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Electrochemical Cell
Half cell on left consists of a zinc metal
electrode (metal conductor) in an
electrolyte (solution which conducts)
containing a solution of zinc ions at a
concentration of 1
...

Half cell on right consists of a copper
metal electrode (metal conductor) in an
electrolyte (solution which conducts)
containing a solution of copper (II) ions at
a concentration of 1
...

The electrolytes in the two half cells are
connected by a salt bridge
...

Left hand of Cell
Zn2+(aq) + 2e-  Zn(s)
Zinc metal releases electrons to flow into the
external circuit
Oxidation
Takes place in left hand half cell as eqm shifts to
left hand side
Right to left

Right Hand of Cell
Cu2+(aq) + 2e-  Cu(s)
Copper metal conducts electrons from the
external circuit to the copper ions
Reduction
Takes place in right hand half cell as eqm shifts
to right hand side
Left to Right

Representation of the Cell
Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)
+
Oxidation
Reduction
Cell diagrams are written by convention with the metal electrodes on the outside
...
The zinc is oxidised
while the copper is reduced
...


Electrode Potentials
 Reactions written as reduction (gain of electrons)
 Short hand is Cu2+/Cu
Measuring Standard Electrode Potentials (Standard REDOX Potentials)
The Standard Hydrogen Electrode (SHE)
H2 gas at 100kPa
Platinum Wire

Platinum foil covered
in porous platinum
[H+] = 1
...
The reaction takes place on the surface of the platinum
...

Standard Conditions
 Hydrogen pressure is at 100kPa
 Temp is 298 K
 All ion concentrations are 1
...
00V
To find the standard electrode potential
of an other element the standard
hydrogen electrode is attached to the
electrode system being investigated so
it form the left hand cell, a high
resistance voltmeter is used to find the
potential difference between two half
cells
...
It is a simple system which generates voltage, each of
the two half cells and their contents are described as half cells
...
May be a strip of filter paper soaked in electrolyte like potassium nitrate
solution or a glass tube filled with potassium nitrate solution
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The two equilibria are set up on the two electrodes (Magnesium and porous platinum)
Mg2+(aq) + 2e-  Mg(s)
2H+(aq) + 2e-  H2(g)
Magnesium has a much greater tendency to form its ions than hydrogen
...
This means that there will be
a much greater build- up of electrons on the piece of magnesium than on the platinum
...
Voltage measured would be 2
...

Eθ (Mg2+/Mg) = -2
...
00M
Cu2+(aq) + 2e-  Cu(s)
2H+(aq) + 2e-  H2(g)
The hydrogen one lies further to the left so there will be less of a build up of electrons on the
copper than there is on the platinum of the hydrogen electrode
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34V
...
The
voltmeter will show the hydrogen electrode as the negative and the copper as positive
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34 V
The Voltmeter
Voltmeter described as high resistance, ideally infinite
...
If there was low resistance electrons would flow from high
concentration to a lower concentration so the voltage measured would drop
...
This is the elctromotive force or emf of the cell
...


Non-metal/ Non-metal Half Cells
Non-metals can be included in the electrochemical series as oxidising and reducing agents have their
standard electrode potentials measured
...

To build this equilibrium into a circuit the chlorine gas is bubbled over a platinum electrode, which is
immersed in a solution of 1 mol dm-3 chloride ions
...

Chlorine is therefore a good (Best) oxidising agent as it is quite good at removing electrons from
other things
...


The Electrochemical Series
Species are arranged in order of their standard electrode potentials so we get a series that tells us
how good each substance is an oxidising agent at picking up electrons
The species with a more positive potential will oxidise one (reverse the equation) with a less positive
potential
...

Positive electrode is on the right, negative electrode on the left
...
Cell reaction that corresponds to a given cell diagram;
 Right hand as reduction, more positive electrode
 Left hand as oxidation, more negative electrode

Various Factors Affecting Cell Potentials
 Real cell potential can only be measured when no current flows in the external circuit
...
A potential
difference measured under zero- current conditions is known as the electromotive force of
the cell
 Concentration of solutions of cell, 1M as standard
 Temperature, 298K as standard
 Pressure, 100kPa as standard

Batteries
Non- Rechargeable Cells
Zinc/Copper Cells
Daniell battery had an emf of 1
...
Positive electrode is carbon
...

The commercial form of this type of cell consists of a zinc canister filled with ammonium chloride
(NH4Cl) and water- the electrolyte
...
Half equations;
Zn(s) + 2e-  Zn2+(aq)
E= -0
...
7V
These are not E values as the conditions are far from standard
At zinc
Zn(s) >>>> Zn2+(aq) + 2eAt carbon
2NH4+(aq) +2e-  H2(g) + 2NH3(g)
So the overall reaction as the cell discharges is;
2NH4+(aq) + Zn(s) >>>> H2(g) + 2NH3(g) + Zn2+(aq)
EMF= 0
...
7= 1
...

As the cell discharges the zinc is used up and the walls of the canister become thin and
prone to leakage
...

A variant of this battery is a zinc chloride but uses zinc chloride as the electrolyte
...

Long life alkaline batteries are also based on the same system but with electrolyte of
potassium hydroxide
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Rechargeable Batteries
Rechargeable batteries can be recharged by reversing the reactions by applying an external voltage
to drive the electrons in the opposite direction
...

They consist of six 2V cells connected in series to give 12V
...
The positive plate is made of lead coated with lead (IV) oxide, PbO2,
and the negative plate is made of lead
...


Portable Batteries
Rechargeable batteries come in all shapes and sizes
Nickel/Cadium
Replace traditional zinc carbon batteries
...

Cd(OH)2(s) + 2e-  Cd(s) + 2OH-(aq)
NiO(OH)(s) + H2O(l) + e-  Ni(OH)2(s) + OH-(aq)
Overall;
Cd(s) + 2NiO(OH)(s) + 2H2O(l) >>>> 2Ni(OH)2(s) + 2H2O(l) +Cd(OH)2(s)
Emf= 1
...

Lithium Ion
Used in mobiles, laptops, cameras
Lithium batteries are based on lithium rather than heavier metals so are lighter than other batteries
...
A single cell give a
voltage of 3
...
0 V

Fuel Cells
Fuel cells generate electricity from an electrochemical reaction in which oxygen (from air) and a fuel
(hydrogen) combine to form water
...
g
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00V (Oxidation)
E= +1
...
23V

The electrolyte carries charged particles from one electrode to the other
...

The H+ ions flow through the electrolyte to a second electrode where they react with oxygen
to form water
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Also fuel
prices rise as resources dwindle so the cost of living increases
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Cost effective and efficient at 7080% conversion rates

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