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ElectrochemistryPart II: The Galvanic Cell

Dr. C. Yau

Spring 2014

Jespersen Chap. 20 Sec 1

1

2

What is a Galvanic Cell?• A galvanic cell is a spontaneous

electrochemical cell in which electricity is produced by a spontaneous redox reaction.

• The resulting electron transfer is forced to take place through a wire.

• It is also known as a voltaic cell.

• Do not confuse it with the electrolytic cell (discussed later) where an electrochemical reaction is forced to take place by passing electricity through the cell.

3

Comparison of Types of Cell

Galvanic or Voltaic Cell:

Spontaneous electrochemical rxn

Electricity produced

Electrolytic Cell:

Non-spontaneous electrochemical rxn

Electric current is passed thru a wire to force the reaction to take place.

44

Anatomy Of A Galvanic Cell

• Half-cells (compartments containing reactants for each half- reaction)

• Electrodes to conduct current through the solution.• Salt bridge to allow ion movement to keep solns neutral.• Supporting electrolyte (spectator ions: NO3

-)• Connecting external circuit (wire and voltmeter)

Zn Zn2+ + 2 e− Cu2+ (aq) + 2e− Cu (s)

Note: Textbk is inconsistent with which is on the left (cathode or anode).There is no convention which is on the left side in a sketch of this sort.

e-

e-

anodecathode

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Electrochemical Cells• In all cells, electrons transfer between the

cathode (the reduction half-cell) and the anode (the oxidation half-cell)

REMEMBER! "Red-Cat and An-Ox”

Reduction at the Cathode

& Oxidation at the Anode

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A closer look at the electrodes:

Zn2+

Zn2+

Zn2+

Zn2+

Zn2+

Zn anode

Cu2+

Cu2+

Cu2+

Cu2+

Cu2+

Cu2+

Cu2+

Cu cathode

Oxidation of Zn to Zn2+

Leaves e- behind on the electrode (soln becomesmore positive)

Reduction of Cu2+ to Cue- extracted from electrode (soln becomesmore negative)

Zn Cu

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Electrochemical Cells

• Electrical current is conducted via the movement of electrons and ions.

• To prevent charge buildup, a salt bridge allows ions to move between the cells.

REMEMBER! • Electrons flow from anode to cathode

through the wire. a to c• Cations move towards the cathode.• Anions move towards the anode.• Red-Cat and An-Ox.

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There is a buildup of what charges at each cell?

The salt bridge often made of KCl or KNO3

(unreactive ions – spectator ions)

What ions in the salt bridge move to which cell?

Zn2+

Zn Cu

KCl or KNO3

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Towards which compartment will electrons flow in an electrochemical cell?

A. Toward the cathode B. Toward the anode C. It depends on the reaction

Through which components of the cell will ions not flow?

A. The electrodes B. The solution C. The salt bridgehttp://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/galvan5.swf

anode to cathode

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Standard Cell Notation (Line Cell Notation)

• Cell reactions separated by || that represents the salt bridge with ANODE on left, CATHODE on right.

• Electrodes appear at the outsides• Reaction electrolytes in inner section• Phases (phys. States) separated with |• Species in the same state separated with ;• Concentrations shown in ( )

Zn (s) | Zn2+ (aq) || Cu2+ (aq) | Cu (s)

salt bridgeanode cathode

anode electrode

cathode electrode

anodeelectrolyte

cathodeelectrolyte

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Standard Cell Notation (Line Cell Notation)

Write the half reactions for the galvanic cell shown above.

Cu (s) Cu2+ (aq) + 2e-

Ag+ (aq) + e- Ag (s)Make a sketch of the galvanic cell and label

it fully.

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Now, consider the reaction of

Al3+(aq) + Zn (s) Al(s) + Zn2+

(aq)

Write the half-reactions.

Balance the electrons and write the balanced net ionic equation.

Sketch the galvanic cell (electrochemical cell). Label it fully.

Write the standard cell notation.

Do Prac Exer 1 & 2 on p. 924

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Given: Mg(s) | Mg2+(aq) || Sn2+(aq) | Sn(s)

Sketch the galvanic cell corresponding to this standard cell notation. Label it fully.

Practice with p. 969 #20.50, 20.5113

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Now, consider the reaction of

Fe3+ + Zn Fe2+ + Zn2+

Write the half-reactions.

Balance the electrons and write the balanced net ionic equation.

Write the standard cell notation.

Sketch the galvanic cell (electrochemical cell). Label it fully.

How can you have an electrode that is an ion (such as Fe3+)?

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Where there are no conductive metals involved in a process, an inert electrode is used. C(gr) and Pt are often used.

2Fe3+ + Zn 2Fe2+ + Zn2+

Zn (s) Zn2+(aq) Fe3+

(aq) Fe2+(aq)

Zn(s) |Zn2+(aq) || Fe3+

(aq); Fe2+(aq)|Pt(s)

Zn inert anode cathode (where Fe3+ reduces to Fe2+ at the surface of the Pt electrode)

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Balance and identify the cathode and anodeH2O2(aq) + CO2(g) → H2C2O4(aq) + O2(g) (acidic)

H2O2(aq) + 2CO2(g) → H2C2O4(aq) + O2(g) (acidic)

H2O2(aq) → O2(g) +2H+ + 2e-

2H+ + 2e- + 2CO2(g) → H2C2O4(aq) + (acidic)

oxid

reduc

Which is at the cathode? At the anode?

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Write Line Notation for the cell:H2O2(aq) + CO2(g) → H2C2O4(aq) + O2(g) (acidic)

Standard Cell notation for the reaction:

C(gr)| H2O2(aq) ;H+|O2(g)||CO2(g)|H2C2O4(aq); H+|C(gr)

oxidationreduction

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Balance and identify the cathode and anodeCrO3(s) + MnO2(s)→MnO4

-(aq) + Cr3+

(aq) (basic)

CrO3(s) + MnO2(s) + H2O(l) →MnO4-(aq) + Cr3+

(aq) + 2OH-(aq)

CrO3(s) + 3H2O(l) + 3e- → Cr3+(aq) +6OH-(aq)

MnO2(s) + 4OH- → MnO4-(aq) + 2H2O + 3e-

Balancing redox equations by half-reaction method is given in Sec 6.2 (p. 222)

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Galvanic Cells without Metal Electrodes

Equation from previous slide:CrO3(s)+ MnO2(s) + H2O(l) →MnO4

-(aq)+Cr3+

(aq) + 2OH-(aq)

Write the Standard Cell Notation:

C(gr);MnO2(s)|MnO4-(aq)||CrO3(s)|Cr3+

(aq);OH-|C(gr)

p. 970 #20.52