electrolysis copper sulfate solution with copper carbon graphite electrodes electroplating half-equations products anode cathode apparatus electrolyte cell anode sludge gcse chemistry KS4 science igcse O level revision notes

4. (a) The electrolysis of copper(II) sulfate solution

The electrolyte copper(II) sulfate, provides a high concentration of copper(II) ions Cu²⁺ and sulfate ions SO₄^2– to carry the current during the electrolysis process. There are tiny concentrations of hydrogen ions H⁺ and hydroxide ions (OH^–) from the self-ionisation of water itself, but these can be ignored in this experiment.

H₂O(l) H⁺(aq) + OH^–(aq)

The electrolysis will only take place when electricity is passed through the copper ion solution.

The technical details of the electrolysis of copper sulfate solution with two different electrodes (a) graphite (carbon) electrodes and (b) copper electrodes are all explained below.

Electrolysis of a aqueous copper(II) sulfate solution CuSO₄(aq)

The electrolysis of copper sulfate solution using inert electrodes

The products of electrolysing copper sulfate solution with inert electrodes (carbon/graphite or platinum) are copper metal and oxygen gas.

Using the simple apparatus (above left diagram) and inert carbon (graphite) or platinum electrodes, you can observe the products of the electrolysis of copper sulfate solution are (i) a copper deposit on the negative cathode electrode and (ii) oxygen gas at the positive anode electrode.

This anode reaction differs from when you use copper electrodes (see section (b) below).

You have to fill the little test tubes with the electrolyte (dil. copper sulfate solution), hold the liquid in with your finger and carefully invert them over the nearly full electrolysis cell.

The very simple apparatus (above right) can be used with two inert wire electrodes.

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(a) The electrode products from the electrolysis of copper sulfate with inert graphite (carbon) electrodes

(or platinum electrodes if you can afford them!)

Note: 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)

The half-equations for the electrolysis of copper(II) sulfate solution.

When the current is switched on, a copper deposit forms on the negative cathode and bubbles of the colourless oxygen come off the positive anode. The concept diagram below illustrates the process.

Electrolysis of copper sulfate solution with carbon electrodes

The electrode reactions and products of the electrolysis of the electrolyte copper sulfate solution (with inert carbon-graphite electrodes) are illustrated by the theory diagram above

(i) a copper deposit on the negative cathode electrode surface

(ii) oxygen gas forms at the positive anode electrode surface

The negative cathode reaction with graphite electrodes

The negative cathode electrode attracts Cu²⁺ ions (from copper sulfate) and H⁺ ions (from water).

Only the copper ion is discharged, being reduced to copper metal.

The less reactive a metal, the more readily its ion is reduced on the electrode surface, copper is below hydrogen in the reactivity series, so copper ions are reduced to a copper deposit, in preference to hydrogen ions being reduced to hydrogen gas.

A brown copper deposit forms as the positive copper ions are attracted to the negative electrode (cathode)

Cu²⁺_(aq) + 2e^– ===> Cu_(s)

The positive copper ion is reduced by electron gain

The traces of hydrogen ions are not discharged, so you not see any gas bubbles collecting on the negative cathode electrode.
The blue colour fades as more and more copper is deposited, depleting the concentration of the blue copper ion Cu²⁺ ions in solution.

The positive anode reaction with graphite electrodes

Oxygen gas is formed at the positive electrode, an oxidation reaction (electron loss).

The negative sulfate ions (SO₄^2-) or the traces of hydroxide ions (OH^– from water) are attracted to the positive electrode.

But the sulfate ion is too stable and nothing happens.

Instead either hydroxide ions or water molecules are discharged and oxidised to form oxygen.

(i) 4OH^–_(aq) – 4e^– ===> 2H₂O_(l) + O_2(g)

The negative hydroxide ion is oxidised by electron loss

or written as: 4OH^–_(aq) ===> 2H₂O_(l) + O_2(g) + 4e^–

(ii) 2H₂O_(l) – 4e^– ===> 4H⁺_(aq) + O_2(g)

The water molecule is oxidised by electron loss

or written as: 2H₂O_(l) ===> 4H⁺_(aq) + O_2(g) + 4e^–

Test for the oxygen gas

The colourless gas should re-ignite a glowing splint - a simple test for oxygen.

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Electrolysis of copper sulfate solution with copper electrodes (b) The electrolysis of copper sulfate solution using copper electrodes

This is a method of purifying copper and extracting other valuable metals from the anode sludge.

The products of electrolysing copper sulfate solution with copper electrodes are copper metal and copper ions (because the copper anode dissolves).

Using the simple apparatus (right diagram) and two copper electrodes the products of the electrolysis of copper sulfate solution are

(i) a copper deposit on the negative cathode electrode surface

(ii) copper dissolves from the positive anode electrode surface

This copper anode reaction differs from when you use an inert graphite electrode for the anode (see section (a) above).

When Copper(II) sulfate is electrolysed with a copper anode electrode (the cathode can be carbon or copper), the copper deposit on the cathode (–) equals the copper dissolves at the anode (+). Therefore the blue colour of the Cu²⁺ ions stays constant because Cu deposited = Cu dissolved.
Both half-reaction involve a two electron transfer so it means mass of Cu deposited = mass of Cu dissolving for the same quantity of current flowing (flow of electrons).
You can check this out by weighing the dry electrodes before and after the electrolysis has taken place.
The experiment works with a carbon anode and you see the blackness of the graphite change to the orange-brown colour of the copper deposit and the anode becomes depleted in copper.

Electrolysis of copper sulfate solution with carbon/copper electrodes

Electrolysis of copper sulfate solution with copper electrodes

The electrode reactions and products of the electrolysis of copper sulfate solution with a copper anode are illustrated by the theory diagram above - it doesn't matter whether the cathode is carbon or copper - you get the same copper deposit and the copper anode is oxidised and dissolves to give the copper ion Cu²⁺(aq).

Electrode products from the electrolysis of copper sulfate with copper electrodes

Refer to the diagrams above when working through the reasoning of the half-reactions for the electrolysis of copper(II) sulfate solution explained below.

(i) The negative cathode reaction with copper or carbon electrodes

The negative cathode electrode attracts Cu²⁺ ions (from copper sulfate) and H⁺ ions (from water).
Only the copper ion is discharged, being reduced to copper metal.
The less reactive a metal, the more readily its ion is reduced on the electrode surface.
A reduction electrode reaction at the negative cathode
Cu²⁺_(aq) + 2e^– ===> Cu_(s)
A copper deposit forms, reduction of the copper ions, each gains 2 electrons.

Note on 'plating' - the formation of the copper deposit:

It doesn't matter what the cathode is made of, as long as it is a conducting material.

This is the basis of copper plating, and plating with any metal from a solution of its salt.

(ii) The positive anode reaction with a copper electrode

Its the copper anode that is the crucial difference than electrolysing copper sulfate solution with a inert carbon/graphite/platinum electrode.
The negative sulfate ions SO₄^2- (from copper sulfate) or the traces of hydroxide ions OH^– (from water) are attracted to the positive electrode.
But both the sulfate ion and hydroxide ion are too stable and nothing happens to them because the copper anode is preferentially oxidised to discharge Cu²⁺ copper ions into the electrolyte solution.
This is fairly unusual, because normally electrodes are 'inert', BUT, this technique is used in electroplating.
An oxidation electrode reaction at the positive anode

Cu_(s) – 2e^– ===> Cu²⁺_(aq)
The copper dissolves, oxidation of the copper atoms, each losing 2 electrons to form blue Cu²⁺ ions in solution - in this case the electrode is NOT inert.
or written as: Cu_(s) ===> Cu²⁺_(aq) + 2e^–

A balancing act !

copper atoms oxidised to copper(II) ions: dissolving of copper in its electrolytic purification or electroplating (must have positive copper anode). The change involves two electrons per copper atom.

copper(II) ion reduced to copper atoms: deposition of copper in its electrolytic purification or electroplating using copper(II) sulfate solution, so the electrode can be copper or other metal to be plated OR any other conducting material. The change involves two electrons per copper ion.

This means for every copper atom that gets oxidised, one copper ion is reduced, therefore ...

When copper electrodes are used in the electrolysis of copper sulfate solution, the mass loss of copper from the positive anode electrode should equal the mass of copper gained and deposited on the negative cathode electrode.

You can show this by weighing both electrodes at the start of the experiment. After the current has passed for some time, carefully extract the electrodes from the solution, wash them, dry them and reweigh them. The gain in mass of the cathode should be about the same as the loss of mass from the anode.

(iii) In industry an anode sludge forms under the depleting impure block of copper

A deposit of dark material gathers below the impure copper anode.

This is the residue left after the copper is oxidised, dissolves and transferred to the cathode.

In the electrolytic refining process, after the pure copper is deposited on the cathode plates insoluble impurities fall to the bottom of the cell as anode mud or anode sludge.

Anode sludge contains gold (Au) and other valuable metals like silver (Ag), platinum (Pt), and palladium (Pd).

These can be extracted from the anode sludge created by the electro-refining process.

In the formation of copper ore veins, copper concentrates often these precious metals and reclamation of these metals from anode slime is economically attractive and also it is environmentally friendly.

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ELECTROPLATING

Appendix ELECTROPLATING with e.g. copper, zinc, chromium or silver.

Metal electrodes dipped in aqueous salt solutions

For electroplating in general:

The negative cathode electrode is made the metal/conducting surface to be coated, and the positive anode electrode is made of the plating metal which dissolves and replaces any deposit formed on the cathode -which is the conducting article to be electroplated.

See also Extraction and purification of copper

INTRODUCTION TO ELECTROPLATING and its APPLICATIONS - diagram and explanatory notes below it.

This section below has some technical details e.g. the electrode equations, or go straight to the industrial applications of electroplating

Electroplating

As already described already the use of a copper positive anode electrode is the basis of the method of electroplating any conducting solid with a layer of copper which can be reproduced by electroplating other conducting materials with zinc (a way of galvanising steel), nickel, silver or chromium ('chromium plating'). Read on in conjunction with the theory diagram above describing the process of electroplating.

The CATHODE object to be electroplated must be a conducting material, usually a metal, and must be made the negative cathode electrode and completely immersed in the electrolyte solution.

The ANODE is usually a bar of the metal that is being electroplated onto the cathode object, giving a continuous supply of the coating metal and ensuring the concentration of electrolyte metal ion does not diminish as the electrolytic plating continues. The metal anode bar must be oxidised to provide a metal ion that can migrate across to the cathode and be discharged as the electrolysis takes place.

The electrolyte solution must contain ions of the metal that will form the electroplated deposit; and the ions come from an appropriate salt solution e.g. copper sulfate for copper, silver nitrate for silver, zinc sulfate for zinc or chromium chloride for chromium coatings.

The anode must be made of the metal that will form the electroplated coating on the positive anode object e.g. copper or silver.

As the metal is coated on the -ve cathode object, simultaneously the metal of the +ve anode is oxidised to refresh the solution of metal ions. so there is no depletion of the crucial ion concentration. These positive ions will migrate towards the negative electrode object to be coated.

The purification of copper by electrolysis amounts to copper plating so all you have to do is swap the pure negative copper cathode with the metal you want to coat (e.g. Ni, Ag or Au or any material with a conducting surface).

Swap the impure positive copper anode with any pure block of the metal you want to form the coating layer on the negative electrode object.

So any conducting (usually metal) object can be electroplated with copper, silver or gold for aesthetic reasons (decorative jewellery objects) or steel with zinc (galvanising) or a shiny chromium as anti-corrosion protective layer on steel. Any dull looking cheap metal can be made to look rather more shiny and attractive by electroplating. So cheap brass objects can be 'silver plated' and 'gold plated' to look more valuable that they really are!

Examples - half-reactions given, but read in conjunction with the general notes and diagram in the introduction.

(i) Copper electroplating (copper plating by electrolysis of a copper salt solution)

(-ve cathode electrode) Cu²⁺_(aq) + 2e^– ==> Cu_(s)

electron gain, reduction, copper deposited (electroplated) on the cathode object, dull object might look a lot prettier!

(+ve anode electrode) Cu_(s) – 2e^– ==> Cu²⁺_(aq)

supplies copper ions, electron loss, copper atoms oxidised

(ii) Zinc electroplating (zinc plating by electrolysis)

a reduction electrode reaction at the negative cathode electrode in zinc salt solution

(– ve cathode electrode) Zn²⁺_(aq) + 2e^– ==> Zn_(s)

electron gain, zinc ion reduced, zinc deposit formed e.g. galvanising steel by electroplating

(+ve anode electrode) Zn_(s) ==> Zn²⁺_(aq) + 2e^–

zinc atoms of the positive zinc anode electrode are oxidised, electron loss, supplying more zinc ions

zinc ions reduced to zinc atoms: galvanising steel (the electrode) by electroplating from aqueous zinc sulfate solution, (or from molten zinc chloride?)

(iii) Silver electroplating (silver plating by electrolysis)

a reduction electrode reaction at the negative cathode electrode in a silver salt solution
(– ve cathode electrode) Ag⁺_(aq) + e^– ==> Ag_(s)

silver deposit as the silver ions are reduced to silver atoms, thereby electroplating the object, from cheaper metals like brass, to good looking silver ones and electroplated brass is much cheaper than pure silver and looks just as good!

(+ ve anode electrode) Ag_(s) ==> Ag⁺_(aq) + e^–

silver atoms oxidised on the surface of the silver anode, re-supplying the electrolyte with silver ions
You can do this using the electrolysis of silver nitrate solution.

Incidentally, as a school experiment, if you use lead nitrate solution you will get a coating of lead, despite lead being more reactive than hydrogen. BUT, who would want to coat anything with lead?!

(– ve cathode electrode) Pb²⁺_(aq) + 2e^– ==> Pb_(s)
In both these cases in a school/college experiment you will get oxygen at the anode:

anode (+): 4OH^–_(aq) – 4e^– ==> 2H₂O_(l) + O_2(g)

However a solution of a gold salt is used to electroplate any other metal surface with a nice looking gold surface - but this is a bit costly for schools!

(iv) Chromium electroplating (chromium plating by electrolysis)

a reduction electrode reaction at the negative cathode electrode in chromium(III) salt solution

(– ve cathode electrode) Cr³⁺_(aq) + 3e^– ==> Cr_(s)

chromium deposit as the chromium ions from a chromium salt solution are reduced to chromium atoms, thereby electroplating the object, from cheaper metals like steel, to good looking shiny chromium plated ones!

(v) Tin electroplating (tin plating by electrolysis, 'tinning')

a reduction electrode reaction at the negative cathode electrode in a tin salt solution

(–ve cathode electrode) Sn²⁺_(aq) + 2e^– ==> Sn_(s)
electron gain, tin ion reduced, tin deposit formed.

(vi) Nickel electroplating (nickel plating by electrolysis)

a reduction electrode reaction at the negative cathode electrode in a nickel salt solution e.g. nickel(II) sulfate

(–ve cathode electrode) Ni²⁺_(aq) + 2e^– ==> Ni_(s)
electron gain, nickel ion reduced, nickel deposit formed.

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Examples of APPLICATIONS of ELECTROPLATING

Please note that examples of electrode equations for plating are given in the previous sections on this page

Feature property Example of electroplating applications (all you need is ANY conducting material !)

Electroplating to forms a protective barrier e.g. to give a material anti-corrosion properties including rust prevention Electroplating can create a barrier on a material that protects it against atmospheric conditions such as corrosion. Electroplated parts can last longer and need to be replaced less frequently and so saving money. Examples of corrosion protection include nickel plating, tin plating and their various alloys are all used for corrosion protection on nuts, bolts, housings, brackets and many other metal parts and components.
Gold electroplating provides a superior corrosion and tarnish protection, but it is more expensive than other plating processes! Plating for anti-corrosion - prevention of tarnishing is used to protect against premature tarnishing in certain kinds of metals and also reduce the likelihood of scratching. Silverware products retain their attractiveness and hold their value over a longer time.

Zinc electroplating plating is used in the manufacture of washers, bolts, nuts, transmission components, armoured personnel carriers and tanks to reduce corrosion.

Tin electroplating or “tinning,” to give a material enhanced surface anti-corrosion properties is a cost-effective alternative to plating with more expensive materials such as gold or silver and used in the manufacture of electronic parts and components, hardware products, fasteners, screws, nuts and bolts.

Electroplating with nickel gives greater corrosion protection, greater wear resistance and increased surface thickness e.g. in the production of electronic and computer parts and components.

Electroplated surfaces to enhance appearance Jewellery can be electroplated with a thin layer of a precious metal to make it more lustrous and attractive to customers. This gives manufacturers a cost-effective way to make products more aesthetically appealing. Jewellers can sell products that look like pure gold or other precious metals like silver at a much lower price!
Electroplating with chromium can be used to refurbish old chrome parts such as bumpers, grills and tire rims of cars to make them look brand new. You can chromium electroplate the plastic lightweight but sturdy parts of a modern car.

It is possible to electroplate copper onto non-metal materials like plastic to enhance their appearance e.g. the fashion industry can convert dull looking plastic into an attractive shiny metallic looking material.

Plating to reduce surface friction Nickel electroplating can reduce the build-up of friction in certain materials such as electrical connectors, so improving performance and reducing premature wear and tear.

Enhancing electrical conductivity Electroplating with silver or tin-lead alloys can increase electrical conductivity, useful in the manufacture of electronics and electrical components.
Economically, it is a cost-effective and efficient electrical conductivity solution.

A silver salt electroplating solution can be used in the production of solar panels.

Electroplating to improve heat resistance Electroplating processes with gold or zinc-nickel alloys can make surfaces capable of withstanding extremely high temperatures.
Electroplating with these metals protects engine parts and components from damage caused by extreme temperatures, and so increasing their lifespan.

Plating to give a surface to promotes adhesion Electroplating with copper gives an undercoating that facilitates adhesion with a variety of additional coatings. Copper plating provides a smooth and uniform surface finish for further treatment.

SUMMARY OF PRODUCTS FROM THE ELECTROLYSIS OF COPPER(II) SULFATE SOLUTION with carbon OR copper electrodes
Electrolyte	negative cathode product	negative electrode cathode half-equation	positive anode product	positive electrode anode half-equation
aqueous copper(II) sulfate CuSO₄(aq) with carbon electrodes	copper deposit	any conducting electrode e.g. carbon rod, any metal including copper itself Cu²⁺_(aq) + 2e^– ==> Cu_(s)	oxygen gas	inert electrode like carbon (graphite rod) or platinum (i) 4OH^–_(aq) – 4e^– ==> 2H₂O_(l) + O_2(g) or 4OH^–_(aq) ==> 2H₂O_(l) + O_2(g) + 4e^– (ii) 2H₂O_(l) – 4e^– ==> 4H⁺_(aq) + O_2(g) or 2H₂O_(l) ==> 4H⁺_(aq) + O_2(g) + 4e^–
aqueous copper (II) sulfate CuSO₄(aq) with copper electrodes	copper deposit	any conducting electrode e.g. carbon rod, any metal including copper itself Cu²⁺_(aq) + 2e^– ==> Cu_(s) this is the copper plating equation	copper(II) ions – the copper anode dissolves	copper anode electrode Cu_(s) – 2e^– ==> Cu²⁺_(aq) or Cu_(s) ==> Cu_(s) + 2e^–
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Electrolysis Quiz (GCSE 9-1 HT Level (harder)

ELECTROCHEMISTRY INDEX

keywords and phrases: revision study notes for AQA Edexcel OCR IGCSE/GCSE chemistry topics modules on explaining the electrolysis copper sulfate solution with copper electrodes carbon graphite electrodes platinum electrodes electroplating half-equations products at the positive anode products at the negative cathode description of apparatus for doing electrolysis experimental investigation electrolyte diagrams of electrolysis cell experiments anode sludge the purification of copper by electrolysis, applications of electroplating in industry preventing corrosion improving appearance electrical conduction nickel chromium tin silver zinc gold plating by electrolysis

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