CHEMISTRY SPM FORM 4 Short Notes Chapter 6 ELECTROCHEMISTRY

8/10/2019 CHEMISTRY SPM FORM 4 Short Notes Chapter 6 ELECTROCHEMISTRY

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CHEMISTRY FORM 4

CHAPTER 6 : ELECTROCHEMISTRY

6.1 Understanding properties of electrolytes and non-

electrolytes

6.2 Analysing electrolysis of molten compounds

6.3 Analysing the electrolysis of aqueous solutions

6.4 Evaluating electrolysis in industry6.5 Analysing voltaic Cell

6.6 Synthesising electrochemical series

6.1 UNDERSTANDING PROPERTIES OF ELECTROLYTES AND NON- ELECTROLYTES

Electrolytes are substance that can conduct electricity when they are in molten state or aqueous

solution and undergo chemical changes.

Non-electrolytes are substance that cannot conduct electricity either in molten states or

aqueous solution.

6.2 ANALYSING ELECTROLYSIS OF MOLTEN COMPOUNDS

Electrolysis is a process whereby compound is molten or aqueous state is broken down into

their constituent elements by passing electricity through them.

The electrolytic cell is the set of apparatus needed to conduct electrolysis. It is consist of a

battery, an electrolyte and two electrodes.

The electrode which is connected to the positive terminal of an electric source is called the

anode while the electrode which is connected to the negative terminal is called the cathode.

In molten electrolyte, the ions move freely.

During electrolysis, the negative ion or anions move to the anode whereas the positive ion or

cations move to the cathode.


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Example : Electrolysis of lead(II) bromide, PbBr2a) Lead(II) Bromide, PbBr2is an ionic compound

PbBr2 Pb2+

+ 2Br-

b) The positive Lead(II) in Pb2+

are attracted to cathode and undergo discharge whereby each of

the ions accepts two electron to form an atom. The half equation below summarized the

process.

Pb2+

+ 2e Pb

c) The negative Bromide ions, Br-are attracted to the anode and undergo discharge, whereby

each of the ionic accepts an electron to form a neutral bromide atom.Br

- Br + e

Two bromine atoms combine to form a bromine gas

Br + Br Br2 or

2Br- Br2 + 2e

d) The overall equation for electrolysis of PbBr2,

Cathode : Pb2+

+ 2e Pb

Anode : 2Br- Br2 + 2e

Overall equation : Pb2+

+ 2Br- Pb + Br2

TRY THIS 1

Complete the chart below


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TRY THIS 2


TRY THIS 3



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6.3 ANALYSING THE ELECTROLYSIS OF AQUEOUS SOLUTIONS

In an aqueous solution, water will dissociate partially to form hydrogen ions and hydroxide ions.

H2O H+ + OH

-

Example : Copper(II) Sulphate, CuSO4consist of copper(II) ions, Cu2+

, Sulphate ion SO42-

,

hydrogen ion, H+and hydroxide ion, OH

-

CuSO4 Cu2+

+ SO42-

H2O H+ + OH

-

There are three factors that may influence the selective discharge of ion during the electrolysis

of an aqueous solution.

Position of ions in the electrochemical series

The ions that are lower in the electrochemical series will be selectively discharge

TRY THIS 4

Select the ion to be discharged from the following pairs of ions. State at which electrode it occurs and

write the half equation for the discharge of ion

a) Hydroxide ion & Chloride Ion : Equation : ______________________________ at the __________

a) Hydroxide ion & Nitrate ion : Equation : ______________________________ at the __________

a) Hydroxide ion & Copper(II) ion : Equation : ______________________________ at the __________

a) Hydroxide ion & Pottasium Ion : Equation : ______________________________ at the __________

a) Hydroxide ion & Silver ion : Equation : ______________________________ at the __________

Position of ions in the electrochemical series

Concentration of ions in the electrolysis

Types of electrodes used in the electrolysis


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TRY THIS 5

Complete the following table for the electrolysis of 0.1 moldm-3

sodium nitrate solution using carbon

electrode.

Set up of apparatus

Write the equation of electrolyte

ionization

ANODE CATHODEWrite the formulae of ions that

are attracted to the anode and

cathode.

Name of the product at anode

and cathode

Observation

Confirmatory test (method and

observation of the test


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TRY THIS 6


sulphuric acid using carbon electrode.

Set up of apparatus


ionization

ANODE CATHODE

Write the formulae of ions thatare attracted to the anode and

cathode.


and cathode

Observation




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TRY THIS 7


copper(II) Sulphate solution using

carbon electrode.

Set up of apparatus


ionization

ANODE CATHODEWrite the formulae of ions that


cathode.


and cathode

Observation




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Electrolyte concentration factor

When electrolysis is carried out using inert electrodes and concentrated solutions, ions that are

more concentrated will be discharged but this only true for halides ions which are Cl-, Br

-, and I

-

TRY THIS 8

State the selected ions to be discharged at the anode and cathode for the following concentrated

solution.

a) Concentrated hydrochloric acid solution, using carbon electrode

Anode : _____________________________ Cathode : _______________________________

b) Concentrated potassium iodide solution, using carbon electrode

Anode : _____________________________ Cathode : _______________________________

c) Concentration of sodium bromide solution, using carbon electrode

Anode : _____________________________ Cathode : _______________________________

TRY THIS 9


hydrochloric acid and 2.0 moldm-3

hydrochloric acid, using carbon electrode.

Set up of apparatus


ionization

0.001 moldm-3

2.0 moldm-3

The ions that move to cathode

Equation of discharged of ion atthe cathode

Name of the product at cathode

The ions that move to anode

Equation of discharged of ion at

the anode


Observation



The electrolyte concentration

after a while


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TRY THIS 10


sodium iodide solution using carbon

electrode.

Set up of apparatus


ionization

ANODE CATHODE

Write the formulae of ions that


cathode.

Name of the product at anodeand cathode

Observation



Types of electrodeThere are 2 types of electrode

- Inert electrodeAn electrode that are acts as conductor only and does not undergo any

chemical changes. Normally they are made of carbon or platinum.

- Reactive electrodeAn electrode that not only acts as conductor but undergoes chemical

changes. During the electrolysis, the metal anode will dissolve while the reaction at the

cathode is similar to the reaction at the carbon electrode. The consist of metal electrodes such

as copper, silver and nickel.


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TRY THIS 11


copper(II) Sulphate solutin, using

carbon electrode and copper electrode.

Set up of apparatus


ionization

Carbon Electrode Copper electrode

The ions that move to cathode


the cathode

Name of the product at cathode

The ions that move to anode


the anodeName of the product at anode

Observation



The electrolyte concentration

after a while

6.4 EVALUATING ELECTROLYSIS IN INDUSTRY

Electrolysis is used in industry for

a) Extraction of metals

b) Purification of metals

c) Electroplating of metals


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A) EXTRACTION OF METALS

Carbon electrode acts as anode and cathode

Name the product formed at Anode:__________________ Cathode: ____________________

Write the half equation for reactions at :

ANODE :______________________________________________________________________

CATHODE :____________________________________________________________________

B) PURIFICATION OF METALS

BEFORE ELECTROLYSIS AFTER ELECTROLYSIS

Ions Present

Cu2+,

H+, OH

-and SO4

2-

Reaction at Anode

Both SO42-

and OH-gets attracted here but not discharged. Instead, the copper anode discharged

by losing electrons to form Cu2+

. So, the electrode size decreases.

Cu(s) Cu2 + 2e-

Reaction at Cathode

Cu2+

produced from anode gains electrons at cathode to become Cu atoms becoming copper.

Hence, the copper is deposited here and the electrode grows.

Cu2+

+ 2e- Cu


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Overall Change

There is no change in solution contents as for every loss of Cu2+

ions at cathode is replaced by

Cu2+

ions released by dissolving anode. Only the cathode increases size by gaining copper and

anode decreases size by losing copper. We can use this method to create pure copper on

cathode by using pure copper on cathode and impure copper on anode. Impurities of anode fall

under it.

C) ELECTROPLATING OF METALS

Electroplating is coating an object with thin layer of metal by electrolysis. This makes the object

protected and more attractive

Object to be plated is made to be cathode and the plating metal is made as anode. The

electrolyte MUST contain plating metal cation.

Half equation that occur at

ANODE :______________________________ CATHODE :_______________________________

To ensure that the surface of the iron spoon is thinly and evenly plated with silver metal, a low

concentration of silver nitrate solution must be used.


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6.5 ANALYSING VOLTAIC CELL

The changes of energy of a voltaic cells

CHEMICAL ENERGY ELECTRICAL ENERGY

A simple voltaic cell can be made by using 2 different metals as electrode immersed to an

electrolyte.

Example

* Daniel Cell is an example of voltaic cell that used zinc and copper as electrodes and immersed

into a solution of its ions.

* The two solution are connected through a salt bridge or porous pot

* The function of a salt bridge or porous pot are:

- To prevent the two electrolytes from mixing

- To allow the flow of the ions so that the electric circuit is completed.

Mg Mg2+

+ 2e (negative terminal)

2H+ + 2e H2 (positive terminal)

Overall Equation

Mg Mg2+

+ 2e

2H+ + 2e H2 .

Mg + 2H+ Mg

2+ + H2

Zinc Electrode (Negative Terminal)

Zn Zn2+

+ 2e

Copper Electrode (Positive Terminal)

Cu2+

+ 2e Cu

Overall Equation

Zn + Cu2+

Zn2+

+ Cu


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6.6 SYNTHESISING ELECTROCHEMICAL SERIES

In the electrochemical series, metals are arranged according to the tendency of their atoms to

donate electrons

The higher the position of a metal in the electrochemical series, the greater the tendency of the

metal atoms to donate electron. Thus, the more electropositive the metal.

K

Na

Ca

Mg

Al

Zn

Fe

Sn

Pb

H

Cu

Ag

Au

The electrochemical series can be constructed based on:

- The potential difference between two metals- The ability of a metal to displace other metal

The Importance of the Electrochemical Series

The electrochemical series can be used to determine the following

* The terminal of a voltaic cell

- more electropositive metal (Negative Terminal)

- Less electropositive metal (Positive Terminal)

The standard cell voltage

* The further the distance between two metals in the electrochemical series, the higher the

voltage of the voltaic cell.

The ability of metal to displace another metal from its salt solution* The more electropositive metal can displace a less electropositive metal from its salt solution.

* Example 1 : The reaction between Zinc and Copper(II) Sulphate solution

Zn Zn2+

+ 2e

Cu2+

+ 2e Cu

OVERALL Zn + Cu2+

Zn2+

EQUATION

* Example 2 : The reaction between silver and Lead(II) Nitrate solution

No reaction occur because silver is less electropositive than lead. Thus it cannot displace Lead

More electropositive

(Ease of releasing of electron

increase)

Least electropositive

CHEMISTRY SPM FORM 4 Short Notes Chapter 6 ELECTROCHEMISTRY

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