Download Electrolysis

Chemistry PAG 2: Electrolysis
Combined Science PAG C1: Electrolysis
Suggested Activity 1: Electrolysis of brine
Instructions and answers for teachers
and technicians
These instructions cover the learner activity section which can be found on page 12. This Practical
activity supports OCR GCSE Chemistry and Combined Science.
When distributing the activity section to the learners either as a printed copy or as a Word
file you will need to remove the teacher instructions section.
This is a suggested practical activity that can be used as part of teaching the GCSE (9-1)
Gateway Science (A) and Twenty First Century Science (B) specifications.
These are not controlled assessment tasks, and there is no
requirement to use these particular activities.
You may modify these activities to suit your learners and centre. Alternative activities are
available from, for example, Royal Society of Biology, Royal Society of Chemistry, Institute of
Physics, CLEAPSS and publishing companies, or of your own devising.
Further details are available in the specifications (Practical Skills Topics), and in these videos.
OCR recommendations:
Before carrying out any experiment or demonstration based on this guidance, it is the
responsibility of teachers to ensure that they have undertaken a risk assessment in
accordance with their employer’s requirements, making use of up-to-date information and
taking account of their own particular circumstances. Any local rules or restrictions issued
by the employer must always be followed.
CLEAPSS resources are useful for carrying out risk-assessments:
(http://science.cleapss.org.uk).
Centres should trial experiments in advance of giving them to learners. Centres may
choose to make adaptations to this practical activity, but should be aware that this may
affect the Apparatus and Techniques covered by the learner.
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Introduction
In this activity, learners carry out electrolysis of brine (sodium chloride solution) to produce
hydrogen and chlorine gas, and sodium hydroxide solution. They will carry out simple gas tests on
the product, and confirm the alkaline nature of the remaining solution.
An alternative activity is available from RSC Learn Chemistry where the electrolysis occurs in a U
shaped tube, and the formation of both the sodium hydroxide and hydrochloric acid can be
observed: http://www.rsc.org/learn-chemistry/resource/res00000735/colourfulelectrolysis?cmpid=CMP00004981.
A wide range of electrolysis reactions are available in this Classic Chemistry Experiment:
http://www.rsc.org/learn-chemistry/resource/res00000466/the-electrolysis-ofsolutions?cmpid=CMP00000536
Production of hydrogen and oxygen can be demonstrated with a Hoffman Voltameter:
http://practicalphysics.org/electrolysis-water.html
DfE Apparatus and Techniques covered
The codes used below match the OCR Practical Activity Learner Record Sheet (Chemistry /
Combined Science) and Trackers (Chemistry / Combined Science) available online. There is no
requirement to use these resources.
By doing this experiment, learners have an opportunity to develop the following skills:
3 [8]: Use of appropriate apparatus and techniques for: i) conducting and monitoring chemical
reactions; ii) conducting and monitoring chemical reactions, including appropriate reagents and/or
techniques for the measurement of pH in different situations
6 [11]: Safe use and careful handling of gases, liquids and solids, including: ii) using appropriate
apparatus to explore chemical changes and/or products
7 [12]: Use of appropriate apparatus and techniques to: i) draw electrochemical cells for separation
and production of elements and compounds; ii) set up and use electrochemical cells for separation
and production of elements and compounds
8 [13]: Use of appropriate qualitative reagents and techniques to analyse and identify unknown
samples or products including: i) gas tests and ii) flame tests
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Aims
To set-up a micro-scale electrolysis reaction of sodium chloride, and to analyse the products
formed.
Intended class time
30 minutes
Links to Specifications:
Gateway Science (Suite A) – including Working Scientifically (WS)
C3.4a recall that metals (or hydrogen) are formed at the cathode and non-metals are formed at the
anode in electrolysis using inert electrodes
C3.4d describe electrolysis in terms of the ions present and reactions at the electrodes
C3.4e describe the technique of electrolysis using inter and non-inert electrodes
C4.1a recall the simple properties of Groups 1, 7 and 0
C4.2a describe tests to identify selected gases
W1.3a presenting observations and other data using appropriate methods
W1.3e interpreting observations and other data
W1.3f presenting reasoned explanations
W1.4a use scientific vocabulary, terminology and definitions
WS2a carry out experiments
WS2b make and record observations and measurements using a range of apparatus and methods
WS2c presenting observations using appropriate methods
Twenty First Century Science (Suite B) – including Ideas about Science
(IaS)
C1.4.2 describe a test to identify chlorine (using blue litmus paper)
C2.2.6 recall the simple properties of Group 7 elements including their states and colours at room
temperature and pressure, their colours as gases, their reactions with Group 1 elements and their
displacement reactions with other metal halides
C3.3.1 describe electrolysis in terms of the ions present and reactions at the electrodes
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C3.3.3 recall that metals (or hydrogen) are formed at the cathode and non-metals are formed at
the anode in electrolysis using inert electrodes
C3.3.4 use the names and symbols of common elements and compounds and the principle of
conversation of mass to write half equations
C3.3.5 explain reduction and oxidation in terms of gain or loss of electrons, identifying which
species is oxidised and which are reduced
C3.3.8 describe the technique of electrolysis of an aqueous solution of a salt
IaS2.1 present observations and other data using appropriate formats
IaS2.11 in a given context interpret observations and other data (presented in diagrammatic,
graphical, symbolic or numerical form) to make inferences and to draw reasoned conclusions,
using appropriate scientific vocabulary and terminology to communicate the scientific rationale for
findings and conclusions
Mathematical Skills covered
No defined mathematical skill is covered in this experiment.
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Technical Requirements – PER GROUP
Chemicals
Identity
0.5 mol dm–3
aqueous
sodium
chloride,
NaCl(aq)
< 1% w/v
universal
indicator
solution *
hydrogen
gas, H2(g)
PRODUCED
chlorine gas,
Cl2(g)
PRODUCED
Approximate
quantity
required or
produced PER
GROUP
c. 50 cm3
Hazard information
Risk information
Currently not classified as
hazardous at this concentration
Wear eye protection.
DANGER: Flammable Ensure laboratory is well
ventilated. Ensure there
are no naked flames in
DANGER: Extremely close proximity to
electrolysis apparatus.
flammable.
access to
dropper bottles
c. 5 cm3
DANGER
May cause or
intensity fire; oxidiser.
Causes skin irritation.
Causes serious eye
irritation. Toxic if
inhaled. May cause
respiratory irritation.
Very toxic to aquatic
organisms.
c. 5 cm3
Ensure the electrolysis is
turned off as soon as
sample of chlorine has
been produced – this will
minimise release of
chlorine into the
laboratory. Be particularly
aware of class members
with respiratory
problems.
sodium
hydroxide
c. 20 cm3 at
Currently not classified as
solution,
< 0.1 mol dm–3
hazardous at this concentration
NaOH(aq)
PRODUCED
* - commercial formulation, or see CLEAPSS Recipe Book #47
Equipment









electrolysis apparatus (see diagram on Learner sheet)
stand, boss and clamp
pipettes
power pack (up to 9 V DC required)
2 × wires
2 × crocodile clips
2 × micro test tubes OR 2 cm3 plastic syringes with tip–end cut off at the 0 cm3 mark
bunsen burner, heat proof mat and splint
blue litmus paper
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Health and Safety
Eye protection should be worn at all times.
Ensure the laboratory is well ventilated. Take particular care if you have members of the group
with respiratory problems.
Learners may need guidance with their setup as they are using electrical equipment and liquids.
Demonstration of a careful set-up beforehand may be useful, and the powerpacks should be as far
as possible from the electrolysis apparatus to minimise the chances of spillages on the powerpack.
9V batteries can be used in place of powerpacks.
In a class with 15 groups, approximately 75 cm3 of chlorine is produced which can get close to the
workplace limit of 1.5 mg dm–3 depending on the size of the laboratory. Ensure learners are only
running the electrolysis for long enough to collect the sample and the room is well ventilated.
Method
Learners will set up and run the electrolysis of brine solution, producing chlorine at the positive
electrode (anode) and hydrogen at the negative electrode (cathode). The remaining solution will be
alkaline due to the presence of sodium hydroxide. They will make a variety of observations
including the production of these substances.
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Images from trials
Figure 1 – micro test-tube method
Figure 2– plastic syringe method
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Analysis of results – Trial results
Test carried out
Product from where?
Observation
squeaky pop test
cathode / anode
a pop and flame travels quickly down
the barrel
damp blue litmus paper
cathode / anode
the paper turns briefly pink/red then
white
universal indicator
remaining solution
turn the solution blue/purple
The questions you set your learners will depend on the focus of the experiment.
1.
Hydrogen is a flammable gas. Describe and explain the evidence you have for the
production of hydrogen gas. [1 mark]
The gas produced at the negative terminal (cathode) was flammable and burnt with a clean
flame and produced a ‘squeaky’ pop, indicating hydrogen was produced.
2.
Chlorine is an acidic gas. Describe and explain the evidence you have for the
production of chlorine gas. [3 marks]
Blue litmus paper tests turns red in the presence of acids , and turns white in the
presence of a bleach. 
Chlorine gas dissolves into the water forming hydrochloric acid, which turns the litmus
paper pink/red. 
3.
Sodium hydroxide forms an alkaline solution. Describe and explain the evidence
you have for the production of sodium hydroxide. [2 marks]
The remaining solution after the electrolysis turned the universal indicator blue/purple,
indicating an alkaline solution. 
This indicates that sodium hydroxide was formed during the electrolysis.
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Extension opportunities
1.
2.
Write word and symbol equations for the different reactions that have occurred:
a.
Half equation at the cathode
b.
Half equation at the anode
c.
Full redox equation in the sodium chloride solution
d.
Squeaky pop test in the hydrogen tube [8 marks]
a.
hydrogen ions + electrons  hydrogen

+
–
2H (aq) + 2e  H2(g)

b.
chloride ions  chlorine + electrons
2Cl– (aq)  Cl2(g) + 2e–


c.
sodium chloride + water  sodium hydroxide + chlorine + hydrogen
2NaCl (aq) + 2H2O(l)  2NaOH(aq) + Cl2(g) + H2(g)


d.
hydrogen + oxygen  water
2H2(g) + 2O2(g)  2H2O(g)


Define each of the half equations as reduction or oxidation. Explain what these
terms mean. [4 marks]
Reduction is the gain of electrons by a substance. Hydrogen ions are reduced to
hydrogen gas. 
Oxidation is the loss of electrons by a substance. Chloride ions are oxidised to
chlorine gas. 
3.
Write a balanced equation for the reaction between chlorine and water showing the
formation of HClO (choric(I)). What property of the ClO– ion is responsible for its
ability to act as a bleach. [4 marks]
Cl2 + H2O  HCl + HClO 
ClO– is an oxidising agent, and can break bonds in chromophores / coloured
chemicals. 
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4.
Research the uses of the products of electrolysis of a sodium chloride solution.
Give at least two examples for each product and make sure to say where you found
the information. Try to use at least one offline (text book / encyclopedia) and one
online (web page) source.
Product
Use
Source
sodium
cleaning
https://hpd.nlm.nih.gov/cgihydroxide
product
bin/household/search?tbl=TblChemicals&quer
yx=1310-73-2, accessed on 26/04/2016
making soap
chlorine
hydrogen
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making
bleaches
Modern Organic Chemistry by ROC Norman
and DJ Waddington, (Mills and Boon, 1972)
https://en.wikipedia.org/wiki/Bleach, accessed
on 26/4/2016
making
plastics
rocket fuel
Chemistry in Context, Graham Hill and John
Holman (Nelson Thornes Ltd, 2000)
http://www.nasa.gov/topics/technology/hydroge
n/hydrogen_fuel_of_choice.html, accessed on
17/11/2016
making
margarine
Essential Science for GCSE, Susanne Lakin
and John Patefield (Nelson Thornes, 1998)
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Chemistry PAG 2: Electrolysis
Combined Science PAG C1: Electrolysis
Suggested Activity 1: Electrolysis of brine
Learner Activity
Introduction
When an electrolytic solution contains more than one cation (positive) and more than one anion
(negative), then more than one product could be formed at each electrode. Which ion gives up or
gains electrons depends on the stability of the ions in solution.
In this activity, you will carry out the electrolysis of sodium chloride solution, collecting two gaseous
products, and testing the pH of the remaining solution. From your results you can then work out
what the products of the electrolysis are.
The ions present in brine solution are:
sodium ions
Na+
chloride ions Cl–
hydrogen ions
H+
hydroxide ions
OH–
Aims
To set-up an electrolysis reaction of sodium chloride
solution, collect two gaseous products and to analyse the
products formed.
Figure 1: The set-up of the apparatus. If you are using micro
test tubes, these replace the syringes.
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Intended class time
30 minutes
Chemicals and equipment (per group)






electrolysis apparatus
stand, boss and clamp
0.5 mol dm–3 sodium chloride solution,
NaCl(aq) (low hazard)
pipettes
power pack (up to 9 V DC required)
OR 9V battery
2 × wires





2 × crocodile clips
2 × micro test tubes OR plastic
syringes with tip–end cut off at the
0 cm3 mark
Bunsen burner, heat proof mat and
splint
blue litmus paper
universal indicator solution
Health and Safety



Eye protection should be worn at all times.
Ensure the laboratory is well-ventilated.
If using a powerpack, ensure it is positioned away from the electrolysis to minimise liquid
getting on the powerpack.
Method
1. Clamp the electrolysis apparatus to the stand with the boss and clamp.
2. Fill the tube of the electrolysis apparatus with sodium chloride solution to within 1 cm of the
top.
3. If using plastic syringes: draw solution in the tube into the syringe barrel and place over one
of the electrodes.
4. If using micro test tubes: use a plastic pipette to fill the test tube with sodium chloride
solution, place your finger over the end, invert the tube and submerge the end of the tube
into the solution in the electrolysis apparatus.
5. Repeat for the second electrode. If necessary, top up the sodium chloride solution to within
1cm of the top.
6. Connect the wires to the power pack and to the electrodes with the crocodile clips.
7. Set the power pack to 9 V D.C. and run the electrolysis until both tubes are full of gas.
If the bubbles are forming very slowly, tip the syringes / test–tube to allow flow within
the solution.
8. Turn off the power pack.
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9. Decide which test tube you think contains hydrogen – carry out the ‘squeaky pop’ test to
confirm this.
Consider the charge on the hydrogen ion, and therefore which electrode the ions
would be attracted to.
SQUEAKY POP TEST: Carefully remove the syringe from the solution and quickly
place your finger over the end to prevent the gas from escaping. Then light a splint,
move your finger and place the splint next to the end of the barrel. If hydrogen is
present, you will hear a ‘squeaky pop’ and see a flame.
10. Decide which test tube you think contains chlorine gas – test the gas with damp blue litmus
paper to confirm this.
Consider the charge on the chloride ion, and therefore which electrode the ions would
be attracted to.
11. Add 2-3 drops of universal indicator solution to the solution remaining in the electrolysis
apparatus.
With the hydrogen and chloride ion concentration decreasing, consider which ions
remain in solution, and therefore the pH of the solution.
12. If you have time, empty the electrolysis apparatus, rinse with tap water, and repeat the
experiment.
13. Dismantle your experimental set up. Place the indicator paper in the bin, and rinse the
apparatus before returning to your teacher.
Analysis of results
You can draw your own table, or use the one below:
Test carried out
Product from where?
squeaky pop test
cathode / anode
damp blue litmus paper
cathode / anode
universal indicator
Remaining solution
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Observation
© OCR 2017
Your ability to analyse your observations may depend on how much of the GCSE
Chemistry/Combined Science course you have studied. Your teacher will let you know which
questions you should focus on:
1.
Hydrogen is a flammable gas. Describe and explain the evidence you have for the
production of hydrogen gas. [1 mark]
2.
Chlorine is an acidic gas. Describe and explain the evidence you have for the
production of chlorine gas. [3 marks]
3.
Sodium hydroxide forms an alkaline solution. Describe and explain the evidence
you have for the production of sodium hydroxide. [2 marks]
Extension opportunities
1.
Write word and symbol equations for the different reactions that have occurred:
a.
Half equation at the cathode
b.
Half equation at the anode
c.
Full redox equation in the sodium chloride solution
d.
Squeaky pop test in the hydrogen tube [8 marks]
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2.
Define each of the half equations as reduction or oxidation. Explain what these
terms mean. [4 marks]
3.
Write a balanced equation for the reaction between chlorine and water showing the
formation of HClO (choric(I) acid). What property of the ClO– ion is responsible for
its ability to act as a bleach. [4 marks]
4.
Research the uses of the products of electrolysis of a sodium chloride solution.
Give at least two examples for each product and make sure to say where you found
the information. Try to use at least one offline (text book / encyclopedia) and one
online (web page) source.
Product
Use
Source
sodium
hydroxide
chlorine
hydrogen
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DfE Apparatus and Techniques covered
If you are using the OCR Practical Activity Learner Record Sheet (Chemistry / Combined Science)
you may be able to tick off the following skills:
3-i
7-ii
Chemistry
3-ii
6-ii
8-i
8-ii
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7-i
8-i
12-ii
17
Combined Science
8-ii
11-ii
13-i
13-ii
12-i
© OCR 2017