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Topic 10 - Making electricity
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A redox reaction occurs to produce products and energy is given out
because it is an exothermic reaction, BUT the energy is released as
electrical energy NOT heat energy so the system shouldn't heat up.
A simple cell can be made by dipping two different pieces of metal (of
different reactivity) into a solution of ions e.g. a salt or dilute acid. If you
use the same metal for both strips, they 'cancel' each other out, so no
voltage, so no current of electrical energy.
All you need is a solution of charged positive and negative particles
called ions e.g. sodium Na+, chloride Cl-, hydrogen H+, sulphate SO42etc.
The greater the difference in reactivity, the bigger the voltage
produced.
BUT a simple demonstration cell can be made by dipping strips of
magnesium and copper into a salt solution and connecting them via a
voltmeter (e.g. as in diagram) and a voltage is readily recorded.
The electrons flow from the more reactive to the less reactive metal.
The reactivity of the metal can be found on the ElectroChemical Series
(ECS) in your data book.
REDOX
Oxidation Is Loss of electrons
Reduction Is Gain of electrons
Easily remembered by the phrase OIL RIG. In a cell the more reactive metal
loses electrons – it is oxidised. The less reactive metal gains electrons – it is
reduced. As both oxidation and reduction happen at the same time in a cell, it
is called a REDOX reaction.
Half Cells
Cells can also be set up using two half cells as shown in the diagram below.
In the above, Zinc, the more reactive metal, loses electrons. It is oxidised to
form positive zinc ions:
Zn (s)  Zn2+(aq) + 2eThe copper ions in the blue solution gain these electrons. The positive copper
ions are reduced to copper metal atoms:
Cu2+(aq) + 2e-  Cu(s)
The overall redox equation is:
Zn (s) + Cu2+(aq) + 2e-  Cu(s) + Zn2+(aq) + 2eAs there are 2 electrons on both sides of the arrow they cancel each other
out, leaving:
Zn (s) + Cu2+(aq)  Cu(s) + Zn2+(aq)
Batteries
Cells or batteries are useful and convenient portable sources of energy but
they are expensive compared to what you pay for 'mains' electricity. We use
batteries as portable forms of energy. The energy comes from a chemical
reaction. Batteries stop working when one or both of the chemicals involved
in the reaction run out. Some batteries however are rechargable, eg a car
battery which is a lead /acid accumulator
Displacement reactions
These reactions occur when one metal reacts with a compound and takes the
place of that metal in the compound. The important rule to remember is that a
metal will only displace another metal from a compound if it is more reactive
than that metal. The list below shows the order of displacement for the metals
in the reactivity series.
Potassium
Sodium
Lithium
Calcium
Magnesium
Aluminium
Carbon
Zinc
Iron
Tin
Lead
Hydrogen
Copper
Silver
Gold
Platinum
To demonstrate the principle of displacement, consider three metals, iron
(most reactive), copper and silver (least reactive). Predict what will happen
when a piece of iron is placed in a beaker of copper sulphate. The iron
displaces the copper from the copper sulphate solution, leaving iron sulphate
and copper metal.
iron + copper sulphate  iron sulphate + copper
This could have been predicted since iron is more reactive than copper. This
is the reaction. Next, what happens when a piece of silver is placed in the
copper sulphate solution? In this situation, no reaction takes place at all
because silver is less reactive than copper.
silver + copper sulphate  No reaction
The displacement reactions can be written as ionic equations.
In the example using iron and copper(II) sulphate:
iron + copper(II) sulphate  iron sulfate + copper.
Fe(s) + CuSO4(aq)

FeSO4(aq) + Cu(s)
Copper(II) sulphate and iron sulphate are ionic compounds. When they are
dissolved in water the ions become separated by the water molecules. If we
write the equation showing the ions we have
Fe(s) + Cu2+(aq) + SO42-(aq) Fe2+(aq) + SO42-(aq) + Cu(s)
In going from reactants to products
iron metal - Fe(s) has become iron ions - Fe2+(aq)
copper ions - Cu2+(aq) have become copper metal - Cu(s)
sulphate ions - SO42-(aq) are not changed during the reaction. Sulphate
ions are the same on the left and the right side of the arrow. Ions which do
not change during the reaction are called spectator ions.
Spectator ions can be left out of the equation, giving
Fe(s) + Cu2+(aq)  Fe2+(aq) + Cu(s)
This is the ionic equation for the reaction between iron and copper(II) sulfate.
Iron is oxidised and copper is reduced.
Similarly, the reaction between tin and lead chloride may be written as
Sn(s) + Pb2+(aq) 
Tin is oxidised and lead is reduced
Sn2+(aq) + Pb(s)