Electrolytic Cell

 
1.4 Electrolytic Cell
 
Definition of Electrolyte
Substances that can conduct electricity in either the molten state or aqueous solution and undergo chemical changes.
 
Definition of Non-Electrolytes
Substances that cannot conduct electricity in all states.
 
Classification of Substances into Electrolytes and Non-Electrolytes
 
Apparatus Set Up for Electrolysis
 

 

 
Ion Discharged at Cation and Anion in An Electrolysis
Gambar menunjukkan KATION. 01. Katod (bercas negatif). 02. Penurunan. 03. Menerima elektron. 04. Penurunan nombor pengoksidaan. Pandai.
 

 

 

 

Gambar menunjukkan ANION. 01. Anod (bercas positif). 02. Pengoksidaan. 03. Melepaskan elektron. 04. Peningkatan nombor pengoksidaan.

 
Comparison between Conductor and Electrolytes

Conductor

Electrolyte

  • Substances that conduct electricity in a solid or molten state, but do not undergo chemical changes.
  • Substances that conduct electricity in molten state or aqueous solution, and undergo chemical changes.
  • Substances that conduct electricity without undergoing decomposition.
  • Substances that conduct electricity and undergo decomposition into their constituent elements.
  • Can conduct electricity due to the presence of free moving electrons.
  • Can conduct electricity due to the presence of free moving ions.
  • Electrical conductivity decreases as temperature increases.
  • Electrical conductivity increases as temperature increases.
  • Examples of conductors are metals and graphite.
  • Examples of electrolytes are ionic compounds, acids and alkalis.
 
Electrolysis of Molten Compounds
Definition of Electrolysis
  • A process whereby compounds in the molten state or an aqueous solution decompose into their constituent elements by passing electricity through them.
Apparatus Set Up for Electrolysis of Molten Lead(II) Bromide

Electrolysis of Molten Lead(II) Bromide

 

Ions yang Hadir \(Pb^{2+},\, Br^-\)
Terminal Anod Katod
Ion Bergerak ke Terminal \(Br^-\) \(Pb^{2+}\)
Setengah Persamaan \(2Br^- \rightarrow Br_2 + 2e^-\) \(Pb^{2+} + 2e^- \rightarrow Pb\)
Pemerhatian Gas perang dibebaskan Pepejal kelabu terbentuk
Nama Hasil Gas bromin Plumbum pepejal
Jenis Tindak Balas Pengoksidaan Penurunan
 
Faktor yang Mempengaruhi Hasil Elektrolisis Larutan Akues 
  • Nilai Eº yang disebutkan dalam jadual adalah nilai Eº dalam siri keupayaan elektrod piawai.
Faktor yang Mempengaruhi Hasil Elektrolisis Larutan Akues
Nilai Eº
Kepekatan larutan 
Jenis elektrod

 

Nilai Eº

 

Elektrod Ion yang Dipilih untuk Dinyahcaskan
Anod Anion dengan nilai E° yang lebih negatif atau kurang positif akan lebih mudah dinyahcaskan dan dioksidakan.
Katod Kation dengan nilai E° yang lebih positif atau kurang negatif akan lebih mudah dinyahcaskan dan diturunkan

 

Kepekatan Larutan

 

Elektrod Ion yang Dipilih untuk Dinyahcaskan
Anod
  • Faktor ini hanya dipertimbangkan untuk pemilihan ion pada anod sekiranya larutan akueus mengandungi ion halida. 
  • Ion halida yang mempunyai kepekatan yang lebih tinggi di dalam elektrolit akan dinyahcas pada anod walaupun nilai Eº ion halida lebih positif.
Katod Kation dengan nilai E° yang lebih positif atau kurang negatif akan lebih mudah dinyahcaskan dan diturunkan.

 

Jenis Elektrod

 

Elektrod Ion yang dipilih untuk dinyahcaskan
Anod
  • Untuk elektrod aktif (cth. kuprum dan argentum)
  • Tiada anion yang dinyahcaskan
  • Atom logam di anod membebaskan elektron untuk membentuk ion logam
Katod Kation dengan nilai E° yang lebih positif atau kurang negatif akan lebih mudah dinyahcaskan dan diturunkan.

 

 
Perbandingan antara Sel Elektrolitik dan Sel Kimia
Kategori Sel Elektrolitik Sel Kimia
Sumber Elektrik Elektrod disambungkan ke bateri atau sumber elektrik Elektrod tidak disambungkan ke bateri atau sumber elektrik
Elektrolit Kedua-dua elektrod dicelupkan ke dalam elektrolit
Jenis logam sebagai elektrod Biasanya elektrod karbon Logam berbeza jika dicelupkan dalam elektrolit yang sama
  Logam yang sama jika dicelupkan dalam elektrolit yang berbeza
Elektrod bercas negatif Katod Terminal positif (logam kurang elektropositif)
Elektrod bercas positif Anod Terminal negatif (logam lebih elektropositif)
Penukaran tenaga Tenaga elektrik → tenaga kimia enaga kimia → tenaga elektrik
Pemindahan elektron Anion melepaskan elektron di anod Atom pada terminal negatif membebaskan elektron
Kation menerima elektron di katod Ion dalam elektrolit menerima elektron
Pengoksidaan Pada anod Terminal negatif 
Penurunan Pada katod Terminal positif
 
Penyaduran Logam
  • Penyaduran logam secara elektrolisis dilakukan dengan objek yang hendak disadur dijadikan katod, logam penyadur dijadikan anod dan menggunakan larutan akueus yang mengandungi ion logam  penyadur.
  • Misalnya, penyaduran cincin besi dengan logam kuprum, Cu.
  •  Anod kuprum mengion menjadi ion kuprum(II), \(Cu^{2+}\).
    • Anod: \(Cu(p) \rightarrow Cu^{2+}(ak) + 2e^-\)
  • Ion kuprum(II), \(Cu^{2+}\) bergerak ke katod, dinyahcas dan terenap lalu membentuk lapisan nipis kuprum, Cu di atas cincin besi.
    • Katod: \(Cu^{2+}(ak) + 2e^- \rightarrow Cu(p) \)
  • Warna biru larutan kuprum(II) sulfat, \(CuSO_4\) tidak berubah kerana kepekatan ion kuprum(II), \(Cu^{2+}\) tidak berubah.
  • Kadar atom kuprum, Cu mengion pada anod adalah sama dengan kadar ion kuprum(II), \(Cu^{2+}\) dinyahcas pada katod.
 
 

 

 

 

Electrolyte and Non-electrolyte

Definition of electrolytes
Substances that can conduct electricity in either the molten state or aqueous solution and undergo chemical changes

 

Definition of non-electrolytes
Substances that cannot conduct electricity in all states

 

Classification of substances into electrolytes and non-electrolytes

 

Apparatus setup for electrolysis

 

Ion discharged at cation and anion in an electrolysis

 

Comparison between conductor and electrolyte

Conductor Electrolyte
Substances that conduct electricity in a solid or molten state, but do not undergo chemical changes.

Substances that conduct electricity in molten state or aqueous solution, and undergo chemical changes.

Substances that conduct electricity without undergoing decomposition. Substances that conduct electricity and undergo decomposition into their constituent elements.
Can conduct electricity due to the presence of free moving electrons. Can conduct electricity due to the presence of free moving ions.
Electrical conductivity decreases as temperature increases. Electrical conductivity increases as temperature increases.
Examples of conductors are metals and graphite.

Examples of electrolytes are ionic compounds, acids and alkalis.

 

Electrolysis of Molten Compounds

 
Definition of electrolysis
A process whereby compounds in the molten state or an aqueous solution decompose into their constituent elements by passing electricity through them.

 

Apparatus setup for electrolysis of molten lead(II) bromide

 

Ions present \(Pb^{2+},\, Br^-\)
Terminal Anode Cathode
Ions move to the terminal \(Br^-\) \(Pb^{2+}\)
Half equation \(2Br^- \rightarrow Br_2 + 2e^-\) \(Pb^{2+} + 2e^- \rightarrow Pb\)
Observation Brown gas released Grey solid deposited
Name of product Bromine gas Solid lead
Type of reaction Oxidation Reduction

 

Factors that Affect the Electrolysis of an Aqueous Solution 

  • The Eº value mentioned in the table is the Eº value in the standard electrode potential series. 

 

Factors affecting the electrolysis of an aqueous solution
E° value
Concentration of solution
Type of electrode

 

E° value

Electrode Ion chosen to be discharged
Anode Anion with a more negative or less positive E° value will be easier to be discharged and oxidised. 
Cathode Cation with a more positive or less negative E° value will be easier to be discharged and reduced. 

 

Concentration of solution

Electrode Ion chosen to be discharged
Anode
  • This factor is only considered for the selection of ions at the anode if the aqueous solution contains halide ions.
  • Halide ions with a higher concentration in the electrolytes will be discharged at the anode, even though the E° value of the halide ions are more positive.
Cathode Cation with a more positive or less negative E° value will be easier to be discharged.

 

Type of electrode

Electrode Ion chosen to be discharged
Anode
  • For active electrodes (e.g. copper and silver)
  • No anions are discharged
  • Metal atoms at the anode releases electrons to form metal ions
Cathode Cation with a more positive or less negative E° value will be easier to be discharged.

 

Comparison between an electrolytic cell and chemical cell

Category

Electrolytic cell

Chemical cell

Electric sources

Electrodes are connected to a battery or any electrical source

Electrodes are not connected to any electrical sources

Electrolyte

Both electrodes are dipped in the electrolyte

Type of metal for electrodes

Usually carbon electrodes

Different metals if being dipped in the same electrolyte

The same metal if being dipped in a different electrolyte

Negatively charged electrode

Cathode

Positive terminal (less electropositive metal)

Positively charged electrode

Anode

Negative terminal (more electropositive metal)

Energy conversion

Electric energy -> chemical energy

Chemical energy → electric energy

Electron transfer

Anion releases electrons at the anode

The atom at the negative terminal releases electrons

Cation receives electrons at the cathode

Ions in the electrolyte receive electrons

Oxidation

At anode

At the negative terminal

Reduction

At cathode

At the positive terminal

 

Electroplating and Purification of Metals

 

Electroplating of metals

  • Electroplating of metals through electrolysis is done by making the object being electroplated as the cathode, the electroplating metal as the anode, and an aqueous solution containing the ions of the electroplating metal as the electrolyte.
  • For example, to electroplate an iron ring with copper, Cu, the copper anode ionises to become copper(II) ions, \(Cu^{2+}\).
    • Anode: \(Cu(s) \rightarrow Cu^{2+}(aq) + 2e^-\)
  • Copper(II) ions, \(Cu^{2+}\) move to the cathode, are discharged and deposited as a thin layer of copper, Cu on the iron ring.
    • Cathode: \(Cu^{2+}(aq) + 2e^- \rightarrow Cu(s) \)
  • The blue colour of copper(II) sulphate, \(CuSO_4\) solution does not change because the concentration of copper(II) ions, \(Cu^{2+}\) remains the same.
  • The rate of ionisation of copper, Cu at the anode is the same as the rate of discharged copper(II) ions, \(Cu^{2+}\) at the cathode.
 

Purification of metal

  • Copper is an important mineral and element in our daily life.
  • It is an important industrial metal due to its ductility, malleability, electrical conductivity and resistance towards corrosion. 
  • Copper used in electrical wiring must have a 99.99% purity.
  • The purity of copper extracted by the process of melting is about 99.5%. 
  • Even a slight difference in copper purity will negatively impact its conductivity.
  • To determine whether a copper metal is pure, one must conduct the purification of metals through electrolysis.
  • The purification of copper by electrolysis is carried out with a piece of pure, thin copper as the cathode; impure copper as the anode; and an aqueous salt solution of copper, such as copper(II) nitrate, \(Cu(NO_3)_2\) as electrolyte.
  • Impure copper anode ionises to form copper(II) ions, \(Cu^{2+}\).
    • \(Cu(s) \rightarrow Cu^{2+}(aq) + 2e^-\)
  • Copper dissolves to become copper(II) ions, \(Cu^{2+}\) and impurities accumulate below the impure copper anode.
  • The anode becomes thinner.
  • At the pure copper cathode, copper(II) ions, \(Cu^{2+}\) are discharged to form copper atoms, Cu.
    • \(Cu^{2+}(aq) + 2e^- \rightarrow Cu(s) \)
  • Solid copper is deposited and the copper cathode becomes thicker.

Electrolytic Cell

 
1.4 Electrolytic Cell
 
Definition of Electrolyte
Substances that can conduct electricity in either the molten state or aqueous solution and undergo chemical changes.
 
Definition of Non-Electrolytes
Substances that cannot conduct electricity in all states.
 
Classification of Substances into Electrolytes and Non-Electrolytes
 
Apparatus Set Up for Electrolysis
 

 

 
Ion Discharged at Cation and Anion in An Electrolysis
Gambar menunjukkan KATION. 01. Katod (bercas negatif). 02. Penurunan. 03. Menerima elektron. 04. Penurunan nombor pengoksidaan. Pandai.
 

 

 

 

Gambar menunjukkan ANION. 01. Anod (bercas positif). 02. Pengoksidaan. 03. Melepaskan elektron. 04. Peningkatan nombor pengoksidaan.

 
Comparison between Conductor and Electrolytes

Conductor

Electrolyte

  • Substances that conduct electricity in a solid or molten state, but do not undergo chemical changes.
  • Substances that conduct electricity in molten state or aqueous solution, and undergo chemical changes.
  • Substances that conduct electricity without undergoing decomposition.
  • Substances that conduct electricity and undergo decomposition into their constituent elements.
  • Can conduct electricity due to the presence of free moving electrons.
  • Can conduct electricity due to the presence of free moving ions.
  • Electrical conductivity decreases as temperature increases.
  • Electrical conductivity increases as temperature increases.
  • Examples of conductors are metals and graphite.
  • Examples of electrolytes are ionic compounds, acids and alkalis.
 
Electrolysis of Molten Compounds
Definition of Electrolysis
  • A process whereby compounds in the molten state or an aqueous solution decompose into their constituent elements by passing electricity through them.
Apparatus Set Up for Electrolysis of Molten Lead(II) Bromide

Electrolysis of Molten Lead(II) Bromide

 

Ions yang Hadir \(Pb^{2+},\, Br^-\)
Terminal Anod Katod
Ion Bergerak ke Terminal \(Br^-\) \(Pb^{2+}\)
Setengah Persamaan \(2Br^- \rightarrow Br_2 + 2e^-\) \(Pb^{2+} + 2e^- \rightarrow Pb\)
Pemerhatian Gas perang dibebaskan Pepejal kelabu terbentuk
Nama Hasil Gas bromin Plumbum pepejal
Jenis Tindak Balas Pengoksidaan Penurunan
 
Faktor yang Mempengaruhi Hasil Elektrolisis Larutan Akues 
  • Nilai Eº yang disebutkan dalam jadual adalah nilai Eº dalam siri keupayaan elektrod piawai.
Faktor yang Mempengaruhi Hasil Elektrolisis Larutan Akues
Nilai Eº
Kepekatan larutan 
Jenis elektrod

 

Nilai Eº

 

Elektrod Ion yang Dipilih untuk Dinyahcaskan
Anod Anion dengan nilai E° yang lebih negatif atau kurang positif akan lebih mudah dinyahcaskan dan dioksidakan.
Katod Kation dengan nilai E° yang lebih positif atau kurang negatif akan lebih mudah dinyahcaskan dan diturunkan

 

Kepekatan Larutan

 

Elektrod Ion yang Dipilih untuk Dinyahcaskan
Anod
  • Faktor ini hanya dipertimbangkan untuk pemilihan ion pada anod sekiranya larutan akueus mengandungi ion halida. 
  • Ion halida yang mempunyai kepekatan yang lebih tinggi di dalam elektrolit akan dinyahcas pada anod walaupun nilai Eº ion halida lebih positif.
Katod Kation dengan nilai E° yang lebih positif atau kurang negatif akan lebih mudah dinyahcaskan dan diturunkan.

 

Jenis Elektrod

 

Elektrod Ion yang dipilih untuk dinyahcaskan
Anod
  • Untuk elektrod aktif (cth. kuprum dan argentum)
  • Tiada anion yang dinyahcaskan
  • Atom logam di anod membebaskan elektron untuk membentuk ion logam
Katod Kation dengan nilai E° yang lebih positif atau kurang negatif akan lebih mudah dinyahcaskan dan diturunkan.

 

 
Perbandingan antara Sel Elektrolitik dan Sel Kimia
Kategori Sel Elektrolitik Sel Kimia
Sumber Elektrik Elektrod disambungkan ke bateri atau sumber elektrik Elektrod tidak disambungkan ke bateri atau sumber elektrik
Elektrolit Kedua-dua elektrod dicelupkan ke dalam elektrolit
Jenis logam sebagai elektrod Biasanya elektrod karbon Logam berbeza jika dicelupkan dalam elektrolit yang sama
  Logam yang sama jika dicelupkan dalam elektrolit yang berbeza
Elektrod bercas negatif Katod Terminal positif (logam kurang elektropositif)
Elektrod bercas positif Anod Terminal negatif (logam lebih elektropositif)
Penukaran tenaga Tenaga elektrik → tenaga kimia enaga kimia → tenaga elektrik
Pemindahan elektron Anion melepaskan elektron di anod Atom pada terminal negatif membebaskan elektron
Kation menerima elektron di katod Ion dalam elektrolit menerima elektron
Pengoksidaan Pada anod Terminal negatif 
Penurunan Pada katod Terminal positif
 
Penyaduran Logam
  • Penyaduran logam secara elektrolisis dilakukan dengan objek yang hendak disadur dijadikan katod, logam penyadur dijadikan anod dan menggunakan larutan akueus yang mengandungi ion logam  penyadur.
  • Misalnya, penyaduran cincin besi dengan logam kuprum, Cu.
  •  Anod kuprum mengion menjadi ion kuprum(II), \(Cu^{2+}\).
    • Anod: \(Cu(p) \rightarrow Cu^{2+}(ak) + 2e^-\)
  • Ion kuprum(II), \(Cu^{2+}\) bergerak ke katod, dinyahcas dan terenap lalu membentuk lapisan nipis kuprum, Cu di atas cincin besi.
    • Katod: \(Cu^{2+}(ak) + 2e^- \rightarrow Cu(p) \)
  • Warna biru larutan kuprum(II) sulfat, \(CuSO_4\) tidak berubah kerana kepekatan ion kuprum(II), \(Cu^{2+}\) tidak berubah.
  • Kadar atom kuprum, Cu mengion pada anod adalah sama dengan kadar ion kuprum(II), \(Cu^{2+}\) dinyahcas pada katod.
 
 

 

 

 

Electrolyte and Non-electrolyte

Definition of electrolytes
Substances that can conduct electricity in either the molten state or aqueous solution and undergo chemical changes

 

Definition of non-electrolytes
Substances that cannot conduct electricity in all states

 

Classification of substances into electrolytes and non-electrolytes

 

Apparatus setup for electrolysis

 

Ion discharged at cation and anion in an electrolysis

 

Comparison between conductor and electrolyte

Conductor Electrolyte
Substances that conduct electricity in a solid or molten state, but do not undergo chemical changes.

Substances that conduct electricity in molten state or aqueous solution, and undergo chemical changes.

Substances that conduct electricity without undergoing decomposition. Substances that conduct electricity and undergo decomposition into their constituent elements.
Can conduct electricity due to the presence of free moving electrons. Can conduct electricity due to the presence of free moving ions.
Electrical conductivity decreases as temperature increases. Electrical conductivity increases as temperature increases.
Examples of conductors are metals and graphite.

Examples of electrolytes are ionic compounds, acids and alkalis.

 

Electrolysis of Molten Compounds

 
Definition of electrolysis
A process whereby compounds in the molten state or an aqueous solution decompose into their constituent elements by passing electricity through them.

 

Apparatus setup for electrolysis of molten lead(II) bromide

 

Ions present \(Pb^{2+},\, Br^-\)
Terminal Anode Cathode
Ions move to the terminal \(Br^-\) \(Pb^{2+}\)
Half equation \(2Br^- \rightarrow Br_2 + 2e^-\) \(Pb^{2+} + 2e^- \rightarrow Pb\)
Observation Brown gas released Grey solid deposited
Name of product Bromine gas Solid lead
Type of reaction Oxidation Reduction

 

Factors that Affect the Electrolysis of an Aqueous Solution 

  • The Eº value mentioned in the table is the Eº value in the standard electrode potential series. 

 

Factors affecting the electrolysis of an aqueous solution
E° value
Concentration of solution
Type of electrode

 

E° value

Electrode Ion chosen to be discharged
Anode Anion with a more negative or less positive E° value will be easier to be discharged and oxidised. 
Cathode Cation with a more positive or less negative E° value will be easier to be discharged and reduced. 

 

Concentration of solution

Electrode Ion chosen to be discharged
Anode
  • This factor is only considered for the selection of ions at the anode if the aqueous solution contains halide ions.
  • Halide ions with a higher concentration in the electrolytes will be discharged at the anode, even though the E° value of the halide ions are more positive.
Cathode Cation with a more positive or less negative E° value will be easier to be discharged.

 

Type of electrode

Electrode Ion chosen to be discharged
Anode
  • For active electrodes (e.g. copper and silver)
  • No anions are discharged
  • Metal atoms at the anode releases electrons to form metal ions
Cathode Cation with a more positive or less negative E° value will be easier to be discharged.

 

Comparison between an electrolytic cell and chemical cell

Category

Electrolytic cell

Chemical cell

Electric sources

Electrodes are connected to a battery or any electrical source

Electrodes are not connected to any electrical sources

Electrolyte

Both electrodes are dipped in the electrolyte

Type of metal for electrodes

Usually carbon electrodes

Different metals if being dipped in the same electrolyte

The same metal if being dipped in a different electrolyte

Negatively charged electrode

Cathode

Positive terminal (less electropositive metal)

Positively charged electrode

Anode

Negative terminal (more electropositive metal)

Energy conversion

Electric energy -> chemical energy

Chemical energy → electric energy

Electron transfer

Anion releases electrons at the anode

The atom at the negative terminal releases electrons

Cation receives electrons at the cathode

Ions in the electrolyte receive electrons

Oxidation

At anode

At the negative terminal

Reduction

At cathode

At the positive terminal

 

Electroplating and Purification of Metals

 

Electroplating of metals

  • Electroplating of metals through electrolysis is done by making the object being electroplated as the cathode, the electroplating metal as the anode, and an aqueous solution containing the ions of the electroplating metal as the electrolyte.
  • For example, to electroplate an iron ring with copper, Cu, the copper anode ionises to become copper(II) ions, \(Cu^{2+}\).
    • Anode: \(Cu(s) \rightarrow Cu^{2+}(aq) + 2e^-\)
  • Copper(II) ions, \(Cu^{2+}\) move to the cathode, are discharged and deposited as a thin layer of copper, Cu on the iron ring.
    • Cathode: \(Cu^{2+}(aq) + 2e^- \rightarrow Cu(s) \)
  • The blue colour of copper(II) sulphate, \(CuSO_4\) solution does not change because the concentration of copper(II) ions, \(Cu^{2+}\) remains the same.
  • The rate of ionisation of copper, Cu at the anode is the same as the rate of discharged copper(II) ions, \(Cu^{2+}\) at the cathode.
 

Purification of metal

  • Copper is an important mineral and element in our daily life.
  • It is an important industrial metal due to its ductility, malleability, electrical conductivity and resistance towards corrosion. 
  • Copper used in electrical wiring must have a 99.99% purity.
  • The purity of copper extracted by the process of melting is about 99.5%. 
  • Even a slight difference in copper purity will negatively impact its conductivity.
  • To determine whether a copper metal is pure, one must conduct the purification of metals through electrolysis.
  • The purification of copper by electrolysis is carried out with a piece of pure, thin copper as the cathode; impure copper as the anode; and an aqueous salt solution of copper, such as copper(II) nitrate, \(Cu(NO_3)_2\) as electrolyte.
  • Impure copper anode ionises to form copper(II) ions, \(Cu^{2+}\).
    • \(Cu(s) \rightarrow Cu^{2+}(aq) + 2e^-\)
  • Copper dissolves to become copper(II) ions, \(Cu^{2+}\) and impurities accumulate below the impure copper anode.
  • The anode becomes thinner.
  • At the pure copper cathode, copper(II) ions, \(Cu^{2+}\) are discharged to form copper atoms, Cu.
    • \(Cu^{2+}(aq) + 2e^- \rightarrow Cu(s) \)
  • Solid copper is deposited and the copper cathode becomes thicker.