31251484 chapter 6 electrochemistry

7/27/2019 31251484 Chapter 6 Electrochemistry

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Electrolytes

Electrolytes are compounds which when molten or dissolved in water conduct electric current and are

decomposed in the process .

Non-electrolyte
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A non-electrolyte is a liquid which does not allow the passage of electricity.

Molten Solution

This is composed of lead(II) ions, Pb2 + , and romide ions, !r-. Its chemical formula is therefore

"!r#.

A suitale apparatus which could e used to carry out this electrolysis is shown in $igure aove.

The ul helps to show when electricity is flowing in the circuit, and until the lead(II) romide is

completely molten, the ul does not light up . This confirms that electrolytes have to e molten

for the ions to start to move to the electrodes and therey conduct electricity.

At the Cathode At the Anode

Observation

When electricity is flowing, a silvery

deposit of lead metal forms on the

cathode. In fact, as it is molten, it is

more likely to drip off in a molten

blob.

Observation

When electricity is flowing,

brown fumes of bromine gas

are seen at the anode.

Half equation

Pb2++ 2e --- Pb

Half equation

2!r---- !r2+ e

Explanation

"he lead#II$ ions, as they are

positive, move to the negative

cathode, where each ion gains two

electrons to form a lead atom.

%ny reaction at a cathode involved is

again in electrons. "his is calledreduction or more e&actly, cathodic

Explanation

"he bromide ions, as they are

negative, move to the positive

anode, where each loses an

electron to form a bromine

atom.

"hen two of these newlyformed atoms combine to
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reduction . form bromine gas.

%ny reaction at an anode

involves a loss of electrons.

In summary, the lead(II) romide is split into its component elements %

"!r#---& " ' !r#

Electrolysis of Aqueous Sulphuric Acid
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As sulphuric acid is aqueous, it is composed not only of hydrogen ions (') and sulphate ions

(*+#-), ut also of hydroide ions (*-) from the water.

#*+' #* --& #'' *+#-' '' *-

The apparatus used to carry out this electrolysis and collect the gases given off is shown in $igure

. .

/hen we have more than one type of ion moving to an electrode, selective discharge (or

preferential discharge) ta0es place.

This means that the ion which can lose or gain electrons with the greatest ease is discharged,

and the other ions, which are harder to discharge, remain in solution .

/ith the electrolyte aqueous sulphuric acid, migration of ions to the electrodes also occurs.

At the Cathode At the Anode

Here we have only one ion,

the hydrogen, H+ (aq),

and each ion gains an

electron to become a

hydrogen atom.

Two of these newly formed

atoms then combine to

form a hydrogen gas

molecule .

Here we have a choice of

either sulphate, S!"#(aq),

or hydro$ide H#(aq) ions.

Hydro$ide is easier to

discharge, so o$ygen gas

is given off at the anode.

Equation%

2'++ 2e --- '2

Equation%

('-+ )e --- (2+ '2(

Notes

&ith electrolysis of aqueous solutions of dilute acids or al'alis,

the volume of hydrogen given off at the cathode is roughly

twice that of the o$ygen gas at the anode.

ccordingly, the elements of water are lost and as the

electrolysis continues, the concentration of the acid or al'ali

increases .

ssentially, the electrolysis of aqueous sulphuric acid is the

electrolysis of water, with hydrogen and o$ygen gas being

given off in a ratio of " % * .


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Etraction of 1etal

The etraction of metals from their ores, in particular aluminium and sodium, is important

industrial uses of electrolysis.

The diagram elow shows the methods of etraction for different metals.

/e can see that those metals which are less reactive than caron in reactivity series are

etracted from their ore y displacement reaction using caron. This will e discussed in detail in

chapter 2, form 3, *idation and 4eduction.

5opper and mercury can e etracted from their ore y urning directly in air.

ilver (Ag) and gold (Au) need no etraction ecause they eist as element in nature.

Those metals which are more reactive than caron are etracted y electrolysis.

Extraction of Aluminium

Aluminium is the most aundant metal found in the earth6s crust. It ma0es up aout 7 y weight

of the Earth8s solid surface.

It is also a very useful metal due to its low density and aility to resist corrosion.

The main source of aluminium is auite ore (Aluminium *ide).

In industry, aluminium is etracted y electrolysis from auite ore.
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Adding Cryolite

In electrolysis, molten aluminium oide must e used to etract aluminium. Aluminium oide

decompose to form aluminium and oide ions when melted.

Al#*2---& #Al2'' 2*#-

owever, the melting point of aluminium oide is very high (over # 999:5), so another

aluminium compound called cryolite (;a2AI$uring electrolysis, the aluminium ions are attracted towards the graphite cathode.

The ions is discharged and ecome molten aluminium metal.

The partial equation of this reaction is as follow%

Al2'' 2e ---& Al

At the anode, oygen gas which also has commercial value is collected. The partial equation

of this reaction is as follow%

#*#----& *#' +e

At the temperature of 9 :5, the oygen urns the caron anode. Therefore the anode has

to e replaced periodically.

Also, this cell uses large quantities of electricity, and therefore needs cheap sources of power.
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Extraction of sodium chloride

In industry, sodium is etracted from molten sodium chloride. 1olten sodium chloride is put

into the apparatus as showing in the diagram aove.

/hen sodium chloride is melted, the sodium and chloride ions disassociate to ecome freely

move ions, as shown in the chemical equation elow.

;a5l ---& ;a'' 5l-

In this electrolytic cell, graphite was used as anode while iron is used as cathode.

The negative chloride ions are attracted to the anode and then discharged to form chlorine

gas.

#5l----& 5l#' #e

ince chlorine gas is also significant in industry, it is collected and stored.

In cathode, the sodium ions are discharged to form sodium atom.

;a'' e ---& ;a
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>ue to high temperature, the sodium metal formed is in molten form.

1etal sodium have lower density. Therefore it moves upward and een collected.

"urification *f 5opper

In the refining or purification of copper, the impure copper is made the anode and a thin, pure

copper plate is used as a cathode.

The electrolyte is usually acidified copper(II) sulphate solution. /hen electricity flows, the copper dissolves from the impure anode and goes into solution as

copper ions.

Impurities in the copper do not dissolve, and instead fall off the anode as anode sludge. At the

cathode, the copper ions are deposited as pure copper metal.

4eaction in anode (impure copper)

In anode, the copper atoms from the electrode are ionised to form copper(II) ions.

5u ---& 5u#'' #e

4eaction in cathode (pure copper)

5u#'5u ---& 5u ' #e

Electroplating
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Electroplating% 5oating with a Thin "rotective ?ayer of 1etal

A very common use of electrolysis is to form a thin protective coating of a metal on the

surface of another which is li0ely to corrode.

The diagram aove illustrate the electroplating of a 0ey with copper.

In this process, we need to ma0e the cathode the o@ect for plating (the 0ey.

The anode is then made of the metal we wish to plate with (copper), and the electrolyte needs

to e a solution of a salt of this metal (copper(II) sulphate).

Anode

In anode, the copper atoms from the electrode are ionised to form copper(II) ions.

5u ---& 5u#'' #e

Cathode

In cathode, the copper ions are discharged to form copper atom and then deposit on the

surface of the 0ey

5u#'---& 5u ' #e
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5ells and !atteries

A device which converts chemical energy into electrical energy is called a cell or attery. !attery

is a collection of cells.

A cell consists of a pair of dissimilar metals in an electrolyte.

$igure aove shows an eample of a simple voltaic cell consist of a magnesium electrode and a

copper electrode immerse in magnesium sulphate solution.

/hen chemical reaction happens, the more reactive metal, magnesium, dissolves in the

magnesium sulphate solution and ecome magnesium ions, therey producing electrons, as

shown in the half equation elow%

1g ---& 1g#'' #e

As electrons are produced, the magnesium acts as the negative electrode.

These electrons then travel to the copper electrode.
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The hydrogen ions around the copper electrode receive the electrons and are discharged to

produce ules of hydrogen gas%

#'' #e ---& #

As electrons are ta0en in, the copper is the positive electrode.

This production and movement of electrons is electricity, so electrical energy has een

generated and the galvanometer is deflected. *verall, the chemical reaction can e

represented y the ionic equation%

1g ' #'---& 1g#'' #

In voltaic cell, the negative electrode is the anode whereas the positive electrode

is the cathode, which is the opposite of the electrolytic cell.

31251484 chapter 6 electrochemistry

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