Electrolysis of Solutions
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What is electrolysis?
Electrolysis involves breaking down ionic substances
into simpler substances using electricity.
An ionic substance contains charged
particles called ions. An ion is an
atom that has gained or lost
electrons and so carries a positive
or negative charge, which is shown
after its formula.
Ions with a positive charge have lost
electrons, and ions with a negative
charge have gained electrons.
Al
Aluminium
atom
loses 3
electrons
Al3+
Aluminium
ion
In electrolysis, the ionic substance must be dissolved in
water or melted so that the ions are free to move.
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Oxidation and reduction
In electrolysis, the substance that the current passes through
and splits up is called the electrolyte.
The electrolyte contains positive and negative ions.
What happens to these ions during electrolysis?
Negative ions move
to the positive electrode
and lose electrons.
This is oxidation.
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heat
Positive ions move
to the negative electrode
and gain electrons.
This is reduction.
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OILRIG
An easy way to remember what happens to the electrons
during oxidation and reduction is to think OILRIG:
Oxidation
Is
Loss of electrons
Reduction
Is
Gain of electrons
Equations written to show what happens to electrons during
oxidation and reduction are called half-equations.
magnesium + oxygen  magnesium oxide
2Mg (s)
+
O2 (g)
oxidation:
Mg  Mg2+ + 2e5 of 22

2MgO (s)
reduction:
O2 + 4e-  2O2© Boardworks Ltd 2011
Sodium chloride solution
The electrolysis of sodium chloride solution
(NaCl solution), produces three very useful products:
 chlorine (Cl2)
 hydrogen (H2)
 sodium hydroxide (NaOH).
Chlorine is expected as a
product of this process, but
hydrogen and sodium
hydroxide are less expected.
Sodium chloride solution is more commonly known as brine.
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Electrolysis of NaCl solution
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The positive electrode
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The negative electrode
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Uses of Cl2, H2 and NaOH
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Why is sodium not formed?
In the electrolysis of sodium chloride solution, the Na+ ions
might be expected to form sodium at the negative electrode.
Instead, hydrogen gas is produced here.
This is because sodium
chloride solution also contains
H+ ions from some of the water:
H2O (l)  H+ (aq) + OH- (aq).
At the negative electrode, H+ ions compete with Na+ ions.
The H+ ions gain electrons; the Na+ ions stay in solution.
For all ionic compounds containing a metal that is more
reactive than hydrogen, electrolysis of a solution of the
compound will produce hydrogen rather than the metal.
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How does sodium hydroxide form?
Sodium chloride solution has four types of ions:
 Na+ and Cl- ions from the sodium chloride
 H+ and OH- ions from the water.
The Cl- ions form chlorine at the positive
electrode and the H+ ions form hydrogen
at the negative electrode.
Na+ and OH- ions are left behind, and so a
solution of sodium hydroxide (NaOH) is formed.
What is the overall equation for the electrolysis of brine?
2NaCl (aq) + 2H2O (l)  H2 (g) + Cl2 (g) + 2NaOH (aq)
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Environmental concerns
The industrial electrolysis of
brine does have some
environmental implications,
including concerns about the
large amount of electricity
used by the process.
In addition, chlorine, a
product of the electrolysis of
brine, is toxic. Its largescale manufacture can
lead to hazards.
Electrodes used to be made from highly toxic mercury and
asbestos, which could contaminate water supplies.
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Fill in the gaps
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Splitting water by electrolysis
Electrolysis can be used to split water (H2O) into its
elements, hydrogen (H2) and oxygen (O).
Water is a covalent compound
and so is a poor conductor of
electricity. However, it does contain
a few free H+ and OH- ions:
H2O (l)  H+ (aq) + OH- (aq).
The conductivity of water can be
improved by adding an acid, such as
dilute sulfuric or hydrochloric acid.
This releases more ions so that more
current flows during electrolysis,
which creates hydrogen and oxygen.
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Using hydrochloric acid
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Redox equations
What happens at the electrodes during
the electrolysis of water?
At the negative electrode:
2H+ + 2e-  H2 (reduction)
At the positive electrode:
4OH-  2H2O + O2 + 4e- (oxidation)
What is the overall equation for the
electrolysis of water?
2H2O (l)  2H2 (g) + O2 (g)
Twice as much hydrogen forms as oxygen. Why is this?
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Glossary
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Anagrams
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Multiple-choice quiz
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