Electrolysis
● It is the breakdown of an ionic compound (molten or aqueous solution) by passing electricity
through them, breaking them down into their elemental form
● Electrolyte
○ A substance that conducts electric current when molten or dissolved in water, with
chemical reactions at the electrodes
● Electrodes
○ The pieces of metal or carbon through which the current enters and leaves the electrolyte
● Cathode
○ Negative electrode(attracts positive ions, cations)
● Anode
○ Positive electrode(attracts negative ions, anions)
● Weak electrolyte
○ A poor conductor of electricity because it is only partially ionised (not a lot of ions present
to conduct the electricity)
● Non-electrolyte
○ A substance that does not conduct electricity in the liquid phase
● Reactions at cathode
○ Electrons flow from the battery to the cathode. Cations(metals and hydrogen ions) in the
electrolyte are attracted to the cathode.
○ Cations accept electrons from the cathode and therefore metals and hydrogen are formed
at the cathode
○ For example
■ Cu2+ + 2e- → Cu
■ 2H+ + 2e- → H2
■ Al3+ + 3e- → Al
● Reactions at anode
○ Electrons flow from anode to the battery. Anions(non-metals except hydrogen) are
attracted to the anode
○ If the anode is inert (carbon or platinum) the negative ions lose electrons to the anode and
form elements
○ For example
■ 2Cl- - 2e- → Cl2
■ 4Oh- - 4e-→ O2 + 2H2O
○ If the anode is not inert (silver, copper or other reactive metals) the metal atoms of the
anode lose electrons and form positive ions. The anode will therefore dissolve and become
smaller
○ For example
■ Cu - 2e-→ Cu2+
■ Zn -2e-→ Zn2+
● Ions of an electrolyte
○ The electrolyte can either be molten or
aqueous
○ A molten substance means that the
substance has been melted down. The ions
therefore come only from the substance
itself
○ NaCl (molten) → Na+ + Cl-
○ An aqueous solution means that the
substance is dissolved in water. The water
molecules themselves can ionize so you will
always find hydrogen and hydroxide ions in
addition to the ions from the solute
○ NaCl(aqueous) → Na+ + Cl- + OH- + H+
● The discharge of ions
○ Ions are discharged at the anode or cathode
○ At the cathode you will find that the H+ will get discharged rather than Na+
○ At the anode you will find that OH- will get discharge rather than Cl-
● Electrochemical series tells us which ions discharge easier than others. The lower ions each
series will be the one to get discharged
● In a concentrated solution, Cl- will be discharged rather than OH- despite what it says on the
electrochemical series. In a dilute solution, OH- will be discharged instead
● Molten sodium chloride (inert electrodes)
○ Ions present → Na+ , Cl-
○ Reaction at cathode = Na+ + e- → Na
○ Reaction at anode = 2Cl- → Cl2 + 2e-
○ Sodium chloride is decomposed
● Concentrated aqueous sodium chloride (inert electrodes)
○ Ions present → Na+ , Cl- , H+ , OH-
○ Reaction at cathode = 2H+ + 2e- → H2
○ Reaction at anode = 2Cl- → Cl2 + 2e-
○ Na+ and OH- remain, making sodium hydroxide
● Concentrated hydrochloric acid (inert electrodes)
○ Ions present → H+ , Cl- , OH-
○ Reaction at cathode = 2H+ + 2e- → H2
○ Reaction at anode = 2Cl- → Cl2 + 2e-
○ Acid therefore gets used up in the electrolyte
● Dilute sulfuric acid (inert electrodes)
○ Ions present → H+, OH- , SO42-
○ Reaction at cathode = 2H+ + 2e- → H2
○ Reaction at anode = 4OH → 2H2O + O2 + 4e-
○ Acid gets more concentrated as water gets used up
● Aqueous copper sulphate (inert electrodes)
○ Ions present → Cu2+ . H+, OH- , SO42-
○ Reaction at cathode = Cu2+ + 2e- → Cu
○ Reaction at anode = 4OH- → 2H2O + O2 + 4e-
○ H+ and SO4 ions remain in the solution (sulfuric acid)
● Aqueous copper sulphate (copper electrodes)
○ Ions present → Cu2+ , H+ , OH- , SO42-
○ Reaction at cathode = Cu2+ + 2e- → Cu
○ Reaction at anode = Cu → Cu2+ + 2e-
○ Copper deposited at the cathode becomes thicker. Copper is removed at the anode and it
gets thinner. The electrolyte remains the same since one electrode produces copper ions
whereas the other removes them. This process is used to electroplate other metals with
copper
● Electroplating
○ This is used to plate one metal with another.
● The metals commonly used to electroplate are copper, chromium, nickel and silver
● The 2 main reasons for electroplating are appearance and protection from corrosion
Refining metals
● Metals can be refined or purified by electrolysis. The impure metal forms the anode, the cathode
is a small piece of pure metal and electrolyte is an aqueous metal salt.
● Cathode
○ Copper ions from solution lose their charge and copper is deposited
○ Cu2+ + 2e- → Cu
● Anode
○ Copper atoms lose their valence electrons and go into solutions as ions
○ Cu → Cu2+ + 2e-
● Overall pure copper is transferred from the anode to the cathode. The impurities from the
copper are left as ‘anode slime’ and the cathode becomes a large piece of pure copper
Brine
● Brine is a concentrated solution of aqueous sodium chloride
● When electrolysed it produces chlorine, hydrogen and sodium hydroxide
● The electrolyte is concentrated sodium chloride which contains H+ , Cl- , OH-
● The H+ ions are discharge at the cathode as hydrogen gas
● The Cl- ion are discharged at the anode as chlorine gas
● The Na+ and OH- ions remain behind and form the NaOH solution
● Uses of products of brine
○ Chlorine → solvents for drycleaning, medicinal drugs, weekillers, paints, bleaches and
hydrogen chloride and hydrochloric acid
○ Sodium hydroxide → soaps, detergents, paper, ceramics, dyes, medical drugs
○ Hydrogen → nylon, hydrogen peroxide, fuel in hydrogen fuel cells
Conductors and Insulators
● Copper and aluminium are commonly used as conductors in electric cables. You need to know why
they are good for this purpose
● Copper
○ Good conductor of electricity
○ Ductile
○ Easily purified
● Aluminium
○ Good conductor
○ Resists corrosion
○ Low density, allowing high diameter cables to be used. This reduces resistance and sagging
● Plastics and ceramics are often used as insulators in electric cables
● Plastics
○ Do not conduct electricity
○ Flexible and easily molded
○ Non-biodegradable
● Ceramics
○ Do not conduct electricity
○ High melting points allowing use at high temperatures
○ Not affected by water or oxygen and can be molded into complex shapes
