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 ,