# OXIDATION REDUCTION. Section I. Cl 2 + 2e. 2. The oxidation number of group II A is always (+) 2.

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1 OXIDATION REDUCTION Section I Example 1: Na Example 2: 2C1 Example 3: K + + e Na + + e Cl 2 + 2e K Example 4: C e 2Cl 1. The oxidation number of group I A is always (+) The oxidation number of group II A is always (+) The oxidation number of oxygen is 2 exception peroxides. In peroxides the oxidation number of oxygen is l. Example of peroxides: H 2 0 2, Na The oxidation number of hydrogen is +1 except in hydrides. In hydrides the oxidation number of H is 1. Example of hydrides: NaH, LiH. 5. The algebraic sum of the oxidation number of all atoms in a neutral compound is zero. 6. The oxidation number of elements in its free state is zero. 7. The oxidation number of the halogens is 1 when they combine with hydrogen or with a metal in a binary compound. We will discuss the ionelectron method first. The following steps are recommended. 1. Write the redox reaction as 2 halfcell reactions. A halfcell reaction representing oxidation and the other halfcell reaction representing reduction. 2. Balance each halfcell individually. 3. To balance oxygen, add H 2 0 molecules to the side where oxygen needs to be added. 4. To balance hydrogen add H + to the opposite side where water molecules were added. 5. The charge on each halfcell reaction is balanced by adding electrons to the appropriate side (usually on the same side where H + are added). 6. To balance the two halfcell reactions we need to make the number of e's lost equal number of e's gained by multiplying the two halfcell reactions by the appropriate number and adding: Example 5: Fe + C1 2 Fe +2 + C1 A: C1 2 C1 red. B: Fe Fe +2 ox.

2 Now we balance each halfcell reaction individually. C: 2e + Cl 2 2C1 D: Fe Fe e Note that halfcell reaction C gained 2e while halfcell reaction D lost 2e. Therefore, the number of e's lost is equal to the number of e's gained. To get the total reaction we simply add reactions C and D. C: 2e + Cl 2C1 D: Fe Fe e Fe + C1 2 Fe C1 Note: C1 2 is the oxidation agent; it oxidizad Fe from oxidation state of zero to +2. The Fe is the reducing agent; it reduced the oxidation number of chlorine from 0 to 1. Also, note that the reducing agent (Fe) was oxidized and the oxidizing agent (C1 2 ) was reduced. Example 6: MnO 4 + C1 Mn +2 + C1 2 A: C1 C1 2 ox. B: MnO 4 Mn +2 red. Balance each halfcell reaction. To balance A multiply C1 by 2; then add 2e to the right side of the reaction to obtain reaction C. C: 2C1 Cl 2 + 2e B: MnO 4 Mn +2 To balance oxygen in halfcell reaction B, add 4 moles of H 2 0 to the right side to obtain reaction D. D: MnO 4 Mn H 2 0 To balance the hydrogen add 8H + to the opposite side where H 2 0 molecules were added and obtain E. E: 8H + + MnO 4 Mn H 2 0 The net charge on the left side of the halfcell reaction E is +7 and on the right side is +2. To balance the charge add 5e to the left side of the reaction to obtain F. F: 5e + 8H + + MnO 4 Mn H 2 0 C: 2Cl C e Halfcell reaction F gained 5e, while halfcell reaction C lost 2e. To balance the gain and loss multiply halfcell reaction F by 2 and halfcell reaction C by 5 and add.

3 10e + 16H + + 2MnO 4 10Cl 2Mn H 2 O 5C e 16H + + 2MnO C1 2Mn H 2 O + 5C1 2 Example 7: Cr 2 O I Cr +3 + I 2 A: Cr 2 O 7 2 Cr +3 B: I I 2 C: Cr 2 O 7 2 D: Cr 2 O 7 2 2Cr +3 E: 14H + + Cr 2 O 7 2 The net charge on the left side of the halfcell reaction is +12 (14 2 = 12), while on the right side is +6 (2 x 3). To balance the charge add 6e to the left side and obtain Reaction F. F: 6e+ 14H + + Cr 2 O 7 2 G: 2I I 2 H: 2I I 2 + 2e To balance the gain and the loss of e s, multiply Reaction H by 3 obtaining Reaction I. Add I and F. I: 6I 3I 2 + 6e F: 6e + 14H + + Cr 2 O H + + Cr 2 O I + 3I 2 BALANCING REDOX REACTIONS BY CHANGE IN OXIDATION NUMBER METHOD: Example 8: Fe + C1 2 FeCl 2 Oxidation number of Fe = 0 Oxidation number of Cl 2 + O Oxidation number of Fe in FeCl 2 = +2 Oxidation number of C1 in FeCl 2 = 1

4 The summary is given by Reaction A: 0 gain of le/atom or a total of 2e Reaction A: Fe + C1 2 FeC1 2 0 loss of 2e +2 The loss of e's is equal to the gain of e's. Example 9: H 2 SO 4 + HI H 2 S + I 2 + H 2 O Establish the oxidation number of each atom in the equation. S in H 2 SO 4 = +6 I in HI = 1 S in H 2 S = 2 The summary is shown by Reaction B. 1 loss of le 0 Reaction B: H 2 SO 4 + 8HI H 2 S + 4I 2 + 4H 2 O +6 gain of 8 2 On the left side we have 8HI; therefore, multiply I 2 on the right side by 4 to balance the I. Left side 4 oxygen; therefore, multiply H 2 O on the right side by 4 to balance the oxygen. Example 10: KMnO 4 + HC1 MnCl 2 + KCl + H 2 O Oxidation number of C1 in HC1 is 1 Oxidation number of Mn in KMnO 4 is +7 Oxidation number of Mn in MnCl 2 is +2 Oxidation number of C1 2 is 0 The summary is shown by Reaction C Reaction C: 1 loss le 0 KMnO 4 + HC1 MnCl 2 + H 2 O + KC1 + C e gain of 5e

5 We multiply the HC1 by 5, but in this case we have to notice that the HC1 was partly used as a reducing agent and partly used to supply the Mn +2 and the K + with C1. Therefore, in addition to the 5 molecules we need to add 2 molecules of HC1 to supply 2C1 for the Mn +2 and one molecule of HC1 to supply one chloride ion for the K +. Adding these gives 8 HC1 as shown by Reaction D. Reaction D: KMnO 4 + 8HC1 MnCl 2 + 5/2C1 2 + KC1 + 4H 2 O The rest of the reaction is balanced by inspection. On the left side we have 4 oxygens; therefore, multiply the H 2 0 by 4. On the left side we have 8 chlorines; therefore, multiply the C1 2 by 5/2. We already have 3 other chlorine (MnCl 2 and KC1). To get rid of the fraction, multiply all compounds in reaction D by 2 to obtain Reaction E. Reaction E:2KMnO HC1 2MnCl 2 + 5C KCl + 8H 2 O Compare example 6 with example 10; it is seen that both of these examples are the same. Example 6: 16H + + 2MnO C1 Example 10: 2KMnO HC1 2Mn C H 2 O 2MnCl 2 + 5C H 2 O + 2KC1 The reason why we have 16H + and 10C1 in the ionelectron method is because we did not add the 4C1 for 2Mn +2 nor did we add the C1 attached to the K +. The equivalent weight of oxidizing or a reducing agent is equal to its molecular weight divided by the number of transferred e' s. Equivalent weight of KMnO 4 in example 10 = = 31.6 Equivalent weight of H 2 SO 4 in example 9 of 98 8 = So then to prepare 1N solution of KMnO 4 used in example 10, dissolve 31.6 g of KMnO 4 in 1 liter of solution.

6 Section II 1. Balance the following redox reactions using the ion electron method: a. Zn + NO 3 b. Zn + NO 3 c. Zn + NO 3 d. Fe 2+ + MnO 4 e. Ag + NO 3 Zn 2+ + NO 2 Zn 2+ + N 2 O Zn 2+ + N 2 Fe 3+ + Mn +2 Ag + + NO f. PbO 2 + C1 Pb 2+ + C1 2 g. Fe +2 + Cr 2 O 7 2 Fe 3+ + Cr 3+ h. Cr 2 O I Cr 3+ + I 2 i. A1 + H + A1 +3 +H 2 j. C1O 3 C1O 4 + C1O 2 2. Balance the following redox reactions using the change in oxidation number method. a. NH 3 + O 2 NO + H 2 O b. CuO + NH 3 N 2 + H 2 O + Cu c. H 2 S +H 2 O 2 S + H 2 O d. Sn + HNO 3 SnO 2 + NO 2 + H 2 O e. CuS + HNO 3 Cu(NO 3 ) 2 + S + NO + H 2 O f. KMnO 4 + HC1 KC1 + MnCl 2 + C1 2 + H 2 O g. Cu + HNO 3 Cu(NO 3 ) 2 + NO + H 2 O h. CdS + I 2 + HC1 CdCl 2 + HI + S i. HC1O 3 HC1O 4 + C1O 2 + H 2 O j. KBr + H 2 SO 4 K 2 SO 4 + Br 2 + SO 2 + H 2 O 3. Calculate the equivalent weight of: a. KMnO 4 in f of number 2 b. HNO 3 in e of number 2 c. HC1O 3 in i of number 2

7 4. Balance the following redox reactions using both the change in oxidation number method and the electron method. (a) Fe 2 O 3 + S Fe + SO 2 (b) NH 3 + O 2 NO + H 2 O (c) N 2 O + H 2 H 2 O + NH 3 (d) Cu + HNO 3 (e) FeS + HNO 3 Cu(NO 3 ) 2 + NO 2 + H 2 O Fe(NO 3 ) 3 + S + NO 2 + H 2 O (f) KMnO 4 + SnF 2 HF MnF 2 + SnF 4 + KF + H 2 O (g) Cr 2 O 3 + Na 2 CO 3 + KNO 3 Na 2 CrO 4 + CO 2 + KNO 2 (h) Na 2 SO 3 + H 2 SO 4 + KMnO 4 (i) KBrO 3 + Fe(NO 3 ) 2 + HNO 3 K 2 SO 4 + MnSO 4 + Na 2 SO 4 + H 2 O KBr + Fe(NO 3 ) 3 + H 2 O (j) HgS + HNO 3 + HCl HgCl 2 + NO + S + H 2 O

8 l. a. Zn Zn e ANSWERS [e + 2H + + NO 3 NO 2 + H 2 O] 2 2/ + Zn + 4H + + 2N03 Zn NO 2 + 2H 2 O + 2/e b. [Zn Zn e] 4 8e + l0h + + 2NO 3 N 2 O + 5H 2 O 4Zn + 8e + l0h + + 2NO 3 4Zn +2 + N 2 O + 5H 2 O + 8e c. [Zn Zn e] 5 l0e + 12H + + 2NO 3 N 2 + 6H 2 O 10e + 5Zn + 12H + + 2NO 3 d. [Fe +2 Fe +3 + e] 5 5Zn +2 + N 2 + 6H 2 O + 10e 5e + 8H + + MnO 4 Mn H 2 O 5e + 5Fe H + + MnO 4 5Fe +3 + Mn H 2 O + 5e e. [Ag Ag + + e] 3 3e + 4H + + NO 3 NO + 2H 2 O 3Ag + 3e + 4H + + NO 3 3Ag + + NO + 2H 2 O + 3e f. 2Cl Cl 2 + 2e 2e + 4H + + PbO 2 Pb H 2 O 2e + 2Cl + 4H + + PbO 2 Cl 2 + Pb H 2 O + 2e g. [Fe +2 Fe +3 + e] 6 6e + 14H + + Cr 2 O 7 = 6Fe e + 14H + + Cr 2 O 7 = 6Fe e h. [2I I 2 + 2e] 3 6e + 14H + + Cr 2 O 7 = 6e + 14H + + Cr 2 O 7 = + 6I + 3I 2 + 6e i. [A1 A e] 2 [2e + 2H + H 2 ] 3 2A1 + 6e + 6H + 2A e + 3H 2

9 j. [H 2 O + C1O 3 [e + 2H + + C1O 3 C1O 4 + 2H + + 2e] C1O 2 + H 2 O] 2 H 2 O + 3C1O 3 + 2e + 4H + C1O 4 + 2H + + 2C1O 2 + 2H 2 O + 2e 3C1O 3 + 2H + H 2 O + C1O 4 + 2C1O 2 0 gain 2e/at or 4e 2. a. 4NH 3 + 5O 2 4NO + 6H 2 O 3 loss of 5c(ox) +2 3 loss of 3e 0(ox) b. 3CuO + 2NH 3 N 2 + 3H 2 O + 3Cu gain of 2e (red) 0 2 loss of 2e 0(ox) c. H 2 S + H 2 O 2 S + 2H 2 O 1 gain of le/atom or 2 e +5 gain of le (red) +4 d. Sn + 4HNO 3 SnO 2 + 4NO 2 + 2H 2 O 0 loss of 4e (ox) gain of 3e (ox) +2 e. 3CuS + 6HNO 3 3Cu(NO 3 ) 2 + 3S + 2NO + 4H 2 O loss of 2e (ox) 0 8 " l loss of le (ox) 0 +3 f. KMnO 4 + 5HC1 KC1 + MnC C H 2 O +7 gain of 5e (red) +2 2 or 2KMnO HC1 2KC1 + 2MnCl 2 + 2MnCl 2 +2C H 2 O

10 +5 gain of 3e (red) +2 g. 3Cu + 8HNO 3 3Cu (NO 3 ) 2 + 2NO + 4H 2 O 0 loss of 2e (ox) gain of le/atom or 2 1 h. 2CdS + 2I 2 + 4HCl 2CdCl 2 + 4HI + 2S 2 2 loss of 2e (ox) 0 or CdS + I 2 + 2HC1 CdCl 2 + 2HI + S +5 gain of le (red) +4 i. 3HClO 3 HClO 4 + 2C1O 2 + H 2 O +5 loss of 2e (ox) gain of 2e (red) +4 j. 2KBr + 2H 2 SO 4 K 2 SO 4 + Br 2 + SO 2 + 2H 2 O 1 loss of le (ox) 0 3. a. Equivalent Weight of KMnO 4 = 158.1/5=31.62 b. Equivalent Weight of HNO 3 = 63/3 = 21 c. Equivalent Weight of HC1O 3 = 84.5/3 = 28.17

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