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School Chemistry: Describing and explaining electrolysis of sodium chloride solution

ELECTROLYSIS of aqueous SODIUM CHLORIDE SOLUTION and sodium bromide and potassium iodide solutions NaCl(aq), NaBr(aq) and KI(aq)

(Suitable for AQA, Edexcel and OCR GCSE level chemistry students, ~US grades 9-10)  (re-edit)


ELECTROCHEMISTRY revision notes on electrolysis: cells, experimental methods, apparatus, batteries, fuel cells and industrial applications of electrolysis.

Here 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.

Sub-index for this page on electrolysis

including using an electrolysis cell - investigating the electrolysis of sodium chloride aqueous solution (brine) and molten sodium chloride and other halide salts

(a) The electrolysis of aqueous sodium chloride solution (brine)

(b) Five further comments on (a) including electrolysis of molten NaCl

(c) The electrolysis of sodium bromide and potassium iodide solutions

(d) Learning objectives for this section on electrolysis

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(a) ELECTROLYSIS 3.

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.

diagram explaining electrolysis of sodium chloride solution apparatus for investigation products electrode equations

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).

(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.

Cathode (-):2H+(aq) + 2e– ==> H2(g)

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

 

(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

Anode (+): 2Cl–(aq) – 2e– ==> Cl2(g)

 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

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.


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(b) 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

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

5(c). 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)


(d) 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.


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