PHYSICAL CHEM - ELECTRODE POTENTIALS + CELLS
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What is an electrochemical cell?
a device made up of 2 half cells, a wire, voltmeter and a salt bridge that uses a redox reaction to convert chemical energy into electrical energy
What is a half cell?
1 part of a cell consisting of an electrode in contact with a solution of its ions
What causes an electric current in an electrochemical cell?
the flow of electrons through an external circuit from the more reactive metal to the less reactive metal
What is the purpose of a salt bridge?
to complete the circuit by allowing ions to move and maintain charge balance
Why is potassium nitrate often used in salt bridges?
its ions are unreactive and do not interfere with the electrode reactions
Why can’t a metal wire be used instead of a salt bridge?
it would form its own electrode system and interfere with the cell reactions
Why does a potential difference form between electrodes?
the two half-cells have different tendencies to lose or gain electrons
In a Zn|Zn²⁺ || Cu²⁺|Cu cell, which metal oxidises?
zinc (Zn)
In a Zn|Zn²⁺ || Cu²⁺|Cu cell, which ion is reduced?
copper(II) ions (Cu^2+)
Which electrode is -ve?
the electrode where oxidation occurs
Which electrode is +ve?
the electrode where reduction occurs
What is electrode potential (E)?
• a measure of the tendency of a half-cell to gain or lose electrons • compares how easily one half-cell is oxidised or reduced compared to another
Why can’t the electrode potential of a single half-cell be measured directly?
because only a potential difference between two electrodes can be measured
Are redox reactions occurring when measuring electrode potential? Why?
• no • the system is at equilibrium
Why is a high-resistance voltmeter used?
to prevent current flowing and maintain equilibrium
Why must no current be flowing when measuring electrode potential?
so the redox reactions do not proceed and disturb equilibrium
What does the measured voltage represent if no current flows?
the max possible potential difference between the electrodes
What happens to voltage if current flows?
it falls to zero as reactants are used up
How is a cell represented?
Reduced form|Oxidised form||Oxidised form|Reduced form
What does a single vertical line ( | ) represent in a cell diagram?
a phase boundary (state change)
What does a double vertical line ( || ) represent in a cell diagram?
the salt bridge
Where is the more positive half-cell written in a cell diagram?
the right hand side
Which species is written next to the salt bridge in a cell diagram?
the most oxidised form
Write the Zn/Cu cell diagram
Zn(s) | Zn^2+(aq) || Cu^2+(aq) | Cu(s)