IONIC THEORY AND ELECTROLYSIS

Tonic Theory

‘To account for the phenomena of electrolysis the lonic ‘Theory was put forward by Arrhenius in 1880, The theory states that electrolytes are made up of ions, which are built up in certain patterns called crystal lattice. When these substances dissolve in water, the structure is destroyed and the ions are set free to move Concentrated mineral acids such as sulphuric acid, hydrochloric acid and nitric acid do not contain ions but they consist of molecules. However. when they are

diluted, the molecular structure is destroyed and ions are formed.

Electrolytes and Non-elecirolytes

Distinguish elecirolyies from non-electrolyres ‘The main purpose of this chapter isto investigate the effects which electricity has on a range of substenees, and to develop a thorough explanation of those effects in terms of our present knowledge of atomic structure. Before we begin, it is important that we familiarize ourselves with different terms that we are going to use to explain different phenomena. It is crucial that the

definitions and meanings of these terms be understood at the outset in order that concepts

defined in this chapter are easily and clearly apprehended. These terms are given hereunder:

• Electrolysis. decomposition of @ compound in solution or molten state by passing electricity

through it

• Conductor: a solid substance that allows electricity to pass through it. All metals are included in

this class.

• Non-conductor or insulator: a solid substance that does not allow electricity to flow through it

All non-metals fall in this class.

• Blectrolyte: a substance which, when dissolved or molten, conducts electricity and is

decomposed by it

• Non-electrolyte: « compound witich cannot conduet electricity, be it in molten or solution state.
• Electrode: a graphite or metal pole (rod) or plate through which the electric eurrent enters or

leaves the electrolyte.

• Cathode: a negative electrode which leads electrons into the electrolyte
• Anode: a positive electrode which leads electrons out ofthe electrolyte
• Ton: a positively or negatively charged atom or radical (group of atoms),
• Cation: a positive ion which moves to the cathode during electrolysis.
• Anion: a negative ion which moves to the anode during electrolysis.

Electrolytes and non-electrolytes Liquids such as ethanol, paraffin, petrol and methylbenzene do not conduct electricity. The bonding in these compounds is covalent. These substances consist of molecules. There are no free electronsor charged particles to flow through them. Solutions of covalent compounds, for

example sugar solution, do not conduct electricity,

‘These compounds are nan-electrolytes. Non-electolytes exist only in the form of molecules and are incapable of ionization Tonic compounds contain charged particles (ions), but in solid state, the ions are firmly held in place and they are not fice to move. An ionic solid does not conduct electricity. However, the ions present can become free to move ifthe solid is melted or dissolved in water. Then they can conduct electricity. For example, solid sodium chloride cannot conduct electricity but when

melted or dissolved in water, the ions, Na” and CI’ are set free. Then these ions are free to move im solution and hence conduct electricity. These compounds are called electrolytes.

Weak and Strong Electrolytes

Categorize weak and strong electrolytes Weak eleciroiyies are compounds that are only partially or slightly ionized in aqueous solutions Some substances, for example, ethanoic acid solution ionize partially CH.COOH gy) CHsCOO ug) + Hay Most of the electrolytes exist in solution in the form of unionized molecules. For example, in ordinary dilute (2M) ethanoic acid, out of every 1000 molecules present, only 4 are ionized and

996 are unionized. A solution of ammonia water is also a weak electrolyte, containing a relatively small proportion of ammonium and hydroxyl ions NH,OH jg) @NHG jag) + OH) Most of the organic acids are weak electrolytes, ¢ . tartaric, citric and carbonic acids However, there is no sharp dividing line between weak and strong electrolytes. Water is also weak electrolyte It ionizes only slightly HO q¢sH ia) + OF ay Study shows that for every molecule of water ionized, there are 6 million molecules of water not

ionized Sirong electrolytes are compounds that are completely ionized in aqueous solutions When sodium chioride is dissolved in adequate water it ionizes completely into Na“and Cr ions.

‘There are no NaCl solid particles left unionized. All strong electrolytes (salts, the mineral acids and caustic alkalis) ionize completely in solutions

The Mechanisms of Electrolysis

Electrolytic Cells of Different Electrolytes in the Molten and Aqueous States Set up electrolytic cells of different electrolytes in the molten and aqueous states The conductivity of ionic compounds is explained by the fact that ions move in a particular direction in an electri field. This ean be shown in experiments with coloured salts. For example, copper (11) chromate (Vi) (CuCrO,) dissolves in water to give a green solution, This solution is

placed in a U-tube. A colourless solution of dilute hydrochloric acid (HCI) is then layered on top of the salt solution in cach arm. Graphite rods are fitted as shown in figure 13.3. These rods (electrodes) carry the current into and out of the solution After passing the current for a short time, the solution around the cathode becomes blue. Around the anode, the solution becomes yellow. These colours are produced by the movement

(migration) of the ions in the salt. ‘The positive copper ions (Cu™) are blue in solution. ‘They are attracted to the cathode (negative electrode). The negative chromate ions (CrO,") are yellow in

solution. They are attracted to the anode (the positive electrode). ‘The use of coloured ions in

solution has shown the direction that positive and negative ions move in an electric field. Always

positive ions (cations) move tothe eathode and negative ions (anions) move tothe anode. battery graphite nh graphite cathode — anode dilute HCI U-tube eee i ne +7-— CuCr05 Ionic Migrations During Electrolysis and the Preferential Discharge of Ions at the Electrodes Explain tonic migrations during electrolysis and the preferential discharge of ions at the electrodes When a salt such as sodium chloride is dissolved in water, its ios are set fee to move. So the

solution can be electrolysed. Since the salts, alkalis and acids are dissolved m water, most of the

Solutions are aqueous. There is then a complication in electrolysis of such substances in aqueous

form, This is because the water used to dissolve them also do ionize partially (it is a weak electrolyte) Then at each electrode, we get more than one ion for discharge, but only one is supposed to be discharged. Take an example of electrolysis of copper (Il) sulphate solution using platinum electrodes. By ionic theory, the solution ionizes thus:CuSO4 gay) > Cu” aa) + SO.7'iag (strong electrolyte) HO = H*+OH (weak electrolyte)

During electrolysis, Cu* and H” tons move to the cathode while SO;*and OH’ ions move to the anode

Cathode Anode

ca so? i on In situations like this, the order of discharge of the ions at the electrode will depend on: 1 the position of the metal ion or radical in the electrochemical series,

• the concentration or nature of the ions (or electrolyte) to be discharged; and
• the nature of the electrodes used

‘The position of ion or radical in the electrochemical series

Cations Anions

e so, ca case of dischargeinereases, No: ease of discharge increases Na® cr

Mg™ Br

art r ae ou Fe Pb? a ee Ag The arrangement of ions above is the same as that of the electrochemical series. If all other

factors are constant, any ion will be discharged from solution in preference to those above it

Cathode Anode

Cu” so,

H OH

Cu*+ 2e + Cusicopper ts discharged (loses its 40H 2H,0q+O.y4€ ydrory! ton is charge) discharged In other words, we can say that the reaction that occurs at the cathode is reduction (electron gain) and that which occurs at the anode is oxidation (electron loss).

Result

• Atcathode, copper is deposited
• Atanode, oxygen is given out (liberated)

‘+ The solution at the end of electrolysis is colourless and acidic because in the electrolyte there are left H! and SOJ* ions, which remain moving freely in the solution as ioni¢ sulphuric acid.

The concentration or nature of electrolyte Take an example of electrolysis of sodium chloride using platinum electrodes when in

• agucous solution;
• concentrated aqueous solution; or
• fsed or molten state

aqueous solution

NaCl Na*+ Cr

H,0oH" + OH

Dunng electrolysis,

Cathode Anode

Na‘and H CTand O1F

Hions are discharged in preference to OHions are discharged im preference to Cl Navions: 2H"+2e'— Hayy ions 4OH2H09)+Onyt4e Result

• At cathode, hydrogen is given out
• Atanode, oxygen is out
• The concentration of the solution remains constant since all Na” and CI ions remain undisturbed

im the solution. This means that the Na” and CT ions that are Te? in the solution are equivalent to sodium chloride solution, <!~[enif]–Concentrated aqueous solution

NaCl Na" + Cr

HOH" + OH

Cathode Anode

Na‘and HH" ions are discharged in CV'and OH{CTions are discharged in preference to preference to Na” ions since Na and OHions since CT and OH ions are very elose to each Hi are very far from each other in e. other in the e. ©. $ (and because there are more CI es2H"+ 26 Hay ions in the solution) 2CT—+ Clay +2 Results

• At cathode Hayy is given off
• Atanode Cla, is given off
• The solution becomes progressively more alkaline as the electrolysis goes on because Na and

OlFions remain in solution as caustic soda (sodium hydroxide) solution. This is the main method used in the manufacture of sodium hydroxide in industry. Na" ags+ OH yay —+ NEOH 0) Fused or molten sodium chloride

NaCl — Na+ Cr

During electrolysis,

Cathode Anode

Nat +e Nay 2CP + Chay) + 28 Result

• At cathode sodium is liberated (deposited)
• Atanode chlorine is given off

‘The nature of electrodes (inert vs active electrodes) <{—[endif]->IF dilute sulphuric acid iscleetrolysed using platinum electrodes:

H,S0, + 2H" + SO?

H,0@H" + OH

Cathode Anode

2H’ +20 + Hay) $0, and OH 40H 2H,0;)#0ay-40 Result

• Atcathode Hayy is given out
• Atanode Ong is given out
• The solution becomes acidic at the end of electrolysis because of the acidic ions (SO?) lef in

the finala solution. Charge flow during electrolysis The coulomb is the electrolytic unit of charge. A current of one ampere is the rate of flow of charge equal to one coulomb per second The charge is calculated from the knowledge of the number of seconds for which a steady current is passed.

Current in cireuit ‘Time taken ‘Total charge Tampere 1 second 1 coulomb ampere 10 seconds 10 coulombs 20 amperes 10 seconds 200 coulombs A amperes t seconds At coulombs Therefore charge ~ quantity of electricity (Q) = Ixt Flow of charge required fo liberate 1 mole of element during electrolysis Electrolysis always produces chemical reactions. Consider a reaction (at cathode) in which one mole of silver Ag” ions is discharged and deposited.Ag’ +e" Ag

In this case, 1 mole of electrons (€°) is required to discharge 1 mole of Ag” jons to produce 1 mole of silver atoms (Aw). mole of electrons is a larue charge and experiments show that it is

faraday. The number of faradays (moles of electrons) required to Tiberate mole of an element during electrolysis is deduced from the equation for the electrode reaction

Example 1

Element Flectrode reaction Faradays

Sovtium Na +e Nay 1 Copper Cur +26 Cy 2 Aluminium AI +3e > Aly 3 Chlorine 2CI + Clyp + 26 2 That is, 1 faraday is needed to deposit 1 mole of sodium atoms (23g), 3 faradays to deposit 1 mole of aluminium atoms (27g) and 2 faradays to liberate mole of chlorine gas (71g)

• The mass of an clement liberated by 1 coulomb of electricity during electrolysis is

called electrochemical equivalent of that element

• The mass of an element deposited or liberated by 1 faraday during electrolysis. is

called chemical equivalent of that clement Experiments to Kentfy the Products of Electrolysis when Different Electrolytes are Used Perform experiments to identify the products of electrolysis when different electrolytes are used

Activity 1

Perform experiments to identify the products of electrolysis when different electrolytes are used Experiments to Mentify the Products of Electrolysis when Different Plecttodes are Used Perform experiments to identify the products of electrotysis when different electrodes are used

Activity 2

Perform experiments to identify the products of electrolysis when different electrodes are used

Laws of Electrolysis

Laws of Electrolysis

Experiments to Relate Masses Liberated and Quantity of Electricity Passed Carry out experiments to relate masses liberated and quantity of electricity passed The laws expressing the quantitative results of electrolysis were first stated by a British chemist called Michael Faraday. The laws assert that the amount (expressed in moles) of an element liberated during electrolysis depends on

• the time of passing the steady current;
• the magnitude of the steady current passed; and
• the charge on the ion of the element.

‘An Experiment to Verify Faraday’s First Law of Electrolysis Carry out an experiment to verify Faraday's First Law of Electrotysis ‘The fact that the amount of a substance liberated during electrolysis depends upon these factors can be proved by conducting experiments, The product of time (measured in seconds) and the ‘current passed (measured in amperes) gives a measure of electricity known as the quantity of electricity

Q-xt Because of this relationship, factors (1) and (2) may be included im the same experiment, The experiment to determine the effect of time on the amount of element deposited or liberated is carried out by passing a steady current through a solution of the compound of that element for different lengths of time The following tabie summarises the specimen results obtained by passing a steady current (0.21 amps) through a solution of copper (II) sulphate for 15, 30, 45 and 60 mimutes. The last column

shows the mass of eopper deposited. Specimen resuits for electrolysis of copper (Il) sulphate Current juantity of electricity Mass of copper deposit Time(s) oes * " ” (amps) (coulombs) (grams)

The relationship between the amount of copper deposited and the quantity of elecwicity passed can be assessed by considering the values in the last two columns in the table. ‘This data may be represented in a graph illustrated in figure 6.4. The shape of the graph shows a stright line passing through the origin. From this fat, itis clear thatthe mass of eopper deposited is directly proportional to the quantity of electricity passed. This is in accordance to what Faraday

formulated in his Fist Law z os af 8 0.10 af 2 0.05 oe — — 0 100 200 300 400 500 600 700 300 Quantity of electricity (coulombs) Graph of mass deposited versus amount of electricity passed Foraday’s First Law of Electrolysis states that the mass of a substance liberated at (or dissolved from) an electrode during electrolysis is directly proportional 10 the quantity of electricity passing through the electrobte.

The quantity of electricity is measured in coulombs where a coulomb is the passage of an electric ‘current of one ampere for one second, Let

• mbe the mass of the substance liberated,
• Me the current passed in amperes: and
• tbe the time in seconds.

the proportionality constant referred to as electrochemical equivalent of the substance liberated Electrochemical equivalent is the mass of a substance (element) liberated by 1 coulomb of electricity during electrolysis, ‘An Experiment to Verify Faraday’s Second Law of Electrolysis Carry out an experiment o verify Faraday’s Second Law of Electrolysis, The third (3) factor mentioned previously as affecting the amount of substance liberated during

lectrolysis may also be investigated experimentally. Because our interest is the effect of the charge on the ions present in solution, we need to keep the quantity of electricity fixed whilst varying the types of the ions in solution, This may be achieved by passing the same quantity of electricity through two cells, with ions of different charges in each eal.

Switeh « bk + oe

• Fl ry ] Resistance

> L 1 Vs Pa Hl to adjust SESE AESE the current

Coppervoltameter Silver voltameter

_/ Atameter EE Apparatus for confirmation of Faraday’s Second Law The experiment is conducted using two voltameters. The two voltameters are connected in series as shown, The first one is a copper voltameter and the second is a silver voltameter The copper woltameter has copper electrodes ina solution of copper (II) sulphate. Hence, in this voltameter, copper ions are discharged and deposited at the cathode.

The silver voltammeter has silver electrodes in a solution ofsilver nitrate, The discharged silver ions are deposited at the cathode fora measured period of time Aer this, the cathode ate removed from the voltameters, cleaned, dried and reweighed. The imerease in mass of the two cathode electrodes represents the respective amounts of copper and clement is calculated using Faraday’s Second Law of Electrolysis.

Specimen results Current flowing – 045A

Mass of silver deposited – 0.755 The results show that the masses of silver and copper deposited are different. A comparison of the amounts of each of the elements deposited can be made simple by calculating the number of moles of atoms of each of the element deposited.

Thus:

It is seen that twice as many atoms of silver are deposited as atoms of copper. The difference in amount of each element deposited arises from the difference in charges on ions of the element concerned.

‘The change on the copper ion is twice that on the silver ionand therefore twice the quantity of electricity will be required to liberate one mole of copper as for the liberation of one mole of silver This relationship is in accordance to Faraday’s Second Law of Electrolysis, which describes the

relationship between the amount of element deposited and the charge on the ions of that element

Faraday’s Second Law of Electrolysis states that witen the same quantity of electricity is passed through solutions of different electrolytes the relative numbers of moles of the elements deposited are inversely proportional to the charges on the ions of each of the elements respectively In order to discharge one mole of monovalent ions such as hydrogen ion, Sodium ion, Silver ion and Chlorine ion,96500 € of electricity are required. This quantity of electricity has been

experimentally determined and is known as the Faraday constant i represents one mole of electrons, which is the same as the quantity of electrons required to discharge one mole of Siliverions to give one mole of silver atom, The validity of Faraday’s Second Law of Blectrolysis is evident from the following observations

• One faraday (IP) discharges one mole of H, Na‘, Ag’, CI and OH ions.
• Two Faradays (2F) discharges one mole of Cu", Pb, Mg”, Ca?', Pe’, ete ions
• Three Faradays (3F) discharge one mole of AP*, Fe, ete. ions

Relationship between the Chemical Equivalents of Elements and Quantity of Electneity Passed

Relate the chemical equivatents of elemenis and quantity of electricity passed A steady current of 4 amperes is passed through aqueous copper (Il) sulphate solution for 1800 seconds using platinum electrodes, Caleulate

• mass of copper deposited
• mass of oxygen liberated

Given:

• Atomic weight of copper = 63.5
• Atomic weight of oxygen = 16
• 1 Faraday = 96500.

Solution

<!-(endif]->Cathode reaction: Cu" + 2e" —> Cys In this case, 2 Faradays of electricity are required to deposit one mole of copper atom 63.54 ‘This means 2 x 96500 C liberates 63.5g of copper

4% 1800C will liberate? 4g of copper.

Anode reaction: 40H 2H,0)* Org, + 4€ The reaction shows that 4 moles of electrons (4 Faradays) are lost during the reaction process. Therefore, 4 * 96500 C are needed to liberate one mole (328) of oxygen.

4x 1800x 32

4×96500

Application of Electrolysis

lectrolysis has several uses in industry. Its main application has been in the fields of manufacture of chemicals and in the purification of metals for which other purification methods prove either too difficult or highly expensive to apply. Some applications of electrolysis are as discussed below: The Industrial Purification of Copper by Electrolysis, Outline the industrial purification of copper by electrolysis Some metals can be purified by means of electrolysis. This process is used in industry to purify

copper. which must be very pure 99.9% for electrical wiring. Copper made by roasting the sulphide ore is about 99.5% pure (so it has an impurity level of 0.5%). This level of impurity euts down clectrical conductivity significantly.

This is how the electrolytic purification (refining) process is carried out"The anode is made of a large block of impure copper. The cathode is a thin sheet of pure copper. ‘The electrolyte is copper (II) sulphate solution During the refining process, the copper atoms of the impure block become ions (the anode dissolves) Cu —» Cu?" + 2" The ions from the solution become atoms.

Cub +22 + Cu They stick onto the cathode. A layer of pure copper builds up on the cathode. As electrolysis takes place, the eathode gains mass as copper is deposited on it. As a result, the cathode gets Staller While the Cathode gets bigger as electrolysis proceed’: Eventually the whole cathode sth aus (impure (pure cy ope) OY ney ofa .

; f in \f i) di y I ly uy wu Pataca ot or vos pac an ei jnBeptien oeoriing of iciAdeli Cary out an experiment on electroplating of metallic materiais ies iS com a a i lc eR EkstGbjeis dan’ Be sed to oat a thin layer ofa Less reactive metal ont a avode reactive wictal, mtu Ky ao make he pra! mot arate ee iin a i Nickel ean be electroplated with silver. This will make nickel more attractive: The diagram below shows how a steal jug is electroplated with silver, The jug becomes the cathode ofan electrolytic

call. The anode is mad of silver. The electrolyte isa solution of a silver compound, for example silver nitrate silver steel jug as anode cathode fe =] silver nitrate solution Sitverplating a stel jug At the anode: The silver dissolves, forming ions in solution:Ag —» Ag” + € At the cathode: The silver ions receive electrons, forming a coat of silver on the jug.Ag™+ & AW) When the layer of silver is thick enough, the jug is removed.In general, to electroplate any object

with metal M, the set up is

• Cathode — object to be electroplated

+ Anode—metal M

• Electrolyte ~ solution ofa soluble compound of M