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Galvanic Cells

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by

Fiona Slavin

on 26 September 2014

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Transcript of Galvanic Cells

what is a galvanic cell?
Galvanic Cell:
is an arrangement of two half-cells that produce electricity spontaneously.
#no filter
the galvanic cell
Half Cell: consists of one electrode and one electrolyte forming half of a complete galvanic cell.
Porus boundary: is a semi-permeable barrier that separates electrolytes while still allowing ions to move through tiny openings between the two solutions.
Electrodes: are a solid conductor.
Electrode
Electrolytes: are aqueous electrical conductors.
Electrolyte
cell notation
Zn | Zn(NO ) || Cu(NO ) | Cu
2+
(s)
3
2
(aq)
3
2
(aq)
2+
(s)
Single line (|) shows a phase boundary
Double line (||) shows a Porus boundary
Proper Cell Notation for Galvanic Cells:
X | XZ || YZ | Y
(s)
(s)
(aq)
(aq)
Theoretical description
Cathode
The electrode where reduction occurs.
Anode
The electrode where oxidation occurs.
+
-
By studying a silver-copper cell, observations can be made that can help explain what is happening. (Please see page 696)
According to the electron transfer theory and the concept of relative strengths of oxidizing and reducing agents:
The strongest oxidizing agent present in the cell always undergoes a reduction at the cathode.
The strongest reducing agent present in the cell always undergoes an oxidization at the anode.
Acronym: "An ox ate a red cat"-> Anode oxidation; reduction cathode.
Electrons released by the anode (Zn) travel through the connected wires to the cathode (Cu).
This direction is decided by the strength of oxidizing agents (Cu is stronger than Zn).
net equation

Identify the strongest oxidizing and reducing agents.
Create an equation for the reduction at the cathode.
Create an equation for the oxidation at the anode.
Combine the equations from step 2 and 3 to create the net ionic equation
The porus boundary permits the redistribution of charge that is needed to maintain neutrality.
If the cations did not move to the cathode, the removal of ions from the cathode would create a net negative charge around the cathode, preventing electron transfer and vice-versa.
Standard Cell
Standard Cell Potential ( E ):

The maximum electric potential difference (voltage) of a cell (the total amount of voltage).
Standard Hydrogen Half-cell
Reference Half-Cell:

a half-cell with an electrode potential of exactly zero volts.
All conditions must be at SATP to be known as a standard cel1. The degree sign ( ) indicates that conditions must be in SATP.
Reduction Potential of S.H.E
2H + 2e H
E = 0.00V
Measuring Standard Reduction Potentials
o
o
o
+
(aq)
-
2(g)
o
When measuring reduction potential, we're looking at the voltage and current.
Voltage (E):

Will determine the numerical value of the half-cell. (measured with a Voltmeter)
Current:
Will determine the sign (+/-) of the half-cell potential.
Pt | H , H ||
Standard Reduction Potential (E ):

Represents the ability of a standard half-cell to attract electron in a reduction half-reaction.
E = E - E
r
r
o
r
r
o
o
Cell
Anode
Cathode
Inert Electrode:

A solid conductor that will not react with any substances present in the cell (the platinum wire).
The half-cell used for this purpose is the
S.H.E (Standard Hydrogen Electrode)
which uses Hydrogen and the inert electrode, Platinum.
(s)
2(g)
(aq)
+
Cell Notation
The Standard Reduction Potential (E ) of a half-cell can be measured by creating a galvanic cell with;
A standard hydrogen half-cell (S.H.E).
The half-cell with the main metal you want to measure.
o
r
example #1
Cathode
Anode
Net
(s)
Zn -> Zn + 2e
2H + 2e -> H
2(g)
-
(aq)
+
2+
(aq)
-
2H + Zn -> H + Zn
+
(aq)
(s)
2(g)
(aq)
2+
E = -0.76V
E = 0.00V
r
o
r
o
(H)
(Zn)
= 0.00V - (-0.76V)
= +0.76V
o
Zn | Zn || H , H | Pt
Example #2
A positive cell potential ( E > 0) indicates that the net reaction is spontaneous - a requirement for all galvanic cells.
Cell Potential under Non-Standard Conditions
Zn | Zn || Cu | Cu
Cathode
Anode
Net
Cu + 2e -> Cu
Zn -> Zn + 2e
Cu + Zn -> Zn + Cu
E = +1.10V
E = +0.76V
E = E - E
= +0.34V
E = E - E
r
o
o
r
*This is a galvanic cell with a S.H.E and Zinc*
= (+1.10V) - (+0.76V)
*This is a galvanic cell with Zinc and Copper*
(s)
(s)
(aq)
(aq)
2(g)
r
o
r
o
o
r
o
-
-
2+
2+
2+
2+
(s)
(s)
(s)
(s)
2+
(aq)
(aq)
2+
(s)
(s)
*Please see appendix C11 (page 805) in the textbook for a list of Standard Reduction Potentials*
(Zn)
If electrons flow for an extended period of time, the electric potential difference of the cell (voltage) will ultimately reach zero (also known as a "dead" cell).
An equilibrium will eventually be reached with no potential energy difference.
A simplified equation that can be used when concentration is not standard (created by Walther Hermann Nernst) is;
E = E - log Q
o
E -
E -
n -

Q -
Non-standard concentration
Standard Concentration
Amount of moles according to cell reaction
Reaction quotient
0.0592V
n

o
o
Ag | Ag || Cu | Cu
(s)
+
(aq)
2+
(s)
(aq)
RA
SOA
OA
SRA
2 [Ag + e -> Ag ]
+
(aq)
-
(s)
Cu -> Cu + 2e
(s)
2+
(aq)
-
Cu + 2Ag -> Cu + 2Ag
(s)
+
(aq)
2+
(aq)
(s)
Electrode
Electrolyte
Electrolyte
Electrode
X - The main metal of one half-cell
Y - The main metal of the other half-cell
+
2+
(aq)
(aq)
(aq)
(aq)
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