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GALVANIC ANDELECTROLYTIC CELLS
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REDOX REACTIONS
OXIDATION REDUCTION
A reaction inwhich a
substance loseselectrons
A reaction inwhich a
substance gainselectrons
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Mg Mg2+ + 2e- oxidation reaction
reducing agent (donates electrons and
so can cause reduction)
Cl2 + 2e- 2Cl- reduction reaction
oxidising agent (accepts electrons andso can cause oxidation)
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Mg Mg2+ + 2e- Mg is oxidised (1)
Cl2 + 2e-
2Cl-
Cl2 is reduced (2)
Mg + Cl2 MgCl2 Redox reaction
The electrons cancel each other out.
MgCl2 is an ionic compound (Mg
2+
2Cl
-
)The equation shows 2 half reactions (1 and2) that add to give the full redox reaction.
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A coil of copper wasplaced in a silver nitratesolution
The solution became bluebecause copper ions
were formed.Solid silver deposited onthe copper wire.
DIRECT ELECTRON TRANSFER
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From the observations we can infer that:
Cu Cu2+ + 2e-
Ag+ + e- Ag
Electrons are transferred from the copperatoms on the piece of copper, to the silver ionsin the silver nitrate solution.
This is a redox reaction.
This is a spontaneous reaction.
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A GALVANIC CELL
THE ZINC-COPPER CELL
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The following observations were made:
the zinc plate decreases in mass.
the copper plate increases in mass.
the voltmeter reading indicates thatelectrons flow from the zinc plate to thecopper plate.
From these observations we can infer that
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The salt bridge:
is a gel solution that connects the twoelectrolyte solutions that the metals dip into.
acts as a transfer medium that allows ions to
flow through but prevents the two solutionsmixing.
completes the circuit allowing electrons to
flow through the connecting wire.
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Zn(s) Zn(aq)2+ + 2e-
Cu(aq)2+ + 2e- Cu(s)
Zn(s) + Cu(aq)2+ Zn (aq)
2+ + Cu(s)
The redox reaction for the cell:
The zinc dissolves the mass of the zinc
plate decreases.
The copper plate increases in mass becausecopper ions deposit as copper metal.
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The electrodes:
Each metal strip is an electrode.
Each electrode with its associated electrolyte
solution, is called a half cell.
The electrode where oxidation occurs is theANODE. It has a negative charge.
The electrode where reduction occurs is theCATHODE. It has a positive charge.
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Cell notation:
Zn / Zn2+
// Cu2+
/ Cu
Zinc atoms are oxidised to Zn2+ ions
(anode).
Cu2+ ions are reduced to copper atoms(cathode). Reduction written on the Right
Write the anode half reaction first salt bridge (//) then the
cathode half reaction
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ELECTROLYTIC CELLS
Source of energy
Positive electrode Negative electrode
A battery is always required in this type of cell.
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Chemical reactions occurring in the cell:
Na+ ions are attracted to the negativeelectrode, where they gain electrons.
Na+
(aq) + e-
Na(s)
Reduction occurs at the negative electrode
the CATHODE.
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Cl- ions are attracted to the positiveelectrode, where they lose electrons.
Cl-
(aq) Cl-
(g) + e-
Oxidation occurs at the positive electrode
the ANODE.
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Comparison of galvanic and electrolytic cells.
Galvanic cell Electrolytic cell
Produces an emf from achemical reaction
Uses the emf of a cell toproduce a chemicalreaction
Chemical potentialenergy converted toelectrical potentialenergy
Electrical potentialenergy converted tochemical potentialenergy
Positive terminal -cathode
Positive terminal - anode
Two containers each with
an electrodes
One container with both
electrodes dipping in
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What should you be able to do?
distinguish between galvanic andelectrolytic cells.
write equations for the reactions occurring
in galvanic and electrolytic cells.
explain the difference between oxidationand reduction and explain why the two types
of reactions go together.
label diagrams of galvanic and electrolyticcells.
