Voltaic Cell
Electrolyte
- Electrolytes are substances that can conduct electricity in molten or aqueous state and undergo chemical changes.
- It can conduct electricity due to the presence of free moving ions.
Example for electrolytes (alkalis, acids, salt solution or molten salt):
- molten lead(II) chloride
- copper(II) sulphate solution
- solution containing ions such as hydrochloride acid
Non-electrolyte
- Non-electrolytes are molecules that cannot conduct electricity and will not undergo any chemical changes.
- It cannot conduct electricity due to the absent of free moving ions.
Example of non-electrolytes (covalent substances):
- molten acetone
- molten naphthalene
- glucose solution
Conductor
- Conductors are substances that can conduct electricity in solid or molten state but do not undergo any chemical changes.
- It can conduct electricity due to the flow of electrons.
Example of conductor:
- iron
- graphite
- mercury
Ionic Compounds
| Solid state | Molten state or aqueous state (dissolved in water) |
| Do not conduct electricity | Can conduct electricity |
| Ions are held in a lattice | - |
| Ions do not move freely | Ions are free to move |
Covalent Compounds
| Solid state | Molten state or aqueous state (dissolved in water) |
| Do not conduct electricity | Do not conduct electricity |
| Exist in molecules | Exist in molecules |
| Molecules do not have free moving ions | Exception: HCl and NH3 exist as free moving ions in water |
Electrolysis of Molten Compounds
- Electrolysis (with battery / electricity current) is a process of decomposition / breaking down / separation of a compound (electrolyte) into its constituent elements when electric current passes through it.
Important definition
| Anode | Electrode connected to the positive terminal (+) of a battery |
| Cathode | Electrode connected to the negative terminal (-) of a battery |
| Anion | Negatively-charged ion. Example: Cl-, SO42- and O2- |
| Cation | Positively-charged ion. Example: Na+, Zn2+ and Al3+ |
Classification of electrodes
| Inert electrodes | Electrodes that do not take part in chemical reactions during electrolysis | Carbon or platinum |
| Active electrodes | Electrodes that take part in chemical reactions during electrolysis | Copper or zinc |
Example 1:
Molten magnesium oxide, MgO
- Ions: Magnesium ions (Mg2+) & oxide ions (O2-)
- Cathode (Negative electrode): Mg2+ move to the cathode
- Anode (Positive electrode): O2- move to the anode
- Electrons flow from anode to the cathode through the wire
- Can conduct electricity
Example 2:
Molten lead(II) bromide, PbBr2
- Ions: Lead(II) ions (Pb2+) & bromide ions (Br - )
- Cathode (Negative electrode): Pb2+ move to the cathode
- Anode (Positive electrode): Br - move to the anode
- Electrons flow from anode to the cathode through the wire
- Can conduct electricity
Example 3:
Molten naphthalene
- Ions: No ions present (naphthalene is covalent compound which consists of molecules = uncharged particles)
- No electrons flows
- Cannot conduct electricity
Electrolysis of Aqueous Compounds (dissolved in water, H2O)
There are three important factors to determine the types of ions to be discharged at the electrodes.
- Positions of ions in the electrochemical series
- Concentration of ions in the solution
- Types of electrodes used
1. Positions of ions in the electrochemical series
The lower the position of the ion in the electrochemical series, the easier the ion is selectivelydischarged.
Electrochemical series:
| Cation | Anion |
| K+ | F- |
| Na+ | SO42- |
| Ca2+ | NO3- |
| Mg2+ | Cl- |
| Al3+ | Br- |
| Zn2+ | I- |
| Fe2+ | OH- |
| Sn2+ | |
| Pb2+ | |
| H+ | |
| Cu2+ | |
| Hg+ | |
| Ag+ | |
| Au+ |
Example 1:
0.5 mol dm3 of potassium chloride, KCl solution
- Positive ions (cations): potassium ions (K+) and hydrogen ions (H+),
- Negative ions (anions): chloride ions (Cl-) and hydroxide ions (OH-)
- Cathode (negative electrode): H+ move to the cathode (ions are selectively discharge)
- Anode (positive electrode): OH- move to the anode (ions are selectively discharge)
Example 2:
0.1 mol dm3 of copper(II) sulphate, CuSO4 solution
- Positive ions (cations): copper ions (Cu2+) and hydrogen ions (H+),
- Negative ions (anions): sulphate ions (SO42-) and hydroxide ions (OH-)
- Cathode (negative electrode): Cu2+ move to the cathode (ions are selectively discharge)
- Anode (positive electrode): OH- move to the anode (ions are selectively discharge)
2. Effect of concentration of ions in the solution
The concentration of a particular type of ion is high = ion more likely to be discharged in electrolysis.
Example:
2.0 mol dm-3 of lead(II) chloride, PbCl2 solution
- Positive ions (cations): lead(II) ions (Pb2+) and hydrogen ions (H+),
- Negative ions (anions): chloride ions (Cl-) and hydroxide ions (OH-)
- Cathode (negative electrode): Pb2+ move to the cathode (ions are selectively discharge)
- Anode (positive electrode): Cl- move to the anode (ions are selectively discharge)
Nevertheless, if the two ions are placed very far apart in the electrochemical series, the concentration aspect becomes insignificant.
Example 1:
2.0 mol dm-3 of sodium bromide, NaBr solution
- Positive ions (cations): sodium ions (Na+) and hydrogen ions (H+),
- Negative ions (anions): bromide ions (Br-) and hydroxide ions (OH-)
- Cathode (negative electrode): H+ move to the cathode (ions are selectively discharge)
- Anode (positive electrode): Br - move to the anode (ions are selectively discharge)
Example 2:
2.0 mol dm-3 of lead(II) nitrate, Pb(NO3)2 solution
- Positive ions (cations): lead(II) ions (Pb2+) and hydrogen ions (H+),
- Negative ions (anions): chloride ions (NO3-) and hydroxide ions (OH-)
- Cathode (negative electrode): Pb2+ move to the cathode (ions are selectively discharge)
- Anode (positive electrode): OH - move to the anode (ions are selectively discharge)
3. Types of electrodes used in the electrolysis
There are 2 important notes:
- Inert electrodes: Carbon and platinum (Both of these electrodes do not react with the electrolytes or products of electrolysis)
- Active electrodes: Silver, copper and nickel (Active anode ionises and concentration of cations in the electrolyte does not change)
Example 1:
Carbon electrode (inert electrode) in the electrolysis of 0.1 mol dm-3 of aqueous copper(II) sulphate, CuSO4 solution
- Positive ions (cations): copper ions (Cu2+) and hydrogen ions (H+),
- Negative ions (anions): sulphate ions (SO42-) and hydroxide ions (OH-)
- Cathode (negative electrode): Cu2+ move to the cathode (ions are selectively discharge). A brown deposited on the cathode.
- Anode (positive electrode): OH- move to the anode (ions are selectively discharge). Gas bubbles are produced. A colourless gas liberates and it ignites the glowing wooden splinter.
- The intensity of the blue colour solution decreases because the concentration of copper(II) ions decreases.
Example 2:
Copper electrode (active electrode) in the electrolysis of 0.1 mol dm-3 of aqueous copper(II) sulphate, CuSO4 solution
- Positive ions (cations): copper ions (Cu2+) and hydrogen ions (H+),
- Negative ions (anions): sulphate ions (SO42-) and hydroxide ions (OH-)
- Anode (positive electrode): OH- and SO42- are not discharged. Copper anode dissolves by releasing electrons to form copper(II) ions, Cu2+. Copper anode becomes thinner.
- Cathode (negative electrode): Cu2+ move to the cathode (ions are selectively discharge) to form copper metal. Copper cathode becomes thicker.
- The intensity of the blue colour solution remains unchanged because the concentration of copper(II) ions remains the same.
Other examples:
3. Copper electrode (active electrode) in the electrolysis of 0.1 mol dm-3 of aqueous copper(II) nitrate, Cu(NO3)2 solution
4. Silver electrode (active electrode) in the electrolysis of 0.1 mol dm-3 of aqueous silver nitrate, AgNO3 solution
Predict the Products of Electrolysis of Aqueous Solutions
Main factor: Position of ions in the electrochemical series
- Cation: The higher the position in the electrochemical series are very stable (remain as cation). Example: K+ and Na+ are never discharged in an aqueous solution in electrolysis.
- Anions: The higher the position in the electrochemical series are very stable (remain as anion). Example: F - and SO42- are never discharged in an aqueous solution in electrolysis.
Second factor: Concentration of the electrolyte
Third factor: Types of electrode as anode
Electrolysis in Industries
A. Extraction of reactive metals
- Reactive metals: Sodium, calcium, magnesium and aluminium extract from their compounds
- Example: extraction of aluminium from aluminium oxide, Al2O3 (bauxite) by using cryolite, Na3AlF6 at 980˚C.
B. Purification of metals
- Anode: impure metal
- Cathode: pure metal
- Electrolyte: solution containing the ions of the metal to be purified
- Example: purification of impure copper metal.
C. Electroplating of metals
- Electroplating is a process of coating the surface of metal objects with a thin and even layer of another metal.
- Importance of electroplating is to prevent corrosion and improve the appearance.
- Cathode: object to be electroplated
- Anode: pure plating metal
- Electrolyte: aqueous solution contains plating metal ions
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