|
 6(a).
Introduction to the electrolysis of copper(II) chloride solution
This pages describes a simple method of
investigating the electrolysis of copper chloride solution is
described.
The formation of the products of electrolysing aqueous copper
chloride is fully explained with the appropriate electrode equations.
So, what are the products of the electrolysis of aqueous copper chloride
solution?
The products of electrolysing copper
chloride solution are copper metal and chlorine gas
In the simple
electrolysis cell (left diagram), the graphite (carbon) electrodes are, through
a large rubber bung, 'upwardly' dipped into an solution of dilute copper
chloride.
In this cheap and
simple apparatus the gaseous product (chlorine) is collected in a small test
tube inverted over the carbon electrodes. You have to fill the little test tubes
with the electrolyte (copper chloride solution), hold the liquid in with your
finger and carefully invert them over the nearly full electrolysis cell.
The electrolysis will
only take place when electricity is passed through the dilute copper(II)
chloride solution. The platinum or carbon electrodes are inert. You can also use
the simple electrolysis set-up illustrated below (right) using two inert metal
wires as electrodes.
 The
electrolyte copper(II) chloride, provides a high concentration of
copper(II) ions Cu2+ and chloride ions Cl–
to carry the current during the electrolysis process.
The tiny traces of
hydrogen ions (H+) and hydroxide ions (OH–)
from water, would not normally be discharged at the electrodes in this
investigation.
The electrolysis will only take place when electricity is passed
through the copper chloride solution.
(b) Details of the electrolysis of copper(II) chloride
solution
The electrode reactions and products of the electrolysis of copper chloride
solution are illustrated by the theory diagram above
Note: The
majority of liquid water consists of covalent H2O molecules,
but there are trace quantities of H+ and OH–
ions from the reversible self–ionisation of water:
H2O(l)
H+(aq) + OH–(aq)
The
half-equations for the
electrolysis of copper(II) chloride solution (the electrolyte)
(a) The negative
cathode electrode reaction
for the electrolysis of copper(II) chloride solution
The positive
copper(II) ions Cu2+
(from copper sulfate) and the H+ ions (from water) are
attracted to the negative cathode.
Only the
copper(II) ion is discharged (preferentially).
The less
reactive a metal, the more readily its ion is reduced on the electrode
surface.
a reduction
electrode reaction
Cu2+(aq)
+ 2e– ==> Cu(s)
(copper deposit)
positive
ion reduction by electron gain
This is a
reduction reaction because the copper ions gain electrons, i.e. the
copper ions are reduced to copper.
(b) The positive
anode electrode reaction
for the electrolysis of copper chloride solution
The (+) anode
attracts the OH– (from water) and Cl–
ions (from copper chloride).
Only the chloride ion
is discharged preferentially in appreciable quantities.
Chloride ions are oxidised
by electron loss to form chlorine gas at the positive electrode.
2Cl–(aq)
– 2e– ==> Cl2(g)
or
2Cl–(aq)
==> Cl2(g) + 2e–
negative
ion oxidation by electron loss
This is an
oxidation reaction because the chloride ions lose electrons i.e. the
chloride ions are oxidised to chlorine molecules.
(c) Extra COMMENTS on the
electrolysis of copper(II) chloride solution
Overall
equation for the electrolysis of aqueous copper chloride:
CuCl2(aq) ===> Cu(s)
+ Cl2(g)
This could be
written more accurately as an ionic equation:
Cu2+(aq) + 2Cl–(aq)
===> Cu(s) + Cl2(g)
(d) Explaining
the product ratio on electrolysis of copper chloride solution
It requires 2
electrons to reduce 1 copper(II) ion, Cu2+, to form one
copper atom.
To form 1 chlorine
molecule, 2 chloride ions, Cl-, must each lose 1 electron
to give 2 chlorine atoms that combine to give 1 chlorine molecule.
Therefore the molar
ratio of copper to chlorine is
1 : 1 for
Cu : Cl2.
This of course,
corresponds with the equations above, summarising the overall
chemical change for the electrolysis of copper(II) chloride
solution.
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and INDEXES
(c) Learning objectives for the
electrolysis of aqueous copper (II) solution
Know that electrolysis requires
a conducting solution of ions (electrolyte of copper chloride) and two inert
solid conducting electrodes e.g. graphite (carbon) or platinum (expensive!).
Know that the electrolyte here
is copper(II) chloride solution containing high concentrations of copper
ions and chloride ions.
Know that electrolysis will
only happen if a d.c. electrical current is passed through the solution of
copper chloride.
Be able to describe the
apparatus required to electrolyse copper chloride solution and be able to
explain and understand the formation of the electrolysis products by:
knowing that the positive
copper ion is reduced by electron gain and discharged at the negative
cathode as copper atoms,
knowing that the negative
chloride ion is oxidised by electron loss and discharged at the positive
anode as chlorine gas molecules,
and be able to write out the
electrode equations for the formation of copper atoms and chlorine gas
molecules.
From the electrode equations,
be able to explain why the mole ratio of copper atoms to chlorine molecules
is theoretically 1 : 1
Know how to test for the chlorine
gas formed from the electrolysis of copper(II) chloride using inert electrodes.
|
SUMMARY OF PRODUCTS FROM
THE ELECTROLYSIS OF COPPER(II) CHLORIDE SOLUTION
with inert carbon
(graphite) electrodes or inert platinum electrodes |
|
Electrolyte |
negative cathode product |
negative electrode
equation
cathode half-equation |
positive anode product |
positive electrode
equation
anode half-equation |
|
Copper(II) chloride
HCl(aq) |
copper deposit |
Cu2+(aq)
+ 2e– ==> Cu(s) |
chlorine gas |
2Cl–(aq)
– 2e– ==> Cl2(g)
or
2Cl–(aq)
==> Cl2(g) + 2e– |
TOP OF PAGE
and INDEXES
(d) Key points about
the electrolysis of copper(II) chloride Solution – core academic content
Process
- Electrolyte:
Aqueous copper(II) chloride solution (CuCl₂).
- Electrodes:
Inert (graphite or platinum).
- Products:
- Cathode (reduction):
Cu2+ + 2e- ==> Cu → Copper metal
deposited.
- Anode (oxidation):
2Cl- ==> Cl2 + 2e- →
Chlorine gas released.
- Overall electrolysis reaction:
CuCl2(aq) ==> Cu(s) + Cl2(g)
Observations
- Cathode:
Reddish-brown copper coating.
- Anode:
Bubbles of chlorine gas (greenish-yellow, pungent smell).
- Gas test:
Chlorine bleaches damp litmus paper.
Applications of the
electrolysis of copper chloride solution
- Copper refining
(purification of copper for electrical wiring).
- Chlorine production
by-product for disinfectants, plastics (PVC), bleaches.
Exam Tips for
questions involving the electrolysis of copper chloride solution
- Always state both products:
copper at cathode, chlorine at anode.
- Write half-equations correctly:
Balance charges and atoms.
- Mention gas tests:
Chlorine bleaches damp litmus paper.
- Link to industry:
Copper refining and chlorine production are common exam contexts.
Common
Misconceptions about the electrolysis of copper chloride solution
- “Hydrogen is produced at the
cathode” → Correction: Copper is less reactive than hydrogen, so copper
is deposited.
- “Electrolysis of CuCl2
produces sodium hydroxide” → Correction: That occurs in brine
electrolysis, not here.
- “Electrons move through the
solution” → Correction: Electrons move in the external circuit; ions
move in the solution.
- “Only chlorine is produced”
→ Correction: Copper metal is also formed.
Revision Strategy
for the electrolysis of copper chloride solution
- Draw annotated diagrams
of copper(II) chloride electrolysis apparatus.
- Practice half-equations
until fluent.
- Use comparative tables
(CuCl₂ versus brine versus molten salts).
- Review exam board command words
(“describe,” “explain,” “evaluate”).
(e)
Practise exam questions based on the electrolysis of copper chloride solution
|
GCSE
level chemistry - practise exam questions on
the electrolysis
of copper(II) chloride solution
Jot
down your responses and check out the answers:
If you think there are
any errors, please email me asap at
chem55555@hotmail.com
I don't mind if students/teachers do a selected printout
of these questions and answers.
I have added extra details, and ,hopefully, helpful
comments in the
ANSWER feedback.
Q1 In the
context of copper chloride solution, what do we mean by
electrolysis?
Q2 In
electrolysis, what is the electrical conducting
solution/melt called and why can it conduct electricity?
Q3 What
are the contacts between the solution/melt and the d.c.
supply called, and what does it mean if they are
described as inert? Quote two examples of inert
electrodes for the electrolysis copper chloride solution
- do NOT say copper!, copper wire is ok for the external
d.c. supply circuit.
Q4 (a)
What are the names of the electrodes and what electrical
charge exists on each surface?
(b) In the
electrolysis of copper chloride solution, when the d.c.
current is switched on, what is the origin of the ions
and which electrode do they move towards?
Q5 Assuming the electrolysis of copper(II) chloride solution
with inert electrodes:
(a) Describe what happens at the cathode in terms of the
half-equation, state whether it is an oxidation or
reduction process, describe and name the product.
(b) Describe what happens at the anode in terms of the
half-equation, state whether it is an oxidation or
reduction process, name the product and describe a
simple chemical test for it.
(c) Write
an equation to show the overall chemical change for the
electrolysis of aqueous copper(II) chloride solution.
I have added extra details, and ,hopefully, helpful
comments in the
ANSWER feedback.
If you think there are
any errors, please email me asap at
chem55555@hotmail.com
Don't forget the wider ranging multiple choice exam
practise questions beyond electrolysis Part 3.
Simple Quiz on the basics of electrolysis
Electrolysis Quiz (GCSE 9-1 FT Foundation Level (easier)
Electrolysis Quiz (GCSE 9-1 HT Higher Level (harder)
Electrolysis Quiz including calculations (GCSE 9-1 HT Higher Level)
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ANSWERS GCSE
level chemistry - practise exam questions on
the electrolysis
of copper(II) chloride solution
Jot
down your responses and check out the answers:
ANSWERS
If you think there are
any errors, please email me asap at
chem55555@hotmail.com
I don't mind if students/teachers do a selected printout
of these questions and answers.
I have added extra details, and ,hopefully, helpful
comments in the
ANSWER feedback.
Q1 In the
context of copper chloride solution, what do we mean by
electrolysis?
ANSWER
Electrolysis uses electrical energy to break down a
compound into simpler substances e.g. an element, using
an energy input of a d.c. electricity supply. It
forces a chemical change to take place in a solution
that wouldn't happen spontaneously. Here chlorine is
split from copper chloride and a copper deposit is
formed..
Q2 In
electrolysis, what is the electrical conducting
solution/melt called and why can it conduct electricity?
ANSWER
The conducting solution/melt is called the electrolyte, and must contain free moving ions to
carry the electrical - current between the
electrodes, e.g. in this case copper(II) ions (Cu2+)and chloride
ions (Cl-) from copper(II) chloride.
Q3 What
are the contacts between the solution/melt and the d.c.
supply called, and what does it mean if they are
described as inert? Quote two examples of inert
electrodes for the electrolysis copper chloride solution
- do NOT say copper!, copper wire is ok for the external
d.c. supply circuit.
ANSWER
Electrodes, and described as inert if they do
not change chemically during the electrolysis process
e.g. carbon/graphite rods or platinum wire. Do NOT say
inert means they are not involved in the electrolysis
process of copper(II) chloride solution.
Q4 (a)
What are the names of the electrodes and what electrical
charge exists on each surface?
(b) In the
electrolysis of copper chloride solution, when the d.c.
current is switched on, what is the origin of the ions
and which electrode do they move towards?
ANSWERS
(a) In electrolysis, the negative electrode is called
the cathode (-ve) and the anode electrode is positive
(+ve).
(b) Most of the positive ions are copper(II) ions (Cu2+)
from the copper chloride (CuCl2) plus a trace of hydrogen
ions from ionised water molecules (H2O
== trace ==> H+ + OH-)
and they move towards the -ve cathode.
Most of the negative ions are chloride ions (Cl-)
from the dissolved copper chloride salt, plus a trace of hydroxide ions
(OH-) from ionised water, and these ions move towards
the +ve anode.
Q5 Assuming the electrolysis of copper(II) chloride solution
with inert electrodes:
(a) Describe what happens at the cathode in terms of the
half-equation, state whether it is an oxidation or
reduction process, describe and name the product.
(b) Describe what happens at the anode in terms of the
half-equation, state whether it is an oxidation or
reduction process, name the product and describe a
simple chemical test for it.
(c) Write
an equation to show the overall chemical change for the
electrolysis of aqueous copper(II) chloride solution.
ANSWERS
(a) -ve cathode
attracts the positive hydrogen ions and copper ions,
which are preferentially discharged before hydrogen
ions, and
is reduced by electron gain to give an orange-brown
deposit of copper on the cathode.
Cu2+(aq)
+ 2e- ===> Cu(s)
(b) +ve anode attracts
the negative hydroxide and chloride ions, BUT, only the
chloride ions are preferentially discharged and oxidised
to give chlorine molecules.
2Cl-(aq)
===> Cl2(g) + 2e-,
the product gas turn litmus red and bleaches it white.
(c) CuCl2(aq)
===> Cu(s) + Cl2(g)
Don't forget the wider ranging multiple choice exam
practise questions beyond electrolysis Part 3.
Simple Quiz on the basics of electrolysis
Electrolysis Quiz (GCSE 9-1 FT Foundation Level (easier)
Electrolysis Quiz (GCSE 9-1 HT Higher Level (harder)
Electrolysis Quiz including calculations (GCSE 9-1 HT Higher Level)
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