Chemistry PAG 2: Electrolysis

Combined Science PAG C1: Electrolysis

Suggested Activity 1: Electrolysis of brine

Instructions and answers for teachersand 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 thesevideos.

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 usefulfor carrying out risk-assessments: ().

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:

A wide range of electrolysis reactions are available in this Classic Chemistry Experiment:

Production of hydrogen and oxygen can be demonstrated with a Hoffman Voltameter:

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

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

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.

Technical Requirements – PER GROUP

Chemicals

Identity / Approximate quantity required or produced PER GROUP / Hazard information / Risk information
0.5 mol dm–3 aqueous sodium chloride, NaCl(aq) / c. 50 cm3 / Currently not classified as hazardous at this concentration
< 1% w/v universal indicator solution* / access to dropper bottles / / DANGER: Flammable / Wear eye protection. Ensure laboratory is well ventilated. Ensure there are no naked flames in close proximity to electrolysis apparatus.
hydrogen gas, H2(g) PRODUCED / c. 5cm3 / / DANGER: Extremely flammable.
chlorine gas, Cl2(g) PRODUCED / c. 5cm3 /

/ 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. / 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 solution, NaOH(aq) PRODUCED / c. 20cm3 at <0.1moldm–3 / Currently not classified as hazardous at this concentration
* - 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 9V DC required)
  • 2 × wires
  • 2 × crocodile clips
  • 2 ×micro test tubes OR 2cm3plastic syringes with tip–end cut off at the 0cm3 mark
  • bunsen burner, heat proof mat and splint
  • blue litmus paper

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 75cm3 of chlorine is produced which can get close to the workplace limit of 1.5mgdm–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.

Images from trials


Figure 1 – micro test-tube method

Figure 2– plastic syringe method

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.

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]
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)
2. / 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. 
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 / cleaning product
making soap / accessed on 26/04/2016
Modern Organic Chemistry by ROC Norman and DJ Waddington, (Mills and Boon, 1972)
chlorine / making bleaches
making plastics / accessed on 26/4/2016
Chemistry in Context, Graham Hill and John Holman (Nelson Thornes Ltd, 2000)
hydrogen / rocket fuel
making margarine / accessed on 17/11/2016
Essential Science for GCSE, Susanne Lakin and John Patefield (Nelson Thornes, 1998)

Document updates

v1.1February 2017Published on qualification page

Version 1.1 – February 20171© OCR 2017

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 ionsNa+hydrogen ionsH+

chloride ionsCl–hydroxide ionsOH–

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.

Version 1.1 – February 20171© OCR 2017

Intended class time

30 minutes

Chemicals and equipment (per group)

Version 1.1 – February 20171© OCR 2017

  • electrolysis apparatus
  • stand, boss and clamp
  • 0.5 moldm–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 ORplastic syringes with tip–end cut off at the 0cm3 mark
  • Bunsen burner, heat proof mat and splint
  • blue litmus paper
  • universal indicator solution

Version 1.1 – February 20171© OCR 2017

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 1cm 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 9V 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.

  1. Turn off the power pack.
  1. 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.

  1. 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.
  1. 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.
  1. If you have time, empty the electrolysis apparatus, rinse with tap water, and repeat the experiment.
  2. 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 usethe one below:

Test carried out / Product from where? / Observation
squeaky pop test / cathode / anode
damp blue litmus paper / cathode / anode
universal indicator / Remaining solution

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]
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

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:

Chemistry / Combined Science
3-i / 3-ii / 6-ii / 7-i / 8-i / 8-ii / 11-ii / 12-i
7-ii / 8-i / 8-ii / 12-ii / 13-i / 13-ii

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