Cathode the electrode at which reduction occurs. Zn (s) + cu2+ (aq) > zn2+ (aq) + cu (s) Cell reaction the net reaction that occurs in the voltaic cell. Potential difference the difference in electric potential (electrical pressure) between two points.

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Related Questions

MC15/20 Animation—Analysis of Copper-Zinc Voltaic Cell

Some oxidation-reduction reactions are spontaneous, and the energy released by them can be used for electrical work. This principle is used in the working of a voltaic cell. Voltaic cells, or electrochemical cells, make use of the electrons that are transferred from the species that releases electrons and undergoes oxidation to the species that accepts electrons and undergoes reduction. If it is possible by any means to separate the oxidation reaction and the reduction reaction and then connect them externally so that the electrons flow from one compartment to the other, you can construct an electrochemical cell.

Watch the video that describes the cell reaction and the cell components of a voltaic cell.

In a copper-zinc voltaic cell, one half-cell consists of a Zn electrode inserted in a solution of zinc sulfate and the other half-cell consists of a Cu electrode inserted in a copper sulfate solution. These two half-cells are separated by a salt bridge.

At the zinc electrode (anode), Zn metal undergoes oxidation by losing two electrons and enters the solution as Zn2+ ions. The oxidation half-cell reaction that takes place at the anode is

Zn(s)→Zn2+(aq)+2e−

The Cu ions undergo reduction by accepting two electrons from the copper electrode (cathode) and depositing on the electrode as Cu(s). The reduction half-cell reaction that takes place at the cathode is

Cu2+(aq)+2e−→Cu(s)

The electrons lost by the Zn metal are gained by the Cu ion. The transfer of electrons between Zn metal and Cu ions is made possible by connecting the wire between the Zn electrode and the Cu electrode. Thus, in the voltaic cell, the electrons flow through an external circuit from the anode to the cathode. For a voltaic cell to work, the solution in the two half-cells must remain electrically neutral. This can happen only if the flow of ions is countered with the flow of electrons. The flow of ions is made possible with the use of a salt bridge. A salt bridge is a solution of some other metal that has common ions. If a copper-zinc voltaic cell utilizes ZnSO4 and CuSO4solution, you will use a saturated Na2SO4 solution in the salt bridge. Thus, the salt bridge will help the migration of ions across the two compartments, or two half-cells.

Part A

Label the diagram according to the components and processes of a voltaic cell.

Drag the appropriate labels to their respective targets.

Hints

Zinc Cathode

Flow of cations

Flow of electrons

Flow of anions

Copper Cathode

Zinc Anode

Copper Anode

Reduction half-cell

Oxidation half-cell

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Correct

The cell reaction for the voltaic cell is

Zn(s)+Cu2+(aq)→Zn2+(aq)+Cu(s)

Here, Zn undergoes oxidation by losing 2e−, and the Cu2+ ions accept 2e− to form metallic Cu, which is deposited at the copper electrode. At the zinc electrode, oxidation occurs, and hence it is known as the anode. Reduction occurs at the copper electrode, and hence it is called the cathode.

The electrochemical cell notation for this cell is Zn(s)|Zn2+(aq)||Cu2+(aq)|Cu(s).

Construction of a voltaic cell

Consider a chromium-silver voltaic cell that is constructed such that one half-cell consists of the chromium, Cr, electrode immersed in a Cr(NO3)3 solution, and the other half-cell consists of the silver, Ag, electrode immersed in a AgNO3 solution. The two electrodes are connected by a copper wire. The Cr electrode acts as the anode, and the Ag electrode acts as the cathode. To maintain electric neutrality, you add a KNO3 salt bridge separating the two half-cells. Use this information to solve Parts B, C, and D.

Part B

The half-cell is a chamber in the voltaic cell where one half-cell is the site of the oxidation reaction and the other half-cell is the site of the reduction reaction.

Type the half-cell reaction that takes place at the anode for the chromium-silver voltaic cell. Indicate the physical states using the abbreviation (s), (l), or (g) for solid, liquid, or gas, respectively. Use (aq) for an aqueous solution. Do not forget to add electrons in your reaction.

Express your answer as a chemical equation.

Hints

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Part C

The half-cell is a chamber in the voltaic cell where one half-cell is the site of an oxidation reaction and the other half-cell is the site of a reduction reaction.

Type the half-cell reaction that takes place at the cathode for the chromium-silver voltaic cell. Indicate physical states using the abbreviation (s), (l), or (g) for solid, liquid, or gas, respectively. Use (aq) for an aqueous solution. Do not forget to add electrons in your reaction.

Express your answer as a chemical equation.

Hints

Magnesium metal is oxidized, and silver ions are reduced in a voltaic cell using Mg2+ (aq, 1 M) |Mg and Ag+ (aq, 1 M)| Ag half-cells.  

  1. Label each part of the cell.
  2. Write equations for the half-reactions occurring at the anode and the cathode, and write an equation for the net reaction in the cell.
  3. Trace the movement of electrons in the external circuit. Assuming the salt bridge contains NaNO3 trace the movement of the Na+ and NO3− ions in the salt bridge that occurs when a voltaic cell produces current. Why is a salt bridge required in a cell?

What are the similarities and differences between Figure 20.3 and Figure 20.4? (b) Why are Na+ ions drawn into the cathode half-cell as the voltaic cell shown in Figure 20.5 operates?

One Class Solution:-

Figure 20.3 shows spontaneous reaction when a strip of zinc is placed in contact with a Cu2+ solution. Cu2+(aq) ions fades and copper metal deposits on the zinc. At the same time, the zinc begins to dissolve.

Same redox reaction: Cu2+(aq) +Zn → Cu +Zn2(aq)+ is occurs in both cases (Figure 20.3 and 20.4). The significant difference between Figure 20.3 and 20.4 is that in the voltaic cell the Zn metal and Cu2+(aq) are not in direct contact with each other. Zn metal is in contact with Zn2+(aq) in one compartment, and Cu metal is in contact with Cu2+(aq) in the other compartment. Cu2+(aq) reduction can occur only by the flow of electrons through an external circuit, namely, a wire connecting the Zn and Cu strips. Electrons flowing through a wire and ions moving in solution both constitute an electrical current.

In Figure 20.5, a salt bridge serves this purpose. The salt bridge consists of a U-shaped tube containing NaNO3(aq), whose ions will not react with other ions in the voltaic cell or with the electrodes. The electrolyte is often incorporated into a paste or gel so that the electrolyte solution does not pour out when the U-tube is inverted. As oxidation and reduction proceed at the electrodes, ions from the salt bridge migrate into the two half-cells—cations migrating to the cathode half-cell and anions migrating to the anode half-cell—to neutralize charge in the half-cell solutions. Whichever device is used to allow ions to migrate between half-cells, anions always migrate toward the anode and cations toward the cathode.