CHEMISTRY SPM FORM 4 Short Notes Chapter 6 ELECTROCHEMISTRY
Transcript of CHEMISTRY SPM FORM 4 Short Notes Chapter 6 ELECTROCHEMISTRY
-
8/10/2019 CHEMISTRY SPM FORM 4 Short Notes Chapter 6 ELECTROCHEMISTRY
1/14
1
Study Smart
www.studysmart.page.tl
STUDYSMART
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.
-
8/10/2019 CHEMISTRY SPM FORM 4 Short Notes Chapter 6 ELECTROCHEMISTRY
2/14
2
Study Smart
www.studysmart.page.tl
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
-
8/10/2019 CHEMISTRY SPM FORM 4 Short Notes Chapter 6 ELECTROCHEMISTRY
3/14
3
Study Smart
www.studysmart.page.tl
TRY THIS 2
Complete the chart below
TRY THIS 3
Complete the chart below
-
8/10/2019 CHEMISTRY SPM FORM 4 Short Notes Chapter 6 ELECTROCHEMISTRY
4/14
4
Study Smart
www.studysmart.page.tl
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
-
8/10/2019 CHEMISTRY SPM FORM 4 Short Notes Chapter 6 ELECTROCHEMISTRY
5/14
5
Study Smart
www.studysmart.page.tl
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
-
8/10/2019 CHEMISTRY SPM FORM 4 Short Notes Chapter 6 ELECTROCHEMISTRY
6/14
6
Study Smart
www.studysmart.page.tl
TRY THIS 6
Complete the following table for the electrolysis of 0.001 moldm-3
sulphuric acid using carbon electrode.
Set up of apparatus
Write the equation of electrolyte
ionization
ANODE CATHODE
Write the formulae of ions thatare attracted to the anode and
cathode.
Name of the product at anode
and cathode
Observation
Confirmatory test (method and
observation of the test
-
8/10/2019 CHEMISTRY SPM FORM 4 Short Notes Chapter 6 ELECTROCHEMISTRY
7/14
7
Study Smart
www.studysmart.page.tl
TRY THIS 7
Complete the following table for the electrolysis of 0.1 moldm-3
copper(II) Sulphate 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
-
8/10/2019 CHEMISTRY SPM FORM 4 Short Notes Chapter 6 ELECTROCHEMISTRY
8/14
8
Study Smart
www.studysmart.page.tl
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
Complete the following table for the electrolysis of 0.001 moldm-3
hydrochloric acid and 2.0 moldm-3
hydrochloric acid, using carbon electrode.
Set up of apparatus
Write the equation of electrolyte
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
Name of the product at anode
Observation
Confirmatory test (method and
observation of the test
The electrolyte concentration
after a while
-
8/10/2019 CHEMISTRY SPM FORM 4 Short Notes Chapter 6 ELECTROCHEMISTRY
9/14
9
Study Smart
www.studysmart.page.tl
TRY THIS 10
Complete the following table for the electrolysis of 2.0 moldm-3
sodium iodide solution using carbon
electrode.
Set up of apparatus
Write the equation of electrolyte
ionization
ANODE CATHODE
Write the formulae of ions that
are attracted to the anode and
cathode.
Name of the product at anodeand cathode
Observation
Confirmatory test (method and
observation of the test
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.
-
8/10/2019 CHEMISTRY SPM FORM 4 Short Notes Chapter 6 ELECTROCHEMISTRY
10/14
10
Study Smart
www.studysmart.page.tl
TRY THIS 11
Complete the following table for the electrolysis of 1.0 moldm-3
copper(II) Sulphate solutin, using
carbon electrode and copper electrode.
Set up of apparatus
Write the equation of electrolyte
ionization
Carbon Electrode Copper electrode
The ions that move to cathode
Equation of discharged of ion at
the cathode
Name of the product at cathode
The ions that move to anode
Equation of discharged of ion at
the anodeName of the product at anode
Observation
Confirmatory test (method and
observation of the test
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
-
8/10/2019 CHEMISTRY SPM FORM 4 Short Notes Chapter 6 ELECTROCHEMISTRY
11/14
11
Study Smart
www.studysmart.page.tl
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
-
8/10/2019 CHEMISTRY SPM FORM 4 Short Notes Chapter 6 ELECTROCHEMISTRY
12/14
12
Study Smart
www.studysmart.page.tl
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.
-
8/10/2019 CHEMISTRY SPM FORM 4 Short Notes Chapter 6 ELECTROCHEMISTRY
13/14
13
Study Smart
www.studysmart.page.tl
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
-
8/10/2019 CHEMISTRY SPM FORM 4 Short Notes Chapter 6 ELECTROCHEMISTRY
14/14
14
Study Smart
www.studysmart.page.tl
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