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Electrochemistry & Fuel Cells
Cells
Half-Cells
Electrochemical Cells
Shorthand Notation
The Standard Hydrogen Electrode
Standard Electrode Potential (E)
Electromotive Force (emf)
Measuring Standard Electrode Potentials
E for Metal/Metal Ion Half-Cell
E for Metal Ion/Metal Ion Half-Cell
E for Non-Metal/Non-Metal Ion Half-Cell
The Electrochemical Series
Oxidising and Reducing Agents
Calculating Cell…

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Cells
An electrochemical cell uses the electron transfer from a redox reaction to produce a voltage
(electrical energy).

The cell consists of two halfcells that correspond to the two halfequations of the redox
reaction, and each halfcell consists of two species of the same element in different
oxidation states.

HalfCells…

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The Copper/Zinc Cell

By convention, the oxidised halfcell is
drawn on the left and the reduced
halfcell on the right.

There are thus two equations, one for
each halfcell:

Zn2+(aq) + 2e Zn(s)
Cu2+(aq) + 2e Cu(s)

Here we know the zinc is being
oxidised and the copper reduced,
hence…

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The Standard Hydrogen Electrode
When measuring the electrode potential of a single halfcell, the standard hydrogen electrode
is used as a reference. It is given an electrode potential of 0.00V.




Components:
H+(aq) from an acid solution (e.g. HCl) 1 moldm=3
H2 (g) at 100kPa and 298K
Platinum as an electrode…

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Measuring Standard Electrode Potentials
The standard electrode potential is found by measuring the voltage of a cell with the
standard hydrogen electrode. The polarity relative to the hydrogen halfcell tells us the sign
of the electrode potential.


E for Metal/Metal Ion HalfCell

E.g. Cu2+/Cu

Measured Voltage: 0.34 V

Polarity of…

Page 6

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E.g. Fe3+/Fe2+





Measured voltage: +0.77 V
Polarity of Iron III/Iron II Cell: Positive
Hence E = +0.77 V

E for NonMetal/NonMetal Ion HalfCell

Platinum electrodes are used for the oxidised and reduced species.




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Measured Voltage: +1.36 V
Polarity of Chloride/chloride HalfCell: Positive
Hence E = +1.36 V




The Electrochemical Series
This is a list of standard electrode potentials in numerical order & all halfequations are
written as reductions.

For example:




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Page 8

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The more positive the E the greater the tendency that the species will be reduced.
The more negative the E the easier it is to oxidise the species.

Oxidising and Reducing Agents

An oxidising agent is reduced itself as it accepts electrons, so strong oxidising agents
have highly positive potentials.…

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If done correctly, the Ecell value will always be positive otherwise the reaction will not
occur.

The Reaction in the Electrochemical Cell

In cells, chemical energy is converted into electrical energy, and once the chemicals are used
up, the reaction stops. You can view the E values to predict what…

Page 10

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Fe2+ Fe3+ + e-

Then check the electrons balance, which they do and combine the equations:

½Cl2 + Fe2+ Fe3+ Cl-

Now compare the result with the original question; "Can chlorine oxidise Iron (II)
to Iron (III)?" The equation corresponds as chlorine is oxidising iron (II) to iron (III). So…

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