Redox Reaction

 

Definition of redox reaction

  • ‘red’ from the reduction reaction and ‘ox’ from the oxidation reaction.
  • Redox reaction is the combination of oxidation and reduction reactions occurring simultaneously.
  • The oxidation reaction is the;
    • Addition of oxygen
    • Loss of hydrogen
    • Release of electron
    • Increase of oxidation number
  • The reduction reaction is the;
    • Loss of oxygen
    • Addition of hydrogen
    • Receiving of electron
    • Decrease of oxidation number

 

  • Oxidising agent: the substance that is reduced in the redox reaction.
  • Examples of oxidising agents are;
    • Acidified potassium manganate(VII) solution
    • Acidified potassium dichromate(VI) solution
    • Chlorine water
    • Bromine water
    • Iron(III) ion

 

  • Reducing agent: the substance that is oxidised in the redox reaction.
  • Examples of reducing agents are;
    • Chloride ion
    • Bromide ion
    • Iodide ion
    • Sulphate ion
    • Sulphur dioxide gas
    • Hydrogen sulphide gas

 

Oxidation number or oxidation state

  • 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;

 

Rule

Example

Oxidation number

The oxidation number of elements as atom or molecule is zero

Sodium, \(Na\)

0

Chlorine gas, \(Cl_2\)

0

The oxidation number of simple ions is the charge of the ion

Sodium ion, \(Na^+\)

+1

The total oxidation number of elements in a compound is zero

Ammonia, \(NH_3\)

0

 

The total oxidation number of elements in a polyatomic ion is the same as the charge of the polyatomic ion

Sulphate ion, \(SO_4\, ^{2-}\)

-2

The oxidation number of all elements in Group 1 is +1

Na in \(NaCl\)

+1

The oxidation number of all elements in Group 2 is +2

Mg in \(MgCl_2\)

+2

The oxidation number of all elements in Group 13 is +3

Al in \(Al_2O_3\)

+3

The oxidation number of all elements in Group 17 is -1

Cl in \(NaCl\)

-1

The oxidation number of oxygen in most of its compounds, except for hydrogen peroxide

O in \(Al_2O_3\)

-2

O in \(H_2O_2\)

-1

The oxidation number of hydrogen in most of its compounds, except for metal hydride

 

H in \(HCl\)

+1

H in \(NaH\)

-1

 

 

Steps in calculating the oxidation number of an element in a compound

 

Step 1

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

Step 2

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

Step 3

  • Write the mathematical equation for the total oxidation number.

Step 4

  • Complete the mathematical equation to determine the unknown number.

 

IUPAC naming system for compounds

  • 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.

 

Redox reaction in the displacement of metal from its salt solution

  • 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

Observation Inference
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
  • \(Zn(s) \rightarrow Zn^{2+} + 2e^-\)

Redox Reaction

 

Definition of redox reaction

  • ‘red’ from the reduction reaction and ‘ox’ from the oxidation reaction.
  • Redox reaction is the combination of oxidation and reduction reactions occurring simultaneously.
  • The oxidation reaction is the;
    • Addition of oxygen
    • Loss of hydrogen
    • Release of electron
    • Increase of oxidation number
  • The reduction reaction is the;
    • Loss of oxygen
    • Addition of hydrogen
    • Receiving of electron
    • Decrease of oxidation number

 

  • Oxidising agent: the substance that is reduced in the redox reaction.
  • Examples of oxidising agents are;
    • Acidified potassium manganate(VII) solution
    • Acidified potassium dichromate(VI) solution
    • Chlorine water
    • Bromine water
    • Iron(III) ion

 

  • Reducing agent: the substance that is oxidised in the redox reaction.
  • Examples of reducing agents are;
    • Chloride ion
    • Bromide ion
    • Iodide ion
    • Sulphate ion
    • Sulphur dioxide gas
    • Hydrogen sulphide gas

 

Oxidation number or oxidation state

  • 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;

 

Rule

Example

Oxidation number

The oxidation number of elements as atom or molecule is zero

Sodium, \(Na\)

0

Chlorine gas, \(Cl_2\)

0

The oxidation number of simple ions is the charge of the ion

Sodium ion, \(Na^+\)

+1

The total oxidation number of elements in a compound is zero

Ammonia, \(NH_3\)

0

 

The total oxidation number of elements in a polyatomic ion is the same as the charge of the polyatomic ion

Sulphate ion, \(SO_4\, ^{2-}\)

-2

The oxidation number of all elements in Group 1 is +1

Na in \(NaCl\)

+1

The oxidation number of all elements in Group 2 is +2

Mg in \(MgCl_2\)

+2

The oxidation number of all elements in Group 13 is +3

Al in \(Al_2O_3\)

+3

The oxidation number of all elements in Group 17 is -1

Cl in \(NaCl\)

-1

The oxidation number of oxygen in most of its compounds, except for hydrogen peroxide

O in \(Al_2O_3\)

-2

O in \(H_2O_2\)

-1

The oxidation number of hydrogen in most of its compounds, except for metal hydride

 

H in \(HCl\)

+1

H in \(NaH\)

-1

 

 

Steps in calculating the oxidation number of an element in a compound

 

Step 1

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

Step 2

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

Step 3

  • Write the mathematical equation for the total oxidation number.

Step 4

  • Complete the mathematical equation to determine the unknown number.

 

IUPAC naming system for compounds

  • 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.

 

Redox reaction in the displacement of metal from its salt solution

  • 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

Observation Inference
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
  • \(Zn(s) \rightarrow Zn^{2+} + 2e^-\)