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Chemistry 30 Notes: Electrochemical cells - Voltaic Components of an electrochemical cell. All redox reactions involve the transfer of electrons from one chemical species to another. It is possible to separate the oxidation and reduction steps in redox reactions and cause them to occur in different physical locations. When we do this we can force the electron transfer through an external circuit and thus use spontaneous reactions to produce electricity. A voltaic cell is a device that generates an electric current from a spontaneous redox reaction. These cells are referred to by two different terms: Voltaic after Alessandro Volta Galvanic after Luigi Galvani All voltaic cells have the same basic structure: Two half-cells, which are separate compartments in which the half- reactions occur. Each half-cell contains the following components: An electrode: a solid conductor that provides a surface for the electron exchange/half-reactions. An electrolyte solution: a solution that usually contains the ion version of the electrode metal. An external conductor: an electrical wire to provide a path for electrons. Can also include a device like a motor or iPod. A salt bridge or porous boundary to allow ion migration but prevent solution mixing. Chemistry 30 Lesson 2-11 Page 1 © 2012 T. de Bruin

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Chemistry 30 Notes: Electrochemical cells - Voltaic

Components of an electrochemical cell.

• All redox reactions involve the transfer of electrons from one chemical species to another.

• It is possible to separate the oxidation and reduction steps in redox reactions and cause them to occur in different physical locations.

• When we do this we can force the electron transfer through an external circuit and thus use spontaneous reactions to produce electricity.

• A voltaic cell is a device that generates an electric current from a spontaneous redox reaction.

• These cells are referred to by two different terms:• Voltaic after Alessandro Volta• Galvanic after Luigi Galvani

• All voltaic cells have the same basic structure:• Two half-cells, which are separate compartments in which the half-reactions

occur.• Each half-cell contains the following components:

• An electrode: a solid conductor that provides a surface for the electron exchange/half-reactions.

• An electrolyte solution: a solution that usually contains the ion version of the electrode metal.

• An external conductor: an electrical wire to provide a path for electrons. Can also include a device like a motor or iPod.

• A salt bridge or porous boundary to allow ion migration but prevent solution mixing.

Chemistry 30 Lesson 2-11 Page 1© 2012 T. de Bruin

HOW TO REMEMBER: A or CCell Polarization and the function of the salt bridge.

The salt bridge (or porous boundary) is required to keep the solutions in both half-cells from mixing, yet allow ion migration to prevent cell polarization, which is a difference in charge between half-cells (keeping overall electrical neutrality in the voltaic cell).

Process of cell polarization occurs as the cell functions:o Cations are produced at the anode resulting in excess positive charge

building up. This causes the electrons not to be released from the anode.o Anion concentration builds up at the cathode as cations are reduced and

removed from the electrolyte solution, resulting in excess negative charge building up. This causes electrons to be repelled from the cathode.

o As the ion build-up progresses, the cell becomes polarized, stopping the electron flow and the half-reactions.

Allowing ion migration between the solutions usin a salt-bridge/porous boundary keeps the net ion charge in each electrolyte solution as zero, preventing cell polarization.

o Anions migrate toward the anode (since positive cations are generated here)

o Cations migrate toward the cathode (since negative anions are generated here).

Example: a Zn-Cu voltaic cell.

Chemistry 30 Lesson 2-11 Page 2© 2012 T. de Bruin

Cell Notation

An electrochemical cell can be described using shorthand called cell notation. The general format of cell notation is shown below:

electrode / electrolyte // electrolyte / electrode↑ ↑phase salt bridge orboundary porous boundary

Example: a standard zinc-copper cell

Cell notation: Zn (s) / Zn2+ (aq) // Cu2+ (aq) / Cu (s)

The cathode and anode are not required to be on any particular side. You must figure that out:

o The cathode half-cell contains the strongest oxidizing agent (SOA).o The anode half-cell contains the strongest reducing agent (SRA).

Chemistry 30 Lesson 2-11 Page 3© 2012 T. de Bruin

Determining net cell reaction and voltage.

The net cell reaction is the sum of the two half-reactions. The cell potential or cell voltage is the sum of the half-reaction potentials (Eo)

of an electrochemical cell. If a half-reaction is reversed (from the redox table) then you must change the sign of

the Eo. NEVER multiply half-reaction potentials by numbers used to balance electrons between

half-reactions. A positive Eo indicates a spontanetous rection (meaning electricity is generated). Voltage is ion concentration dependent. As a voltaic cell operates, the voltage output

starts to drop (decrease) as the SOA ions decrease in concentration. Example problem:

Determine the cell potential of the following electrochemical cell.

Entities list: Ag Ag+ NO31- Cu2+ Cu H2O

2(Ag++ e- � Ag) Eo= +0.80 VCu � Cu2++ 2 e- Eo= -0.34 V

2 Ag++ Cu � Cu2+ + 2 Ag Eo= +0.46 V

Chemistry 30 Lesson 2-11 Page 4© 2012 T. de Bruin

Half-cell Potentials

All half-cell potentials (Eo) are determined by measuring against the standard hydrogen half-cell.

The standard hydrogen half-cell is the reference half-cell whose potential is assigned 0.00 V.

The standard hydrogen half-cell:

Any half-reaction can be chosen as the 0.00 v reference. All you have to do is change all the other voltages on the table by the same amount. This does not change the voltage of a voltaic cell since the voltage is the difference in half-reaction potentials and both half-reaction potentials would have been changed by the same amount if a different zero reference point was chosen.

Chemistry 30 Lesson 2-11 Page 5© 2012 T. de Bruin

Half-cells are connected to a hydrogen half-cell and a voltmeter is used to record the cell potential.

Example 1

since the copper half-cell is undergoing reduction, the reaction is written as a reduction and the voltage is recorded as a positive value in the redox table.

Chemistry 30 Lesson 2-11 Page 6© 2012 T. de Bruin

Example 2

Since the zinc half-cell is undergoing oxidation and redox tables use reduction half-reactions, the zinc half-reaction is reversed and the voltage is changed to a negative.

Chemistry 30 Lesson 2-11 Page 7© 2012 T. de Bruin

Inert electrodes

• Some electrolytes are not compatible with their solid form in a half-cell (you have to determine this yourself using the redox table). That is, they react spontaneously. This occurs mostly with multivalent metals and their highest charged ion. e.g. Cr3+ (aq) and Cr (s) 2(Cr3+ + 1 e- ® Cr2+) Eo = -0.41 V Cr ® Cr2+ + 2 e- Eo = +0.91 V 2 Cr3+ + Cr ® 3 Cr2+ Eo = +0.50 V

(remember: positive Eo indicates reaction is spontaneous)• In these cases, you must use an inert electrode in the half-cell in order to avoid a

spontaneous reaction within the half-cell itself.• An inert electrode is a non-reactive solid conductor. This allows electrons to

conduct in or out of the half-cell without a short circuit within the half-cell.• The two used are C (s) and Pt (s)

Chemistry 30 Lesson 2-11 Page 8© 2012 T. de Bruin