OXIDATION
REDUCTION
REDOX REACTION IN TERMS OF ELECTRON TRANSFER REACTION
- A few examples of redox reaction on the basis of electronic concept are given below:
- According to the electronic concept every redox reaction consists of two steps known as half reactions.
(i) Oxidation reaction: Half reactions that involve loss of electrons are called oxidation reactions.
(ii) Reduction reaction: Half reactions that involve gain of electrons are called reduction reactions. - Oxidizing agent: Acceptor of electrons.
- Reducing agent: Donar of electrons.
OXIDATION NUMBER
Rules for Assigning Oxidation Numbers
(i) The oxidation number of an element in its elementary form is zero.
For example, H2, 02, N2 etc. have oxidation number equal to zero.
(ii) In a single monatomic ion, the oxidation number is equal to the charge on the ion. For example, Na+ ions have an oxidation number of +1 and Mg2+ ions have +2.
(iii) Oxygen has oxidation number -2 in its compounds.
(iv) In non-metallic compounds of hydrogen like HCl, H2S, H2O oxidation number of hydrogen = + 1 but in metal hydrides oxidation number of hydrogen = -1 [LiH, NaH, CaH2 etc.]
(v) In compounds of metals and non-metals metals have positive oxidation number while non-metals have negative oxidation number. For example, In NaCl. Na has +1 oxidation number while chlorine has -1.
(vi) If in a compound there are two non-metallic atoms the atoms with high electronegativity are assigned negative oxidation number while other atoms have positive oxidation number.
(vii) The algebraic sum of the oxidation number of all atoms in a compound is equal to zero.
(viii) In polyatomic ion the sum of the oxidation no. of all the atoms in the ion is equal to the net charge on the ion.
For example, in (C03)2—Sum of carbon atoms and three oxygen atoms is equal to -2.
Fluorine (F2) is so highly reactive non-metal that it displaces oxygen from water.
DISPROPORTIONATION REACTION
FRACTIONAL OXIDATION NUMBERS
- Elements as such do not have any fractional oxidation numbers.
- When the same elements are involved in different bonding in a species, their actual oxidation states are whole numbers but an average of these is fractional. For example, In C302
BALANCING OF REDOX REACTIONS
Oxidation Number Method:
- Following steps are involved:
(a) Write the correct formula for each reactant and product.
(b) By assigning the oxidation change in oxidation number can be identified.
(c) Calculate the increase and decrease in oxidation number per atom with respect to the reactants. If more than one atom is present then multiply by suitable coefficient.
(d) Balance the equation with respect to all atoms. Balance hydrogen and oxygen atoms also.
(e) If the reaction is carried out in an acidic medium, use H+ ions in the equation. If it is in basic medium use OH– ions.
(f) Hydrogen atoms in the expression can be balanced by adding (H20) molecules to the reactants or products. - If there are the same number of oxygen atoms on both sides of the equation then it represents the balanced redox reaction.
Half Reaction Method:
REDOX REACTIONS AS THE BASIS FOR TITRATION
Potassium Permanganate Titration:
- In these titrations potassium permanganate (pink in colour) acts as an oxidising agent in the acidic medium while oxalic acid or some ferrous salts acts as a reducing agent.
- The ionic equation can be written as:
- These are the examples of redox titration.
- On both these titrations, potassium permanganate itself acts as indicator.
- It is commonly known as a self indicator.
- The appearance of pink colour in the solution represents the end points.
Potassium Dichromate Titration :
- In place of potassium permanganate, potassium dichromate can also be used in the presence of dil. H2S04.
- The ionic equation for the redox reaction with FeS04 (Fe2+ ions) is given.
