ELECTROCHEMICAL CELLS
There are two types of electrochemical cells, they are:
1.
2.
Galvanic, developed by Luigi Galvani, and
Voltaic, developed by Allessandra Volta.
When redox equations can be made to cause electrons to flow through a wire or when a flow of
electrons makes redox reactions happen, the processes are referred to as electrochemical
changes. The study of these changes is called Electrochemistry.
The Electrochemical Cell
Zn(s) + Cu2+ (aq) 6 Zn2+ (aq) + Cu(s)
Zn(s) 6 Zn2+(aq)+ 2e2e- + Cu2+ (aq) à Cu(s)
oxidation
reduction
Zn and Cu are placed in different compartments (half –cells)
There are two parts to the circuits of an electrochemical cell – an internal circuit and an
external circuit
External Circuit
Anode – The site of oxidation and electron release
Cathode – The site of reduction and electron consumption
The electrons move from the anode to the cathode in through the external circuit.
Anode: As the zinc electrode loses electrons zinc ions are released from the zinc rod into the
solution.
Zn(s)
6 Zn2++ 2e-
Cathode: Copper ions in solution gain electrons that are released at the copper electrode and
copper atoms are deposited on the copper rod.
2e- + Cu2+ (aq) 6 Cu(s)
Internal Circuit
The two half-cells must be connected internally for any reaction to be maintained. One way to
connect the compartments is through a salt bridge. The salt bridge contains an electrolyte but
care must be taken in the choice (KNO3 or KCl are common). If the salt reacts with other ions in
the half-cells or within in the electrode, it will interfere with the redox reaction, i.e. NaSO4, - all
group IA elements are soluble and Zn/Cu sulfates are soluble. The salt bridge allows negative or
positive ions to move to the other container.
One easy way to rememebr where oxidation and reduction takes place in the cell is by the letter
each begins with.
Oxidation takes place at the anode, both begin with vowels(O and A).In the case of reduction,
which takes place at the cathode, both begin with consonants(R and C).
Cell Notation
This is a method of describing a galvanic cell without drawing a diagram
Anode / anode electrolyte / salt bridge / cathode electrolyte / cathode
Zn / [Zn2+ ] / / [Cu2+] / Cu
Example 1:
Sketch and label a galvanic cell that makes use of the following spontaneous redox reaction.
Include the cell notation as well.
Mg
(s)
+ Fe2+ (aq) 6 Mg2+ (aq) + Fe(s)
Mg (s) 6 Mg2+ (aq) + 2e2e- + Fe2+ (aq) 6 Fe(s)
oxidized at the anode
reduced at the cathode
Electrochemical cell diagram
Cell notation
Mg / [Mg2+] // [Fe2+] / Fe
Example 2:
For the following reaction : Fe(s) + CuSO4 (aq) 6 Fe2(SO4)3 (aq) + 3Cu(s)
a) Balance the equation
b) Give the cell notation
c) Draw the schematic diagram of the cell and be sure to indicate the flow of ea)
Fe(s) 6 Fe3+
Cu2+ 6 Cu
lost egained e-
oxidized
reduced
(Fe(s) 6 Fe3+ + 3e-) x 2
(2e- + Cu2+ 6 Cu) x 3
2Fe(s) 6 2Fe3+ + 6e6e- + 3Cu2+ 6 3Cu
_______________
2Fe(s) + 3Cu2+(aq) 6 2Fe3+(aq) + 3Cu(s)
b)
Anode loses e-
Cathode gains e-
Anode / anode electrolyte // Cathode electrolyte / Cathode
Fe / Fe3+ // Cu2+ / Cu
c)
Example 3:
A voltaic cell uses aluminum and copper electrodes and aqueous solutions of Aluminum sulfate
and copper(II)sulfate.
Unbalanced equation: Al(s) + CuSO4 (aq) 6 Al2(SO4)3
(aq)
+ Cu(s)
a) Write the half-reactions and balance the equation.
.
b) Write the cell notation
c) Draw a diagram of the cell and label the anode, cathode. direction of flow and pick a suitable
salt bridge
a)
(Al(s) 6 Al3+ + 3e-) x 2
(2e- + Cu2+ 6 Cu(s)) x 3
2Al(s) 6 2Al3+ + 6e6e- + 3Cu2+ 6 3Cu(s)
________________
2Al(s) + 3Cu2+ 6 2Al3+ + 3Cu(s)
c)
Al(s) / Al3+ // Cu2+ / Cu
Example 4:
The following is the cell notation for a galvanic cell
Zn(s) / Zn2+ // Ni 2+ / Ni
a) Draw a diagram of the cell and label the anode, cathode and direction of flow of electrons
and pick a suitable salt bridge.
b) Write the half reactions and balance the equation.
(a)
b)
Zn 6 Zn2+
Ni2+ 6 Ni
Zn 6 Zn2+ +2elost e2e- + Ni2+ à Ni
gained e____________
Zn (s) + Ni2+ (aq) 6 Zn2+(aq)+ Ni(s)
oxidized
reduced
Example 5:
A voltaic cell was based on the following reaction:
Fe + Sn2+6 Sn + Fe2+
a) Write the equations for the half reactions and balance the equation.
b) Draw a diagram of the cell and label the anode, cathode, direction of flow and salt bridge
c) Write the cell notation
(a)
2+
Sn
6
Fe
Sn
6 Fe2+
Fe 6 Fe2+ + 2eSn2+ + 2e- 6 Sn
_____________
Fe + Sn2+ 6 Fe2+ + Sn
(b
(c)
)
Fe / Fe2+ // Sn2+ / Sn
Chemistry 3202
Worksheet - Electrochemical Cells
1.
An electrochemical cell used the tow half-reactions
Cu2+ + 2e- 6 Cu(s)
Sn(s) 6 Sn2+ + 2ea.) Select a soluble compound that could be used to make a one molar solution for each
of the half-cells and the salt bridge.
b.) Write the two half-reactions, labeling one reduction and one oxidation.
c.) Draw the diagram for the electrochemical cell and label all parts. Show the direction
of electron movement.
d.) Write the net reaction.
e.) Draw the cell notation for this electrochemical cell.
2.
Given the following overall reaction:
IO3-(aq) + Fe2+(aq) 6Fe3+(aq) + I2 (s)
a.) Write the half-reactions
b.) Identify the oxidizing agent and the reducing agent.
c.) Sketch the galvanic cells and indicate the direction of electron flow.
d.) Pick a suitable salt bridge.
e.) Draw the cell notation for this galvanic cell.
3.
A student in the lab, immersed some aluminum foil in a beaker containing 1.0 mol/L
Al(NO3)3 (aq). Some silver foil was also placed in a beaker containing 1.00 mol/L AgNO3(aq).
The two beakers were connected with a KNO3 (aq) salt bridge and the metal foil electrodes
were connected with a wire through a voltmeter. A reaction occurred.
a.)
b.)
c.)
d.)
e.)
4.
For the following cell notation:
Ag(s) | Ag+ || Zn2+ (aq) | Zn (s)
a.)
b.)
c.)
d.)
5.
Write the net ionic equation.
Sketch the galvanic cells and identify the anode and cathode.
Which direction will the electrons travel in the wire?
What cations are present in the cell?
Draw the cell notation for the cells.
Write the half-reactions
Write the overall reaction
Sketch the galvanic cells
Identify the cathode and anode
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