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Summary Galvanic Cells
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ELECTROCHEMISTRY – GALVANIC CELLSOXIDATION AND REDUCTION REACTIONS
In a redox reaction there is a transfer of electrons.
In the oxidation half reaction electrons are lost [OIL]
In the reduction half reaction electrons are gained [RIG].
An oxidising agent accepts electrons and causes oxidation.
A reducing agent donated electrons and causes reduction.
USING THE TABLE OF STANDARD ELECTRODE POTENTIALS
― The reactions on the table show only the atoms, molecules or ions involved in the
transfer of electrons. Some reactions include H+ ions which indicate the presence
of an acid and H2O molecules which form from the H+ ions.
― Spectator ions are not involved in electron transfer. There are no spectator ions
on the table.
― The reactions are shown as half reactions which are reversable. All reactions in
the table are written as reduction processes when reading from left to right and
oxidation processes when reading from right to left.
― Substances on the left are oxidising agents. Substances on the right are reducing
agents.
Reduction read from left to right.
Oxidation read from right to left.
Above hydrogen equation is more likely to undergo oxidation, anything below
is likely to undergo reduction.
SPONTANEOUS REACTIONS
A spontaneous reaction is one which takes place without added energy.
Spontaneous reactions are exothermic. Spontaneous redox reaction will take place when
a stronger oxidising agent is added to a stronger reducing agent.
GALVANIC [VOLTAIC] CELLS
In this type of cell a chemical reaction is used to produce an electric current.
Chemical energy → Electrical energy
All cells, which produce electricity [batteries], are based on spontaneous redox reactions.
Study following spontaneous reaction in which zinc metal [Zn] displaces copper ions
[Cu2+] from solution to form zinc ions [Zn2+] and copper metal [Cu].
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
THE OXIDATION HALF CELL
Zn → Zn2+ + 2e-
― Oxidation takes place at the anode.
― An electrode made of Zn metal is in contact w. a solution
of Zn2+ ions.
― Atoms of zinc change to Zn2+ ions and 2electrons on the
electrode.
― The electrode becomes negatively charged.
― The concentration of Zn2+ ions in the solution increases
and the electrode decreases in mass.
, THE REDUCTION HALF CELL
Cu2+ + 2e- → Cu
― Reduction takes place at the cathode.
― An electrode made of Cu metal is in contact w.
a solution of Cu2+ ions.
― Cu2+ ions accept 2 electrons from the
electrode and change to copper atoms.
― The electrode becomes positively charged.
― The concentration of the Cu2+ ions in solution
decreases and the electrode increases in
mass.
When the electrodes are joined, electrons flow from the anode to the cathodes. Since the
cell maintains a negative charge on one electrode and a positive charge on the other
electrode, there is a potential difference / emf between the two electrodes. This potential
difference [emf] can be measured using a voltmeter.
The potential difference between the two electrodes varies with temperature, pressure
and concentration of the solutions. To make comparisons between cell emfs we use
standard conditions of concentration [1 mol.dm-3] and temperature [25°C / 298 K]. [Must
be at STP].
THE SALT BRIDGE
To complete the circuit a salt bridge that contains a solution of an ionic salt [NaCl / KCl]
joins the two solutions.
In the anode half-cell, positive ions [Zn2+] are constantly produced and there is a build up
of positive charge in the solution.
In the cathode half-cell, positive ions are constantly removed [Cu2+] and there is a build
up of negative charge in the solution.
The salt bridge completes the circuit and maintains the electrical neutrality of the
solutions by allowing ions to move between them.
CELL NOTATION
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