REDOX REACTIONS. Dear Reader

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1 REDOX REACTIONS Dear Reader You have studied about different types of reactions in your previous class. One of those reactions are oxidation-reduction reactions. In order to understand these reactions, let us first learn about the process of oxidation and reduction with the help of following examples. When charcoal burns in air, it form carbon monoxide and carbon dioxide depending upon the availability of oxygen. 2C(s) + O2(g) C(s) + O2(g) 2CO(g) CO(g) A piece of magnesium burns in air to form magnesium oxide 2Mg(s) + O2(g) 2MgO(s) In the above reactions, an element combines with oxygen to form an oxide. This process of combining an element with oxygen is known as oxidation. Reverse of this process, that is when a compound loses oxygen, is termed as reduction. Now look at the following reactions. F2(g) + H2(g) N2(g) + 3 H2(g) 2HF(l) 2NH3(g) We find that in these reactions hydrogen has added to fluorine or nitrogen. The processes of oxidation or reduction are also defined on the basis of reaction with hydrogen. A reaction in which hydrogen is added, is known as reduction and the process which involves the loss of hydrogen is called oxidation. Let s investigate the following reactions Fe2O3(s) + 2 Al (s) Al2O3(s) + 2 Fe(s) 2 NH3(g) + 3 Cl2(g) N2(g) + 6 HCl(g) In the first reaction, we find that ferric oxide has lost oxygen, hence it is reduced and that oxygen has been taken up by aluminium, so aluminium is oxidised. In the second 179

2 reaction ammonia has lost hydrogen, so it is oxidised and chlorine is reduced because it has taken that hydrogen. So we observe that in the above reactions, the process of oxidation and reduction is occurring simultaneously. Such reactions are termed as redox reactions. Oxidation and reduction in terms of electron transfer Let us look again at the reaction of magnesium with oxygen in a way as you have studied in bond formation. We split the reaction in two half reactions as given below. Mg Mg e O2 + 4 e 2 O 2- In the first half reaction, each magnesium atom loses two electrons to form an Mg 2+ ion and in the second half reaction,each O2 molecule gains four electrons to form two O 2- ions. These ions exist together as magnesium oxide. The complete reaction can also be written as given below 2 Mg + O2 2 [Mg 2+ ][O 2 ] As we have discussed, magnesium is said to be oxidised in this reaction because it has combined with oxygen but in this process it has lost electrons. So the process of oxidation can be defined in terms of electrons as the process involving loss of electrons. The reverse process where the electrons are gained is termed as reduction. In the above reaction, magnesium is oxidised and oxygen is reduced. The processes of oxidation and reduction always occur together because if one species loses the electrons,there must be another species to accept these electrons. It is impossible to have one without the other, as shown in the figure below. Another type of reactions involving displacement of metals, with which you are familiar, can also be classified as redox reactions. If a nail of iron is placed in a solution 180

3 of copper sulphate, metallic copper is deposited on the iron nail and some iron passes into the solution as Fe 2+ ions and forming ferrous sulphate. Fe(s) + CuSO4(aq) Cu(s) + FeSO4(aq) In terms of electrons, this reaction can be divided into oxidation and reduction halves. Fe(s) Fe 2+ (aq) + 2e (Oxidation) Cu 2+ (aq) + 2e Cu(s) (Reduction) The substance which accepts the electrons and gets reduced is known as oxidising agent because it has oxidised the other species. For example in the above reaction copper ions are oxidising agent. The species which loses electrons, gets oxidised in the process and reduces the other species, hence is called reducing agent. So in the above reaction Fe is a reducing agent. The relationship between oxidising and reducing agents can be clearly understood with the help of the following diagram. 181

4 The process of oxidation and reduction can also be looked into by using the concept of oxidation number which you will study in class XI. SUGGESTED VIDEO LINKS DO IT YOURSELF 1. Fill in the blanks with appropriate words: (i) The process of oxidation involves... of oxygen. (ii) The process involving gain of electrons is called... (iii) A substance that... electrons is known as oxidising agent. (iv) A reducing agent itself gets... during the reaction. (v) A... reaction can be written as two half reactions. 2. In the reaction: Fe(s) + CuSO4(aq) Cu(s) + FeSO4(aq) identify the species (i) (ii) which is oxidized. which is reduced. (iii) which acts as reducing agent. 3. In terms of electrons, oxidation involves: gain of electrons 182

5 (d) loss of electrons exchange of electrons conversion of a metal ion into the metal atom. 4. In a reaction between zinc and iodine, zinc iodide is formed. What is being oxidized? Zinc ions Iodide ions Zinc atom (d) Iodine 5. Conversion of PbSO4 into PbS is- (d) reduction reaction neutralization reaction oxidation reaction dissociation reaction 6. Which of the following is a redox reaction? (d) reaction between NaOH and H2SO4 O3 from O2 by lightening in atmosphere Nitrogen oxides from nitrogen and oxygen by lightening in atmosphere. Evaporation of water 7. The number of electrons lost or gained during the reaction: (d) 8 3Fe + 4 H2O Fe3O4 + 4H2 183

6 8. In the following equation: 3Br2 + 6CO H2O 5Br + BrO3 + 6HCO3 Bromine is oxidized and carbonate is reduced Bromine is reduced and carbonate is oxidized Bromine is neither reduced nor oxidized (d) Bromine is reduced as well as oxidized ANSWERS 1. (i) gain (ii) reduction (iii) loses (iv) oxidized (v) Redox 2. (i) Fe(s) (ii) Cu 2+ (aq) (iii) Fe(s) (d) 8. (d) 184