ELECTROLYSIS of SODIUM CHLORIDE SOLUTION

Doc Brown's Chemistry KS4 science–chemistry GCSE/IGCSE/O Level/A Level Revision

ELECTROCHEMISTRY revision notes on electrolysis, cells, experimental methods, apparatus, batteries, fuel cells and industrial applications of electrolysis Full descriptions of the apparatus and detailed explanations are provided for the electrolysis of sodium chloride solution (brine) with carbon electrodes.

See APPENDIX 1. for the electrolysis of sodium bromide and potassium iodide solutions

3. The electrolysis of sodium chloride solution (brine) and molten sodium chloride

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)?

Reminders: Electrolysis (of sodium chloride) is a way of splitting up (decomposition) of the compound (sodium) using electrical energy. The electrical energy comes from a d.c. (direct current) battery or power pack supply. A conducting liquid, containing ions, called the electrolyte (molten or aqueous sodium chloride) must contain the compound (sodium chloride) that is being broken down. The electricity must flow through electrodes dipped into the electrolyte, to complete the electrical circuit with the battery. Electrolysis can only happen when the circuit is complete, and an electrical current (electricity) is flowing, then the products of electrolysing aqueous sodium chloride solution (brine) OR molten sodium chloride are released on the electrode surfaces where they can be collected. Electrolysis always involves a flow of electrons in the external wires and electrodes and a flow of ions in the electrolyte and there is always a reduction at the negative cathode electrode (which attracts positive ions, cations) and an oxidation at the positive anode electrode (which attracts negative ions, anions) and it is the ions which are discharged to give the products. These revision notes on the electrolysis of aqueous sodium chloride solution (electrolysis of brine) should prove useful for the new AQA chemistry, Edexcel chemistry & OCR chemistry GCSE (9–1, 9-5 & 5-1) science courses.

My ELECTROCHEMISTRY INDEX

 *

3. The electrolysis of sodium chloride solution (brine)

Aqueous solutions with inert electrodes (carbon or platinum)

The products of electrolysing aqueous sodium chloride solution are hydrogen gas, chlorine gas and sodium hydroxide solution

The simple 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 Voltameter (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.

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.

Brine is moderately concentrated 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).

 

(a) 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.

2H+(aq) + 2e– ==> H2(g)

positive ion reduction by electron gain

other 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).

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

 

(b) 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

2Cl–(aq) – 2e– ==> Cl2(g)

 or  2Cl– ==> Cl2(g) + 2e–

negative ion oxidation by electron loss

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 and then bleaches it white – 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, the residual solution contains sodium hydroxide. In fact this is how sodium hydroxide is manufactured in the chemical industry.

Na+ + OH– = NaOH, a familiar formula! The presence of the alkali sodium hydroxide, can be shown by adding universal indicator to the brine solution (aqueous sodium chloride). The indicator will turn from green to purple.

 

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.

2H2O(l) – 4e– ==> 4H+(aq) + O2(g)

or

4OH–(aq) – 4e– ==> 2H2O(l) + O2(g) (oxygen gas)

For more, see Extra COMMENTS 2.

 

Summary of the possible products from the electrolysis of sodium chloride

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)    (since the sodium ions are spectator ions)

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.

For the industrial electrolysis of brine and the uses of the products see The Halogens and Salt page.

 

Extra COMMENTS on the electrolysis of sodium chloride solution

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 voltameter 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. 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.

Advanced Level Student Note on the ratio of chlorine to oxygen production:

The increase in oxygen to hydrogen ratio through the electrolysis is essentially a concentration effect. 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. So, 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. BUT, as the brine (NaCl(aq)) becomes depleted in chloride ions, and the hydroxide ion is increasing (also a 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.

 

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 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 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.

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

Na+(l) + e– ==> Na(l)   a reduction electrode reaction (electron gain)

positive ion reduction by electron gain

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

2Cl–(l) – 2e– ==> Cl2(g)

 or  2Cl–(l) ==> Cl2(g) + 2e–   an oxidation electrode reaction (electron loss)

negative oxidation by electron loss

See The extraction of sodium from molten sodium chloride using the 'Down's Cell'

 

SUMMARY OF PRODUCTS FROM THE ELECTROLYSIS OF SODIUM CHLORIDE solution or melt

with inert electrodes like carbon (graphite) or platinum

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) chlorine gas

2Cl–(l) – 2e– ==> Cl2(g)

 or  2Cl–(l) ==> Cl2(g) + 2e–

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)

chlorine gas

2Cl–(aq) – 2e– ==> Cl2(g)

 or  2Cl–(aq) ==> Cl2(g) + 2e–

************************* ************ **************************************************** ********** ****************************************

APPENDIX 1. The electrolysis of sodium bromide and potassium iodide solutions

The concept diagrams for aqueous sodium chloride are equally valid.

Because sodium and potassium are reactive metals, you will get hydrogen ions discharged at the negative cathode giving hydrogen gas.

Sodium Bromide

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

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 may be a dark solid precipitate if sufficient iodine is formed.

cathode (-):   2H+(aq) + 2e– ==> H2(g)

anode (+):   2I–(aq) – 2e– ==> I2(aq/s)

 


ELECTROCHEMISTRY INDEX:  1. INTRODUCTION to electrolysis - electrolytes, non-electrolytes, electrode equations, apparatus 2. Electrolysis of acidified water (dilute sulfuric acid) and some sulfate salts and alkalis 3. Electrolysis of sodium chloride solution (brine) and bromides and iodides 4. Electrolysis of copper(II) sulfate solution and electroplating with other metals e.g. silver 5. Electrolysis of molten lead(II) bromide (and other molten ionic compounds) 6. Electrolysis of copper(II) chloride solution 7. Electrolysis of hydrochloric acid 8. Summary of electrode equations and products 9. Summary of electrolysis products from various electrolytes 10. Simple cells (batteries) 11. Fuel Cells e.g. the hydrogen - oxygen fuel cell 12. The electrolysis of molten aluminium oxide - extraction of aluminium from bauxite ore & anodising aluminium to thicken and strengthen the protective oxide layer 13. The extraction of sodium from molten sodium chloride using the 'Down's Cell' 14. The purification of copper by electrolysis 15. The purification of zinc by electrolysis 16. Electroplating coating conducting surfaces with a metal layer 17. Electrolysis of brine (NaCl) for the production of chlorine, hydrogen & sodium hydroxide AND 18. Electrolysis calculations


give the apparatus and electrode equations for the electrolysis of sodium bromide, give the apparatus and electrode equations for the electrolysis of potassium iodide, information on electrolysis of sodium chloride solution brine explained electrodes equations products theory diagrams KS4 Science electrolysis of sodium chloride solution brine explained electrodes equations products theory diagrams GCSE chemistry guide notes on electrolysis of sodium chloride solution brine explained electrodes equations products theory diagrams for schools colleges academies science course tutors images pictures diagrams of apparatus for electrolysis of sodium chloride solution brine explained electrodes equations products theory diagrams investigations word balanced symbol equations of electrolysis of sodium chloride solution brine explained electrodes equations products theory diagrams science chemistry revision notes on electrolysis of sodium chloride solution brine explained electrodes equations products theory diagrams revising the chemistry of electrolysis of sodium chloride solution brine explained electrodes equations products theory diagrams help in chemical understanding of electrolysis of sodium chloride solution brine explained electrodes equations products theory diagrams description of electrolysis of sodium chloride solution brine explained electrodes equations products theory diagrams experiments for chemistry courses university courses in chemistry careers in chemistry jobs in the chemical industry laboratory assistant apprenticeships in chemistry technical internship in chemistry IGCSE chemistry electrolysis of sodium chloride solution brine explained electrodes equations products theory diagrams USA US grade 8 grade 9 grade10 electrolysis of sodium chloride solution brine explained electrodes equations products theory diagrams chemistry explanations of electrolysis of sodium chloride solution brine explained electrodes equations products theory diagrams how do you do the experiment for the electrolysis of sodium chloride (brine) solution? what happens in the electrolysis of sodium chloride solution using carbon electrodes, give the electrode equations for the electrolysis of sodium chloride solution, draw a diagram of the apparatus for electrolysing sodium chloride solution, electrode products chlorine hydrogen sodium hydroxide, explain how you can split sodium chloride by electrolysis of molten salt, describe some industrial applications of the electrolysis of brine, give the electrode equation for what happens at the negative cathode electrode in the electrolysis of sodium chloride solution (brine), give the electrode equation for what happens at the positive anode electrode in the electrolysis of sodium chloride solution (brine) gcse chemistry revision free detailed notes on electrolysis of sodium chloride solution to help revise igcse chemistry igcse chemistry revision notes on electrolysis of sodium chloride solution O level chemistry revision free detailed notes on electrolysis of sodium chloride solution to help revise gcse chemistry free detailed notes on electrolysis of sodium chloride solution to help revise O level chemistry free online website to help revise electrolysis of sodium chloride solution for gcse chemistry  free online website to help revise electrolysis of sodium chloride solution for igcse chemistry free online website to help revise O level electrolysis of sodium chloride solution chemistry how to succeed in questions on electrolysis of sodium chloride solution for gcse chemistry how to succeed at igcse chemistry how to succeed at O level chemistry a good website for free questions on electrolysis of sodium chloride solution to help to pass gcse chemistry questions on electrolysis of sodium chloride solution a good website for free help to pass igcse chemistry with revision notes on electrolysis of sodium chloride solution a good website for free help to pass O level chemistry electrolysis of sodium chloride solution (brine) for AQA AS chemistry, electrolysis of sodium chloride solution (brine) for Edexcel A level AS chemistry, electrolysis of sodium chloride solution (brine) for A level OCR AS chemistry A, electrolysis of sodium chloride solution (brine) for OCR Salters AS chemistry B, electrolysis of sodium chloride solution (brine) for AQA A level chemistry, electrolysis of sodium chloride solution (brine) for A level Edexcel A level chemistry, electrolysis of sodium chloride solution (brine) for OCR A level chemistry A, electrolysis of sodium chloride solution (brine) for A level OCR Salters A level chemistry B electrolysis of sodium chloride solution (brine) for US Honours grade 11 grade 12 electrolysis of sodium chloride solution (brine) for pre-university chemistry courses pre-university A level revision notes for electrolysis of sodium chloride solution (brine)  A level guide notes on electrolysis of sodium chloride solution (brine) for schools colleges academies science course tutors images pictures diagrams for electrolysis of sodium chloride solution (brine) A level chemistry revision notes on electrolysis of sodium chloride solution (brine) for revising module topics notes to help on understanding of electrolysis of sodium chloride solution (brine) university courses in science careers in science jobs in the industry laboratory assistant apprenticeships technical internships USA US grade 11 grade 11 AQA A level chemistry notes on electrolysis of sodium chloride solution (brine) Edexcel A level chemistry notes on electrolysis of sodium chloride solution (brine) for OCR A level chemistry notes WJEC A level chemistry notes on electrolysis of sodium chloride solution (brine) CCEA/CEA A level chemistry notes on electrolysis of sodium chloride solution (brine) for university entrance examinations describe the electrolysis of sodium bromide solution, describe the electrolysis of potassium iodide electrolysis of brine for AQA AS chemistry, electrolysis of brine for Edexcel A level AS chemistry, electrolysis of brine for A level OCR AS chemistry A, electrolysis of brine for OCR Salters AS chemistry B, electrolysis of brine for AQA A level chemistry, electrolysis of brine for A level Edexcel A level chemistry, electrolysis of brine for OCR A level chemistry A, electrolysis of brine for A level OCR Salters A level chemistry B electrolysis of brine for US Honours grade 11 grade 12 electrolysis of brine for pre-university chemistry courses pre-university A level revision notes for electrolysis of brine  A level guide notes on electrolysis of brine for schools colleges academies science course tutors images pictures diagrams for electrolysis of brine A level chemistry revision notes on electrolysis of brine for revising module topics notes to help on understanding of electrolysis of brine university courses in science careers in science jobs in the industry laboratory assistant apprenticeships technical internships USA US grade 11 grade 11 AQA A level chemistry notes on electrolysis of brine Edexcel A level chemistry notes on electrolysis of brine for OCR A level chemistry notes WJEC A level chemistry notes on electrolysis of brine CCEA/CEA A level chemistry notes on electrolysis of brine for university entrance examinations

ALPHABETICAL SITE INDEX for chemistry   

ELECTROCHEMISTRY INDEX

 Doc Brown's Chemistry 

*

For latest updates see https://twitter.com/docbrownchem

TOP OF PAGE

visits since January 2000