Chemistry Form 5: Chapter 3 - Redox Reaction in Displacement of Metal

Reactivity Series of metals

Displacement of metals from solution is a redox reaction whereby a less reactive metal ion is displaced from its salt solution by a more reactive metal. As a result, the less reactive metal ion is deposited as a solid metal while the more reactive metal dissolves in the solution. The general formula for a displacement reaction is: M (s) + Xn+ (aq)  ----->   Mn+ (aq) + X (s) where metal M is the more reactive than metal X.

Most reactive K Na Ca Mg Al Zn Fe Pb [H] Cu Ag Least reactive

Example of displacement reaction:

Fe (s) + CuSO₄ (aq)     ----->     FeSO₄ (aq) + Cu (s)

Iron displaces copper from the solution because it is more reactive than copper metal. Iron, being more reactive, loses its electrons readily. The electrons are transferred from the iron atoms to the copper(II) ions in the solution. Copper(II) ions are reduced to copper metal and iron atoms become oxidised to iron (II) ions. Iron acts as reducing agent whereas copper (II) ions act as a oxidising agent. 

Oxidation reaction:     Fe (s)     ----->     Fe²⁺ (aq) + 2e^-
Reduction reaction:    Cu²⁺ (aq) + 2e^-     ----->     Cu (s)
Overall Redox reaction:   Fe (s) + Cu²⁺ (aq)     ----->   Fe²⁺ (aq) + Cu (s)

Click on the diagram below to play!

Chemistry Form 4: Chapter 6 - Daniell Cell

Daniell cell is also known as Galvanic cell which using same working principle as voltaic cell. It converts chemical energy to electrical energy. Daniell cell is a part of a battery consisting of two metals joined by a salt bridge or a porous pot.

• More electropositive metal acts as negative terminal which undergoes oxidation process (release electron)
• Less electropositive metal acts as positive terminal which undergoes reduction (gain electron).
• Electron flow from negative terminal to positive terminal.
• The function of salt bridge or porous pot is to allow the movement of ions to pass through it to complete electrical circuit. Another function is to separate two electrolyte solutions. 
• Examples of salt bridge are sodium chloride or potassium nitrate.

At Negative Terminal: More electropositive metal ( Zinc )

Observation:  Zinc plate dissolves or becomes thinner, its mass decreases

                        

Half chemical equation :     Zn  ------>     Zn²⁺   +    2e

At Positive Terminal: Less electropositive metal ( Copper )

                                               

Observation:  Copper plate becomes thicker

                        

Half chemical equation :     Cu²⁺     +    2e   ------>    Cu

Click on the diagram below to play!

Physics Form 4: Chapter 3 - Hydraulic Jack

 

Click on the diagram below to play!

Chemistry Form 4: Chapter 6 - Extraction of Aluminium From its Ore

The ores of aluminium is called bauxite. Bauxite is a composition of aluminium oxide, (Al₂O₃).

The position of aluminium in the reactivity series indicates that, it could not be extracted by reducing the oxide using carbon or carbon monoxide. Electrolysis of molten aluminium oxide ("alumina") will, however, give aluminium and oxygen.

A = carbon anode       B = molten aluminium    

C = molten aluminium oxide and cryolite     D = carbon cathode  

• The function of cryolite is to lower the melting point of aluminium oxide.
• Aluminium metal forms at the cathode electrode and sinks to the bottom of the tank, where it is tapped off.
• Oxygen forms at the anode electrodes. This oxygen reacts with the carbon of the anode electrodes, forming carbon dioxide, and they gradually burn away. Consequently, the anode electrodes have to be replaced frequently.

Physics Form 5: Chapter 4 - Thermionic Emission

• Thermionic emission is the release of electrons from a heated metal.
• The electrons in the metal gain kinetic energy from heat. Electrons that gain sufficiently high kinetic energy will be able to escape from the surface of metal.

• The tungsten filament act as cathode
• Cathode ray is a beam of electrons moving at high speed in vacuum.

Physics Form 4: Chapter 3 - Pascal Principle

• Liquid cannot be compressed, thus its pressure only can be transmitted to all direction with equal magnitude.
• Pascal principle states that the pressure in a liquid is transmitted equally to all direction throughout enclosed liquid.

Pressure at small piston = Pressure at large piston

Chemistry Form 5: Chapter 3 - Rusting of Iron

• Rusting is a corrosion of iron.
• For iron to rust, oxygen and water must be present.
• In the presence of acids and salts, rusting occurs faster because these substances increase the electrical conductivity of water, making water a better electrolyte. 
• Oxygen acts as the oxidizing agent and iron acts as the reducing agent.

 

·     The surface of iron at the middle of the water droplet serves as the anode at which oxidation occurs. The iron atoms lose electrons to form iron (II) ions.

·     The electrons flow to the edge of the water droplet where there is plenty of dissolved oxygen. The iron surface there serves as cathode at which reduction occurs. Oxygen gains the electrons and is reduced to hydroxide ions.

·     The iron (II) ions produced combine with the hydroxide ions to iron (II) hydroxide. The Fe(OH)₂ is then further oxidized by oxygen to form iron (III) oxide, Fe₂O₃ known as rust

 

Chemistry Form 4: Chapter 6 - Purification of Metal

• The anode is a impure copper.
• The cathode is a pure copper.
• When electricity is passed through the cell, the copper is dissolved at anode where Cu²⁺ ions go into solution. Anode copper becomes thinner.

• At cathode, copper ions are selectively discharge and deposited. Cathode copper becomes thicker.

The blue intensity of solution remains unchanged because concentration of copper (II) ions in the solution remains unchanged. The rate at which copper enter into electrolyte from anode is equal to the rate at which the copper ions discharged at cathode.