|
(a)
ELECTROLYSIS 3. Introduction to the electrolysis of sodium chloride solution
ELECTROCHEMISTRY revision notes on
electrolysis:
cells, experimental methods, apparatus, batteries, fuel cells and
industrial applications of electrolysis.
Here are full descriptions of the
apparatus and detailed explanations are provided for the electrolysis of
sodium chloride solution (brine) with carbon electrodes (and also
aqueous sodium bromide and potassium iodide.
The electrolysis of aqueous solutions
with inert electrodes (carbon or platinum)
In this case the products of electrolysing aqueous sodium chloride solution are
hydrogen gas, chlorine gas and sodium hydroxide solution
The electrolysis of aqueous
sodium chloride (often referred to as 'brine' solution) is described in
terms of apparatus and products formed. What are the products of the
electrolysis of aqueous sodium chloride solution (brine)?
The above apparatus is the simplest possible way to demonstrate
electrolysis and show that a chemical change takes place when an electric
current is passed through solutions of ions e.g. solutions of sodium chloride,
sodium bromide and potassium iodide.
  The simple,
but more elaborate apparatus
illustrated on the right can be used in simple school or college
experiments for the electrolysis of sodium chloride solution (often
referred to as 'brine' in the chemical industry). The graphite (carbon) electrodes are, through a large
rubber bung,
'upwardly' dipped into
an solution of the sodium chloride solution (the electrolyte).
The cell can be made from plastic pipe and
a big rubber bung with two holes in it. In the simple apparatus
the gaseous products (hydrogen and chlorine) are collected in small test
tubes inverted over the carbon electrodes and chemical tests performed
on them. You have to fill the little test tubes with the electrolyte (sodium
chloride solution), hold the liquid in with your finger and carefully invert
them over the nearly full electrolysis cell.
A
more elaborate format is to use a Hoffman Voltammeter (above left
diagram)
using
platinum electrodes and accurately calibrated collecting tubes like
burettes. The Hofmann voltammeter is filled with the electrolyte (aqueous
sodium chloride solution) by opening the taps at the top of the outer tubes to
allow any gas to escape. The gases formed on the electrolysis of the dilute
'brine' solution can be collected via the same taps. The students should note that nothing happens until you switch
on the electricity supply (see simple animation above!). The platinum or carbon
electrodes are inert. The
industrial electrodes must be made of an inert material like
platinum/titanium which is not attacked by chlorine or alkali, but
in the school /college laboratory, the Hofmann voltammeter is a good
demonstration (platinum electrodes) and the 'simple cell' for students uses
carbon/graphite electrodes which are reasonably inert.
However a
simple cell using carbon electrodes can be used by students/pupils to demonstrate the
industrial process in the laboratory and the simple apparatus
(above right) can also be used in schools using two inert wire electrodes.
The electrolysis will only take place when electricity is passed through the
sodium chloride solution.
(b) Details of products and explanation
of the electrolysis of sodium chloride solution
There is quite a lot to 'digest' for this
particular electrolysis investigation.
The electrode reactions and products of the
electrolysis of sodium chloride solution (brine) are illustrated by the theory
diagram above
The electrolyte sodium chloride solution (brine),
provides a high concentration of sodium ions Na+ and chloride
ions Cl– to carry the current during the electrolysis process.
Initially there are only traces of hydrogen ions
H+ and hydroxide
ions OH– from the self-ionisation of water.
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)
Brine is moderately concentrated aqueous sodium chloride solution
(brine) with carbon (graphite) gives equal volumes of hydrogen gas (hydrogen
ions H+ discharged at the –ve cathode) and green
chlorine gas (chloride ions Cl– discharged at the +ve
anode) with sodium hydroxide left in solution. The electrolysis will only
take place when electricity is passed through the sodium chloride solution.
The
electrode equations and the theory of what happens in the
electrolysis of aqueous sodium chloride
The
half-equations for the electrolysis of sodium
chloride solution (the electrolyte brine).
(1)
The
negative cathode electrode reaction for the electrolysis of
brine (sodium chloride solution)
The negative (–) cathode
attracts the Na+ (from sodium chloride) and H+ ions
(from water). Only
the hydrogen ions are discharged at the cathode. The more reactive a
metal, the less readily its ion is reduced on the electrode surface.
The hydrogen ions are reduced by
electron (e–) gain to form hydrogen molecules
at the negative electrode which attracts positive ions.
Cathode (-ve):2H+(aq)
+ 2e– ==> H2(g)
(reduction
half-reaction)
half-equation: positive ion reduction by
electron gain, hydrogen ions to neutral hydrogen molecules
other possible equations
2H2O(l) + 2e–
==>
H2(g) + 2OH-(aq)
or 2H3O+(aq)
+ 2e– ==> H2(g) + 2H2O(l)
Nothing happens to the sodium ion, but it is
still important (see after the anode reaction has been described about what
else left and formed).
In fact, if sodium was released (which it isn't), it
would immediately react with water to give hydrogen, the same product you get
from the reduction of the hydrogen ion.
Test for the cathode gas - colourless gas gives
a squeaky pop with a lit splint – hydrogen
(2) The positive anode electrode
reaction for the electrolysis of brine (sodium chloride solution)
The positive anode
attracts the negative hydroxide OH– ions (from water) and chloride
Cl– ions
(from sodium chloride). Only
the chloride ion is discharged in appreciable quantities i.e. it is
preferentially oxidised to chlorine.
The
chloride ions are oxidised by electron loss to give chlorine
molecules at the positive electrode which attracts negative ions.
an oxidation
electrode reaction
Anode (+):
2Cl–(aq)
– 2e– ==> Cl2(g)
(oxidation
half-reaction)
or
2Cl– ==>
Cl2(g) + 2e–
negative ion oxidation
by electron loss, chloride ion to neutral chlorine molecules
Note that you can write
these anode oxidation reactions either way round
The chloride
ion is oxidised to chlorine gas molecules in any chloride salt solution
electrolysed,
hydrochloric acid
and in any electrolysis
of a molten chloride salt.
Test for the anode gas - pale green gas turns
damp blue litmus red (slightly acidic gas) and then bleaches it white (strong
bleaching agent) – chlorine (test 2 gas 2)
Usually nothing happens to the hydroxide ion BUT
it is important, because, the hydroxide ion, with the unchanged sodium ion,
means the
residual solution contains sodium hydroxide. In fact this is how sodium
hydroxide is manufactured in the chemical industry.
Residual Na+ + OH– = NaOH, a
familiar formula! The presence of the alkali sodium hydroxide, can be shown by
adding universal indicator/red litmus to the residual brine solution (aqueous sodium chloride)
at the end of the experiment.
The
indicator will turn from green to purple because of the formation of alkaline
sodium hydroxide.
Note that, if most of the chloride ions
have been discharged as chlorine molecules, you can then get some
oxygen gas formed at the anode i.e. like in the electrolysis of
water, and chloride ions are being replaced by hydroxide ions which
can be oxidised to oxygen at the anode.
Anode (+):
2H2O(l)
– 4e– ==> 4H+(aq)
+ O2(g)
or the more likely
half-reaction equation
Anode (+):
4OH–(aq)
– 4e– ==> 2H2O(l)
+ O2(g) (oxygen gas)
For more, see Extra COMMENTS 2.
Summary of the possible products from the
electrolysis of aqueous sodium chloride solution
The three products from the electrolysis of
sodium chloride solution are all of industrial significance:
hydrogen, chlorine and sodium hydroxide.
Overall equation for the electrolysis of brine:
2NaCl(aq) + 2H2O(l) ==> H2(g) + Cl2(g) +
2NaOH(aq)
and the ionic equation is ...
2H2O(l) + 2Cl-(aq)
+ 2Na+(aq) ==> 2Na+(aq) + 2OH-(aq)
+ H2(g) + Cl2(g)
or more correctly
2H2O(l)
+ 2Cl-(aq) ===> 2OH-(aq) + H2(g)
+ Cl2(g)
by treating the sodium ion as a
spectator ion, though it is an important end product, in combination
with the other residual ion, the hydroxide ion, they constitute sodium hydroxide, the third major product important for the
chlor-alkali chemical industry.
Another complication in the electrolysis of
sodium chloride solution, is that the chlorine will react with sodium hydroxide
to form sodium chlorate(I) NaOCl, which is how a bleach is made - but this
situation is usually studied at a more advanced level of chemistry.
For the industrial electrolysis of brine and the
uses of the products see
The Halogens
and Salt
page.
TOP OF PAGE
and INDEXES
(c)
Five extra comments on the
electrolysis of sodium chloride solution and other related electrolysis
reactions
Some comments make reference to the diagram of
the electrolysis of brine above.
1. Tests for the gases formed in the electrolysis
of sodium chloride solution
The (–) cathode gas - colourless gas gives
a squeaky pop with a lit splint – hydrogen (test 1 gas 1)
The (+) anode gas - pale green gas turns
damp blue litmus red and then bleaches it white – chlorine (test 2 gas 2)
For the industrial electrolysis of brine and the
uses of the products see
The Halogens
page.
You can collect samples of gases through the
taps on the Hofmann voltammeter or from the little test tubes in the simple
school electrolyse cell. The universal indicator changes from green (~ pH
7 for the salt solution) to blue-purple (Ph > 7) as the alkali sodium hydroxide
is formed.
2. In very dilute sodium chloride
solution, oxidation of hydroxide ions or water molecules can produce oxygen gas
as well as chlorine gas.
Advanced Level Student Note on the
ratio of chlorine to oxygen production:
At low concentrations of chloride ion a competing
oxidation of water or hydroxide ion can occur, particularly as the concentration
of hydroxide ion is increasing as the electrolysis proceeds.
The increase in oxygen to
hydrogen ratio through the electrolysis is
essentially a concentration effect and occurs as
most of the chloride ions have been oxidised to
chlorine.
If you consider
the electrode potentials:
O2/OH-
Eθ = +0.40 V and for Cl2/Cl-
Eθ = +1.36 V,
then, logically, the hydroxide ion OH- is
more easily oxidised than the chloride Cl-
ion.
BUT, initially the concentration-kinetic factor
wins out, the much higher concentration of chloride
ions over hydroxide ions leads to the much more
probable oxidation of the chloride ion to form
chlorine.
So, as the brine (NaCl(aq))
becomes depleted in chloride ions, and the hydroxide
ion is increasing (a by-product of the
electrolysis), the probability of OH- ion
oxidation to give oxygen is more likely, so you
begin to get an increase in the O2/Cl2
ratio in the product gases at the positive anode
electrode the longer the electrolysis continues - at
least until no chloride ions are left to be
discharged.
3. Theoretically, in the
electrolysis of sodium chloride solution, the gas volume ratio for H2
: Cl2 is 1 : 1
BUT chlorine is slightly soluble in water and also
reacts with the sodium hydroxide formed (the residual solution).
Therefore the volume of chlorine gas
observed is seems to be less than predicted.
Why a theoretical 1 : 1 gas volume ratio? It takes
two electrons to reduce two hydrogen ions to a hydrogen molecule.
It takes the
removal of two electrons, one from each chloride ion, to form a chlorine
molecule.
So, for the same quantity of current passing (electron flow), you
should expect to form equal numbers of hydrogen and chlorine molecules.
4.
Electrolysis of molten
sodium chloride
This
gives
silvery sodium metal and pale green chlorine gas.
This is a simpler electrolysis situation where
the ionic compound sodium chloride on melting provides a highly concentrated
mixture of positive sodium ions and negative chloride ions.
It also illustrates
the difference sometimes, between electrolysing the pure molten salt and its
aqueous solution in water. Here there is no possibility of hydrogen being
formed, so you get sodium metal formed at the cathode.
The electrode reactions and products of the
electrolysis of the molten ionic compound sodium chloride are illustrated by the
theory diagram above
molten sodium chloride electrolyte NaCl(l)
(i)
molten sodium formed at the negative cathode electrode which
attracts the positive sodium ions
Cathode (-):Na+(l)
+ e– ==> Na(l)
Half equation: a reduction
electrode reaction (electron gain)
positive ion reduction by
electron gain, sodium ion to neutral sodium atoms
sodium
ion reduced to sodium metal atoms:
typical of
electrolysis
of molten chloride salts to make chlorine and the metal.
(ii) chlorine gas formed at the positive anode electrode which
attracts the negative chloride ions
Anode (+):
2Cl–(l)
– 2e– ==> Cl2(g)
or
2Cl–(l)
==> Cl2(g) + 2e–
Half equation: an oxidation electrode reaction (electron loss),
negative chloride ions lose electrons to give neutral chlorine
molecules.
See
The extraction of sodium
from molten sodium chloride using the 'Down's Cell'
|
SUMMARY OF PRODUCTS FROM THE ELECTROLYSIS
OF SODIUM CHLORIDE:
aqueous solution or molten salt with inert electrodes like carbon
(graphite) or platinum
Note the
alternative ways (or) of writing the half reactions - the
electrode equation
|
|
Electrolyte |
negative cathode
product |
negative electrode
cathode half-equation |
positive anode
product |
positive electrode
anode
half-equation |
|
molten sodium chloride
NaCl(l) |
molten sodium |
Na+(l)
+ e– ==> Na(l)
reduction,
electron gain |
chlorine gas |
2Cl–(l)
– 2e– ==> Cl2(g)
or
2Cl–(l)
==> Cl2(g) + 2e–
oxidation, electron loss |
|
aqueous
sodium chloride solution (brine)
NaCl(aq) |
hydrogen |
2H+(aq)
+ 2e– ==> H2(g)
or
2H3O+(aq)
+ 2e– ==>
H2(g) + 2H2O(l)
or
2H2O(l)
+ 2e– ==>
H2(g) + 2OH–(aq)
reduction,
electron gain |
chlorine gas |
2Cl–(aq)
– 2e– ==> Cl2(g)
or
2Cl–(aq)
==> Cl2(g) + 2e–
oxidation, electron loss |
(d)
The electrolysis of aqueous
solutions of sodium bromide and potassium iodide
The concept diagrams for aqueous sodium
chloride are equally valid, just substitute in your head Br or I for Cl.
Because sodium and potassium are reactive
metals, so you will get hydrogen ions preferentially discharged at the negative cathode giving
hydrogen gas.
Sodium bromide and potassium iodide give colourless
solutions when dissolved in water, therefore it is quite easy to spot if the
coloured halogen elements are formed on the positive anode electrode.
You can use any simple electrolysis apparatus to these
experiments.
Sodium Bromide, NaBr(aq)
Sodium bromide gives hydrogen at the
cathode and the element bromine at the anode - you would see a
orange-brown colouration appearing around the positive electrode.
cathode (-): 2H+(aq)
+ 2e– ==> H2(g)
anode (+):
2Br–(aq)
– 2e– ==> Br2(aq)
Potassium Iodide, KI(aq)
Potassium iodide gives hydrogen at the
cathode and the element iodine at the anode - you would see a brown
colouration appearing around the positive electrode and the solution may become
very dark or even a dark solid
precipitate if sufficient iodine is formed.
cathode (-):
2H+(aq)
+ 2e– ==> H2(g)
anode (+):
2I–(aq)
– 2e– ==> I2(aq/s)
(e)
Learning objectives for the electrolysis of
sodium chloride, sodium bromide and
potassium iodide solutions
Know the similarities and differences in the electrolysis of aqueous sodium
chloride solution and molten sodium chloride solution.
Know that the electrolysis of sodium bromide and potassium iodide solutions
are similar to electrolysis of sodium chloride solution.
Know that electrolysis requires a conducting solution of ions (electrolyte
of sodium chloride solution or molten sodium chloride)
and two inert solid conducting electrodes e.g. graphite (carbon) or platinum
(expensive!).
Know that the electrolyte here contains free moving metal ions and
non-metal ions from the melted salt e.g. sodium and potassium metal ions and
chloride, bromide and iodide non-metal halide ions.
Know that electrolysis will only happen if a d.c. electrical current is
passed through the sodium chloride solution/melt and reduction and oxidation
reactions occur on passage of the electric current and the ions discharged
to give the products e.g. hydrogen, sodium or chlorine depending on
conditions.
Where practicable in a school or college laboratory, be able to describe
the apparatus required to electrolyse sodium chloride, sodium bromide and
potassium iodide solutions and be able to explain and understand the formation of the electrolysis
products by:
knowing that the positive ions are reduced by electron gain and
discharged at the negative cathode as hydrogen (solution) or sodium atoms
(molten salt),
knowing that the negative chloride, bromide or iodide ions are oxidised by electron loss and
discharged at the positive anode as chlorine, bromine or iodine molecules,
and be able to write out the electrode equations (half equations) for the formation of
hydrogen or neutral metal atoms by electron gain reduction and chlorine,
bromine and iodine molecules from the oxidation of chloride, bromide
or iodide ions by electron loss.
From the electrode equations, be able to explain why the mole ratio e.g.
of hydrogen to chlorine molecules is
theoretically 1
: 1 in the electrolysis of chloride salt solutions.
Be able to recognise from observations whether chlorine, bromine or iodine is
formed at the anode from the electrolysis of halide salt solutions.
Know how to test for chlorine gas formed from the electrolysis of a chloride
salt using inert electrodes and recognise brown vapour/solution formed at the
anode is an indication that bromine was formed in the electrolysis of bromide
salts, and a very dark coloured solution or black precipitate indicating the
formation of iodine.
TOP OF PAGE
and INDEXES
(f)
Key
revision points about the Electrolysis of Brine – Core Academic Content
Process
- Electrolyte:
Concentrated sodium chloride solution (brine).
- Electrodes:
Inert (graphite or titanium).
- Products:
- Cathode (reduction):
2H+ + 2e- ==> H2 →
Hydrogen gas.
- Anode (oxidation):
2Cl- ==> Cl2 + 2e- →
Chlorine gas.
- Remaining solution:
Sodium hydroxide (NaOH) forms in solution.
- Overall reaction:
2NaCl(aq) + 2H2O(l) ==> H2(g) + Cl2(g)
+ 2NaOH(aq)
Industrial
Importance
- Hydrogen:
Used in fuel, margarine manufacture (hydrogenation).
- Chlorine:
Used in disinfectants, plastics (PVC), bleaches.
- Sodium hydroxide:
Used in soap, paper, detergents.
Applications of the
electrolysis of sodium chloride solution
- Production of ...
- Hydrogen:
Clean fuel, ammonia production.
- Chlorine:
Water treatment, plastics.
- Sodium hydroxide:
Soap, paper, detergents.
Exam Tips for
questions involving the electrolysis of sodium chloride solution
- Always state all three products:
hydrogen, chlorine, sodium hydroxide.
- Write half-equations correctly:
Balance charges and atoms.
- Mention industrial uses:
Examiners often ask “Why is this process important?”
- Gas tests:
Hydrogen → “pop” test; Chlorine → bleaches damp litmus paper.
Common
Misconceptions about the electrolysis of sodium chloride solution
- “Electrolysis of brine produces
sodium metal” → Correction: Sodium is too reactive; hydrogen is
produced instead.
- “Only chlorine is produced”
→ Correction: Hydrogen and sodium hydroxide are also formed.
- “Electrons move through the
solution” → Correction: Electrons move in the external circuit; ions
move in solution.
- “Pure water is electrolysed in
brine electrolysis” → Correction: It’s concentrated sodium
chloride solution, not pure water.
Revision Strategy
for electrolysis of sodium chloride solution
- Draw annotated diagrams
of brine electrolysis apparatus.
- Practice half-equations
until fluent.
- Use comparative tables
(brine versus water versus molten salts).
- Review exam board command words
(“describe,” “explain,” “evaluate”).
|
(g) GCSE
level chemistry - practise exam questions on
the electrolysis
of sodium 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 sodium 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 sodium 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 sodium 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 sodium 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, name the product and describe a
simple chemical test for it.
(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) Which ions are left in solution and what is the
solution product and a test for it, after electrolysis
is complete? Write a balanced equation to summarise the
overall chemical change.
(d) Why
wouldn't sodium metal be discharged at the cathode?
(e) Is it
possible for another gas to form at the anode as the
electrolysis proceeds?
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.
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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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 sodium 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 sodium 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 sodium chloride and sodium hydroxide 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 sodium ions and chloride
ions from sodium 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 sodium chloride solution
- do NOT say copper!, copper wire is ok for the external
d.c. supply circuit.
ANSWERS
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 sodium chloride solution.
Q4 (a)
What are the names of the electrodes and what electrical
charge exists on each surface?
(b) In the
electrolysis of sodium 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 sodium ions (Na+)
from the sodium chloride (NaCl) 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 sodium chloride, plus a trace of hydroxide ions
(OH-) from ionised water, and these ions move towards
the +ve anode.
Q5 Assuming the electrolysis of sodium 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, name the product and describe a
simple chemical test for it.
(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) Which ions are left in solution and what is the
solution product and a test for it, after electrolysis
is complete? Write a balanced equation to summarise the
overall chemical change.
(d) Why
wouldn't sodium metal be discharged at the cathode?
(e) Is it
possible for another gas to form at the anode as the
electrolysis proceeds?
ANSWERS
(a) -ve cathode
attracts the positive hydrogen ions and sodium ions, but
the hydrogen ions are preferentially discharged and
reduced by electron gain to give hydrogen gas molecules.
2H+(aq)
+ 2e- ===> H2(g),
the product gas gives a squeaky pop with a lit splint.
(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) Sodium ions and
hydroxide ions are left in solution which gets more
alkaline as the electrolysis proceeds i.e. the alkali
sodium hydroxide is formed which turns litmus or
universal indicator blue.
2NaCl(aq) + 2H2O(l) ===>
2NaOH(aq) + H2(g) +
Cl2(g)
Note you need two of
equation (a) to balance up equation (b), 4 electrons
worth of change!
(d) If sodium was formed it would immediately react
with water to form hydrogen and sodium hydroxide.
Sodium is too reactive to be discharged from aqueous
solution, different if molten sodium chloride.
(e) As the chloride ions decrease in concentration,
there is an increasingly greater chance that the
hydroxide ions become oxidised to oxygen gas at the
anode.
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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