CHEM 120 Lecture Notes - Lecture 22: Electric Current, Eflow, Voltmeter

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15 Jun 2018

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Voltmeter:

- Electrochemical Cell: e-flow associated with a redox reaction routed through ext. circuit

- Galvanic/Voltaic Cell: produces current (electricity) from a spontaneous chemical reaction

that do work

- Electrolytic Cell: must be supplied with current to drive a nonspontaneous reaction

- Electrode: conducting substance that allows electrons to transfer between aq. phase & ext.

circuit, can be active/passive depending on redox rxn.

- Anode: electrode oxidation, [-] charge

- Cathode: electrode reduced, [+] charge

Salt Bridge: checks for over accumulation of charge; maintains neutrality

- over abundance of [+] charges on the anode, flow of Cl-

- over abundance of [-] charges on the cathode, flow of K+

- anions to anode, cations to cathode

Half Cell: compromised of container, electrolyte & electrode, where either oxidation/reduction

occurs

Passive Electrodes: do not take place in redox rxn

Cell Notation: oxidation half cell written on the left (anode), reduction half cell on the right

(cathode)

- m represents double line barrier on exam separating anode & cathode

- single line separates different phase

EMF: force that drives current

Cell Potential/Voltage Ecell – the difference in electrical potential between the anode & cathode

- Zinc has more oxidation potential; it can lose more electrons than the cell on the right

(pulling)

- Cu has more reduction potential; it wants to gain electrons (pushing)

V = J/C A = C/s

Cell Potential at Standard Conditions E0cell- only measured with respect to standard H electrode

Standard H electrode: made up of H gas (1 atm) & a passive platinum electrode

2H+ + 2e → H2 (1 atm) E0= 0V

(the sum of reduction – the sum of oxidation potential)

*Standard Cell Potential is always for the reduction reaction even if there is an oxidation present

+E0 → more likely to be reduced, strong oxidizing agent

-E0→ more likely to be oxidized, strong reducing agent

E0red = -E0ox

Changing the coefficients of the half reactions does not change the value of E0

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Electrochemical cell: e-flow associated with a redox reaction routed through ext. circuit. Galvanic/voltaic cell: produces current (electricity) from a spontaneous chemical reaction that do work. Electrolytic cell: must be supplied with current to drive a nonspontaneous reaction. Electrode: conducting substance that allows electrons to transfer between aq. phase & ext. circuit, can be active/passive depending on redox rxn. Salt bridge: checks for over accumulation of charge; maintains neutrality. Over abundance of [+] charges on the anode, flow of cl- Over abundance of [-] charges on the cathode, flow of k+ anions to anode, cations to cathode. Half cell: compromised of container, electrolyte & electrode, where either oxidation/reduction occurs. Passive electrodes: do not take place in redox rxn. Cell notation: oxidation half cell written on the left (anode), reduction half cell on the right (cathode) M represents double line barrier on exam separating anode & cathode single line separates different phase.

Related Questions

MC15/20 AnimationâAnalysis 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)âZn2+(aq)+2eâ

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)+2eââCu(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

Help

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Correct

The cell reaction for the voltaic cell is

Zn(s)+Cu2+(aq)âZn2+(aq)+Cu(s)

Here, Zn undergoes oxidation by losing 2eâ, and the Cu2+ ions accept 2eâ 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

You need to create and electrochemical cell that reduces Cr2+ to Cr2+ and meets the following description

One half- cell is Pt electrode immeresed in 0.750 M CrSo4;

One half -cell is a Au electrode immersed in 0.0550 M Aucl ;

The two half cells are connected by a salt bride and voltemeter

The cell is spontanenous (galvanic)

Sketch a cel and label the following, anode cathode, salt bride (indentify the salt you would used), voltemeter and the electrodes (with the appropraite electrodes).

b. show the direction of current (electrons ) in the circuit

c. show the direction of cation flow through the salt bridge

d. write the half reaction equation (include spectators ions and oxidation number for all elemets)

e. Write the overall reaction equation( include spectator ions and oxidation number for all elements

f. write the line diagram that represents this galvanic cell

g. Calculate values of Eau , Ecr , and Ecell

g. Calculate the value of keq for this 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.

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CHEM 120 Lecture Notes - Lecture 22: Electric Current, Eflow, Voltmeter

CHEM 120 Full Course Notes

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