Stock notation: the oxidation number is expressed by putting a Roman numeral representing the oxidation number in parenthesis after the symbol of the metal in the molecular formula. Thus aurous chloride and auric chloride are written as Au( I) Cl and Au( III) Cl3. Similarly, stannous chloride and stannic chloride are written as Sn( II) Cl2 and Sn( IV) Cl4.
Oxidation: An increase in the oxidation number Reduction: A decrease in the oxidation number
Oxidizing agent: A reagent which can increase the oxidation number of an element in a given substance. These reagents are called as oxidants also. Reducing agent: A reagent which lowers the oxidation number of an element in a given substance. These reagents are also called as reductants.
Redox reactions: Reactions which involve change in oxidation number of the interacting species
Balancing of redox reactions: Oxidation Number Method:
Write the net ionic equation for the reaction of potassium dichromate( VI),
K2Cr2O7 with sodium sulphite, Na2SO3, in an acid solution to give chromium( III) ion and the sulphate ion.
Step 1: The skeletal ionic equation is: Cr2O7 2 –( aq) + SO3 2 –( aq) → Cr 3 +( aq)+ SO4 2 –( aq)
Step 2: Assign oxidation numbers forCr and S + 6 – 2 + 4 – 2 + 3 + 6 – 2
Cr2O7 2 –( aq) + SO3 2 –( aq) → Cr 3 +( aq)+ SO4 2 –( aq)
Step 3: Calculate the increase anddecrease of oxidation number, and make them equal:
+ 6 – 2 + 4 – 2 + 3 + 6
Cr2O7 2 –( aq) + 3SO3 2 –( aq) → 2Cr 3 +( aq)+ 3SO4 2 –( aq)
Step 4: Balance the charge by adding H + as the reaction occurs in theacidic medium,