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Summary AQA Alevel Chemistry Electrode Potential Notes
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AQA Alevel Chemistry Electrode Potential Notes
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Electrode PotentialWhen a piece of metal is dipped into a solution of its metal ions an equilibrium is set up. There is
a tendency for the metal ions to gain electrons and form and metal, and the metal to lose
electrons and form metal ions.
If this equilibrium lies to the left - the metal acquires a negative charge due to the build up
of electrons. This causes a negative electrode potential.
If the equilibrium lies to the right - the metal builds up a positive charge as the electrons
have been used to form a metal from metal ions - electrode as a positive electrode
potential.
Metal Electrodes A metal surrounded by a solution of its ions. (Zn and
Zn2+)
Gas Electrodes A gas and a solution of its ions. (H2 and HCl)
Redox Two ions of the same element (Fe2+/Fe3+)
Electrodes
The electrode potential of a cell cannot be measured directly. Instead the cell must be
connected to another half cell of a known potential so the potential difference can be measured
and - potential difference of the cell calculated. Usually the SHE cell is used for this - standard
hydrogen electrode.
Setting up an electrochemical cell
Two half cells are joined together to give a complete circuit.
Two metals joined by a wire where electrons flow through a wire.
Two solutions joined by a salt bridge where ions flow through the salt bridge.
Voltmeter allows the potential difference to be measured.
Salt Bridge is a piece of filter paper soaked in a solution of ions which are un-reactive
but can carry charge.
Must be measured under standard conditions:
298K
100kPa
1 mol/dm3
The potential should be measured under zero current conditions to measure the full potential
difference so no current is drawn from the cell. A high resistance voltmeter is used to achieve
this.
Standard conditions are needed because otherwise the position of equilibrium would change
with conditions causing the potential to change.
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