Cells and half cells

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Cells and half cells
Electricity from chemical reactions
In redox reactions, electrons are transferred. An electrochemical cell controls the electron transfer to
produce electrical energy. The controlled transfer of electrons is the base of all cells and batteries.
Cells and half cells
A halfcell comprises an element in two oxidation states. The simplest halfcell has a metal placed in an
aqueous solution containing Cu (aq) ions (oxidation state +2) into which is placed a strip or rod of copper
metal (oxidation state 0).
Equilibrium exists at the surface of the copper between these oxidation states of copper:
Cu (aq) + 2e Cu(s)
× The forward reaction involves electron gain and is reduced.
× The reverse reaction involves electron loss and is oxidised.
By convention, the equilibrium is written with the electrons on the lefthand side. The electrode potential of
the halfcell indicates its tendency to lose or gain electrons in this equilibrium.
Cells from metal/metal ion electrodes
A simple electrochemical cell can be made by connecting together two halfcells with different electrode
× One half cell releases electrons.
× The other halfcell gains electrons.
The difference in electrode potential is measured with a voltmeter.
The two half cells are joined together using a salt bridge.
× The wire connects the two metals, allowing electrons to be transferred between the two halfcells. In
this cell, the wire connects the copper electrode in the copper halfcell in the zinc electrode in the
copper halfcell to the zinc halfcell. The voltmeter has a high resistance and is used to minimise the
current that flows. If a small bulb were to replace the voltmeter, the bulb would light up.
× The salt bridge connects the two solutions, allowing ions to be transferred between the half cells. A
simple salt bridge can be made out of a strip of filter paper soaked in aqueous solution of an ionic
compound that does not react with either of the halfcell solutions. Usually the solutions used are
KNO (aq) or NH NO (aq).
The redox equilibrium in each half cell is:
Zn (aq) + 2e Zn(s)
Cu (aq) + 2e Cu(s)
The Zn /Zn equilibrium releases electrons more readily than the Cu /Cu equilibrium.
× The Zn /Zn equilibrium releases electrons into the wire, making zinc the negative electrode.

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× Electrons flow along the wire to the Cu electrode of the Cu /Cu halfcell.
× The Zn /Zn equilibrium loses electrons and moves to the left.
Zn (aq) + 2e Zn(s)
× The Cu /Cu equilibrium gains the electron and moves to the right:
Cu (aq) + 2e Cu(s)
The reading on the voltmeter measures the potential difference of the cell ­ this measures the difference
between electrode potentials and the half cells.…read more


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