ELECTROLYSISELECTROLYSIS

BONDING, STRUCTURE BONDING, STRUCTURE & PROPERTIES& PROPERTIES

After completing this topic you should be able to :

BONDING, STRUCTURE & BONDING, STRUCTURE & PROPERTIESPROPERTIES

ELECTROLYSISELECTROLYSIS

• Explain the conduction of ionic compounds in terms of movement of ions and the non-conduction of covalent substances in terms of molecules which are uncharged.

• Explain that an ionic compound breaks down into its elements during conduction and this reaction is called electrolysis.

• State a dc supply must be used if the products of electrolysis are to be identified.

• Explain the formation of atoms /molecules at each electrode during electrolysis in terms of positive ions gaining electrons and negative ions losing electrons.

• Write the ion-electron equations for the reactions which take place on the electrodes during electrolysis.

ELECTROLYSISELECTROLYSISElectrolysis is the break down of an ionic compound using electricity. The reaction occurs on the electrodes during conduction.

ELECTROLYSIS OF SOLUTIONSELECTROLYSIS OF SOLUTIONSElectrolysis of copper(II) bromide and zinc iodide solutions.

RESULTSRESULTS

carbonrodscopper(II)

bromide solution or zinc iodide solution

6 V d.c.6 V d.c.

power packpower pack

+ ve+ ve- ve- veCompoundCompound

Product at:Product at:

+ ve+ veelectrodeelectrode

- ve- veelectrodeelectrode

copper(II) copper(II) bromidebromide

zinc iodidezinc iodide

brominebromine

iodineiodine

coppercopper

zinczinc

For electrolysis to take place the ions in an ionic compound have to be free For electrolysis to take place the ions in an ionic compound have to be free to move. The ions are free to move when the ionic compound is dissolved in to move. The ions are free to move when the ionic compound is dissolved in water and when it’s molten (liquid.)water and when it’s molten (liquid.)

ELECTROLYSIS OF A LIQUIDELECTROLYSIS OF A LIQUID

carboncarbonrodsrodslead(II)lead(II)

bromidebromide

6 V d.c.6 V d.c.

power packpower pack

+ ve+ ve- ve- ve

HEATHEAT

RESULTSRESULTSLead forms at the negative electrode Lead forms at the negative electrode and bromine forms at the and bromine forms at the positive positive electrodeelectrode..

CONCLUSIONCONCLUSIONDuring electrolysisDuring electrolysis::

A metal forms at the negative electrode as metal ions are positively charged A metal forms at the negative electrode as metal ions are positively charged and are attracted to the negative electrode.and are attracted to the negative electrode.

A non - metal forms at the positive electrode as non - metal ions are negatively A non - metal forms at the positive electrode as non - metal ions are negatively charged and are attracted to the positive electrode.charged and are attracted to the positive electrode.

A liquid which contains ions is called an A liquid which contains ions is called an ELECTROLYTE.ELECTROLYTE.

Electrolytes conduct electricity.Electrolytes conduct electricity.

All ionic compounds make electrolytes when melted or in solution.All ionic compounds make electrolytes when melted or in solution.

During electrolysis the ions get changed into atoms when the ions touch the During electrolysis the ions get changed into atoms when the ions touch the electrodes. electrodes.

ELECTROLYSIS EXPLAINEDELECTROLYSIS EXPLAINEDUsing the electrolysis of copper(II) chloride solution to explain the process of Using the electrolysis of copper(II) chloride solution to explain the process of electrolysis.electrolysis.

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CuCu2+2+

ClCl--

ClCl--

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Ions presentIons present

ClCl--

ClCl--

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CuCu2+2+

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The copper(II) ion, CuThe copper(II) ion, Cu2+2+, is , is attracted to the negative attracted to the negative electrode. electrode.

When the CuWhen the Cu2+2+ touches the touches the electrode, 2 electrons electrode, 2 electrons transfer onto the ion, transfer onto the ion, changing it into a copper changing it into a copper atom.atom.

CuCu2+2+(aq)(aq) 2e-2e-++ Cu(s)Cu(s)

The chloride ion, ClThe chloride ion, Cl ‒ ‒, is , is attracted to the attracted to the positive electrode.positive electrode.

When 2 ClWhen 2 Cl‒‒ ions touch ions touch the electrode, 1 the electrode, 1 electron each transfers electron each transfers to the electrode to the electrode changing them into Cl changing them into Cl atoms, which then join atoms, which then join to form a Clto form a Cl22 molecule. molecule.

2Cl2Cl--(aq)(aq) 2e-2e-++ClCl22(g)(g)

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CATHODECATHODEANODEANODE

In electrolysis the positive electrode is called the In electrolysis the positive electrode is called the ANODE and the negative and the negative electrode is called the CATHODE.electrode is called the CATHODE.

ELECTROCHEMICAL SERIESELECTROCHEMICAL SERIES

The equations showing the ions being discharged are called “ion-electron The equations showing the ions being discharged are called “ion-electron equations.”equations.”

These are listed in the These are listed in the ELECTROCHEMICAL SERIESELECTROCHEMICAL SERIES..

In the electrolysis of molten lead(II) bromide, lead formed at the negative In the electrolysis of molten lead(II) bromide, lead formed at the negative electrode and bromine formed at the positive electrode.electrode and bromine formed at the positive electrode.

Here is the ion-electron equation for the discharge of the lead(II) ions:Here is the ion-electron equation for the discharge of the lead(II) ions:

-ve-veelectrodeelectrode PbPb2+2+((ll)) 2e-2e-++ Pb(Pb(ll))

This equation is just copied from the Electrochemical Series with a change This equation is just copied from the Electrochemical Series with a change to the state symbols.to the state symbols.

2Br2Br--((ll)) 2e-2e-++BrBr22(g)(g)+ve+veelectrodeelectrode

In the Electrochemical Series the listed equations for In the Electrochemical Series the listed equations for chlorine, bromine and iodine have to be reversed to show the discharge of their ions.have to be reversed to show the discharge of their ions.

BrBr22((ll)) 2e-2e-++ 2Br2Br-- ( (aqaq))

Here is the bromine equation from the Electrochemical Series.Here is the bromine equation from the Electrochemical Series.

Writing the equation in reverse shows the discharge of bromide ions to form Writing the equation in reverse shows the discharge of bromide ions to form bromine. For this to happen a total 2 electrons have to be lost.bromine. For this to happen a total 2 electrons have to be lost.

Bromine (BrBromine (Br22) is a gas because the molten lead(II) bromide is so hot it boils ) is a gas because the molten lead(II) bromide is so hot it boils the bromine.the bromine.

COVALENT SUBSTANCESCOVALENT SUBSTANCESCovalent substances do not conduct electricity (Covalent substances do not conduct electricity (except carbon as graphiteexcept carbon as graphite).).

Covalent bonding involves sharing electrons between the bonding non - metal Covalent bonding involves sharing electrons between the bonding non - metal atoms to form molecules. The atoms in a molecule do not become charged. atoms to form molecules. The atoms in a molecule do not become charged. They do not form ions.They do not form ions.

The neutral molecules do The neutral molecules do not get attracted to the not get attracted to the electrodes.electrodes.

-ve-veCATHODECATHODE

+ve+veANODEANODE water moleculeswater molecules

Covalent substances do not conduct as molecules are not charged, and Covalent substances do not conduct as molecules are not charged, and therefore do not get attracted to the electrodes.therefore do not get attracted to the electrodes.

DIRECT CURRENTDIRECT CURRENT

Electrolysis will only take place when a Electrolysis will only take place when a DIRECT CURRENT (d.c.) is passed is passed into aninto an ELECTROLYTE.

A direct current ensures that there is a POSITIVE and NEGATIVE electrode. A direct current ensures that there is a POSITIVE and NEGATIVE electrode. [ANODE and CATHODE.][ANODE and CATHODE.]

There is another connection on the power pack - There is another connection on the power pack - ALTERNATING CURRENT (a.c.).

In a.c. the current changes direction 50 times per second, so there is not a In a.c. the current changes direction 50 times per second, so there is not a permanent positive and negative electrode. As a result, electrolysis does not permanent positive and negative electrode. As a result, electrolysis does not occur when a.c. is used.occur when a.c. is used.

-ve-veCATHODECATHODE

+ve+veANODEANODE water moleculeswater molecules

-ve-ve+ve+veee--

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CuCu2+2+

ClCl--

ClCl--

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Ions presentIons present

ClCl--

ClCl--

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CuCu2+2+

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CATHODECATHODEANODEANODE