Download Chapter 26 – Electricity from Chemical Reactions

Chapter 26 – Electricity
from Chemical Reactions
A ______________ cell is designed so
that half reactions occur in two separate
compartments of the cell. Because the
oxidant and reductant do not come into
direct contact with each other, electrons
can only be transferred by travelling
through an external circuit connecting the
negative and positive electrodes. This
flow of electrons creates an electric
current. Chemical energy of the reactants
is transformed into electrical energy.
Half Cells
Each half cell contains an electrode in
contact with a solution. The Daniell Cell
(at right) shows each half cell forming a
conjugate redox pair.
 The electrode at which oxidation occurs at is the ____________ and in a galvanic cell,
where electrons are produced, the charge is _____.
 The electrode at which reduction occurs at is the _____________ and in a galvanic cell,
where electrons are consumed, the charge is _______.
Salt Bridge
If no salt bridge was present, the solution in each half cell would accumulate charge which
would prevent further reaction and prevent the production of electricity. The salt bridge contains
ions that are free to move so that they can balance charges formed in the half cells.
_________________ (Positive ions) move towards the ______________ and _____________
(negative ions) move towards the _____________. This part of the cell is known as the internal
circuit.
Half Equations:
In balanced half equations the number of atoms of each element is equal on both sides and the
total charge on each side is equal. For conjugate redox pairs they are generally fairly simple
however, the KOHES method is used for more complex half equations.
Electrochemical Series:
Chemists constructed an electrochemical series by constructing cells from various combinations
of half cells and ranking them in order of their reactivity. The strongest oxidants are found up the
top left and the strongest reductants are found at the bottom right of the table. The
electrochemical series is strictly valid only for the conditions in which it was determined:
 1M concentrations of solutions
 25oC temperature
 1 atm pressure
The electrochemical series can be used to predict what will happen when two half cells are
combined to form a cell. It is important that predictions be checked by experiments as they are
predictions only!
Potential Difference:
A current flows in a galvanic cell because
one half cell as a greater tendency to push
electrons into the external circuit than the
other half – a potential difference exists
between the two half cells (sometimes
called emf). The potential difference is
measured in volts. A rough indication of cell
voltage can be obtained from the
electrochemical series. Large potential
differences are recorded between cells
containing a strong oxidant connected to a
strong reductant. It is impossible to measure
the potential difference of an isolated halfcell as both oxidation and reduction must be
occurring for potential difference to occur.
The potential difference of a cell at standard
conditions is the difference between EO
values of its two half cells.
Cell potential difference = higher
half cell EO – lower half cell EO
Predicting Direct Redox Reactions:
If the contents are mixed directly, the principle is
the same. A chemical on the left of the
electrochemical series must react with a chemical
on the right that is lower in the series. Predictions
should be checked by experiment. Reactions that
occur in galvanic cells or when directly mixed are
known as spontaneous reactions.
Limitations of predictions:
Half cell potentials vary if you move away from standard conditions. If conditions change
significantly, the order of half equations will change from that of the electrochemical series.
The electrochemical series gives no info about the rate at which reactions occur.
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