• ‘red’ from the reduction reaction and ‘ox’ from the oxidation reaction.

• Redox reaction is the combination of oxidation and reduction reactions occurring simultaneously.
• Initially, a redox reaction refers to a reaction involving oxygen.

• Acidified potassium manganate(VII) solution.
• Acidified potassium dichromate(VI) solution.
• Chlorine water.
• Bromine water.
• Iron(III) ion.

• Chloride ion.
• Bromide ion.
• Iodide ion.
• Sulphate ion.
• Sulphur dioxide gas.
• Hydrogen sulphide gas.

• The oxidation number of an element is the charge of an element if the element exists as an ion.
• Also known as oxidation state.
• The rules in assigning the oxidation number of an element are as follows;

 Natrium, \(Na\)

 Gas klorin, \(Cl_2\)

 Ion natrium, \(Na^+\)

 Ammonia, \(NH_3\)

 Ion sulfat, \(SO_4\, ^{2-}\)

 Na dalam \(NaCl\)

 Mg dalam \(MgCl_2\)

 Al dalam \(Al_2O_3\)

 Cl dalam \(NaCl\)

 O dalam \(Al_2O_3\)

 O dalam \(H_2O_2\)

 H dalam \(HCl\)

 H dalam \(NaH\)

• Write the oxidation number of each element according to the rules.
• Use the symbol X to represent the unknown number.

• Multiply the oxidation number with the subscript of each element based on the molecular formula.

• Write the mathematical equation for the total oxidation number.

• Complete the mathematical equation to determine the unknown number.

• An ion can have a few oxidation numbers.
• For such ions, the oxidation number is written in the name of the compound to avoid confusion.
• If the oxidation number of an element is one, there is no need to write the oxidation number in the name of the atom or compound.

• A more reactive metal displaces the metal or cation in a salt solution.
• This reaction is a redox reaction.
• The oxidation number of the cation in the salt solution is reduced, and the oxidation number of the more reactive metal is increased.

Example: Addition of Zinc Plate into Copper(II) Nitrate, \(CuNO_3\) Solution

• The intensity of blue colour of copper(II) nitrate solution decreases

• The concentration of copper(II) ion, \(Cu^{2+}\) decreases as the ion is reduced into solid

• Brown solid deposited

• Copper(II) ion is reduced 
• \(Cu^{2+} (aq)+2e^- \rightarrow Cu(s)\)

• The zinc plate becomes thinner

• The zinc metal is oxidised as it releases electrons to form zinc ion
• \(Zn(s) \rightarrow Zn^{2+} + 2e^-\)

• ‘red’ from the reduction reaction and ‘ox’ from the oxidation reaction.

• Redox reaction is the combination of oxidation and reduction reactions occurring simultaneously.
• Initially, a redox reaction refers to a reaction involving oxygen.

• Acidified potassium manganate(VII) solution.
• Acidified potassium dichromate(VI) solution.
• Chlorine water.
• Bromine water.
• Iron(III) ion.

• Chloride ion.
• Bromide ion.
• Iodide ion.
• Sulphate ion.
• Sulphur dioxide gas.
• Hydrogen sulphide gas.

• The oxidation number of an element is the charge of an element if the element exists as an ion.
• Also known as oxidation state.
• The rules in assigning the oxidation number of an element are as follows;

 Natrium, \(Na\)

 Gas klorin, \(Cl_2\)

 Ion natrium, \(Na^+\)

 Ammonia, \(NH_3\)

 Ion sulfat, \(SO_4\, ^{2-}\)

 Na dalam \(NaCl\)

 Mg dalam \(MgCl_2\)

 Al dalam \(Al_2O_3\)

 Cl dalam \(NaCl\)

 O dalam \(Al_2O_3\)

 O dalam \(H_2O_2\)

 H dalam \(HCl\)

 H dalam \(NaH\)

• Write the oxidation number of each element according to the rules.
• Use the symbol X to represent the unknown number.

• Multiply the oxidation number with the subscript of each element based on the molecular formula.

• Write the mathematical equation for the total oxidation number.

• Complete the mathematical equation to determine the unknown number.

• An ion can have a few oxidation numbers.
• For such ions, the oxidation number is written in the name of the compound to avoid confusion.
• If the oxidation number of an element is one, there is no need to write the oxidation number in the name of the atom or compound.

• A more reactive metal displaces the metal or cation in a salt solution.
• This reaction is a redox reaction.
• The oxidation number of the cation in the salt solution is reduced, and the oxidation number of the more reactive metal is increased.

Example: Addition of Zinc Plate into Copper(II) Nitrate, \(CuNO_3\) Solution

• The intensity of blue colour of copper(II) nitrate solution decreases

• The concentration of copper(II) ion, \(Cu^{2+}\) decreases as the ion is reduced into solid

• Brown solid deposited

• Copper(II) ion is reduced 
• \(Cu^{2+} (aq)+2e^- \rightarrow Cu(s)\)

• The zinc plate becomes thinner

• The zinc metal is oxidised as it releases electrons to form zinc ion
• \(Zn(s) \rightarrow Zn^{2+} + 2e^-\)