Ch. 5 Oxidation-Reduction Reactions Brady & Senese, 5 th Ed. 1
Index 5.1. Oxidation-reduction reactions involve electron transfer 5.2. The ion-electron method creates balanced net ionic equations for redox reactions 5.3. Metals are oxidized when they react with acids 5.4. A more active metal will displace a less active one from its compounds 5.5. Molecular oxygen is a powerful oxidizing agent 5.6. Redox reactions follow the same stoichiometric principles as other reactions 5.1. Oxidation-reduction reactions involve electron transfer 2
Oxidation Reduction Reactions Involve two processes: Oxidation the loss of electrons, and Reduction the gain of electrons Ca (s) + 2 H 2 O (l) Ca(OH) 2(s) + H 2(g) Oxidizer = Oxidizing agent = received the electrons Reducer = Reducing agent = donated the electrons 5.1. Oxidation-reduction reactions involve electron transfer 3
Guidelines For Redox Reactions Oxidation and reduction always occur together Total number of e - lost by one substance is the same as the total number of e - gained by the other For a redox reaction to occur, something must accept the e - that are lost by another substance 5.1. Oxidation-reduction reactions involve electron transfer 4
OXIDATION NUMBERS Oxidation numbers (also called oxidation states) are positive or negative numbers assigned to elements in chemical formulas according to a set of rules. The term oxidation number is abbreviated O.N. Rule 1: The O.N. of any uncombined element is 0. For example: Fe (0), Cl 2 (0), and Ca(0) Rule 2: The O.N. of a simple ion is equal to the charge on the ion. For example: Mg 2+ (+2), O 2- (-2), and Cl - (-1).
OXIDATION NUMBERS (continued) Rule 3: The O.N. of group IA and IIA elements when they are in compounds are always +1 and +2, respectively. For example: Na 2 S (Na = +1) and Ca(NO 3 ) 2 (Ca = +2) Rule 4: The O.N. of hydrogen is +1. For example: HBr (H = +1) and H 2 SO 4 (H = +1) Rule 5: The O.N. of oxygen is -2 except in peroxides, where it is -1. For example: MgO (O = -2), HBrO 3 (O = -2), and H 2 O 2 (O = -1) A closer look at H 2 O 2 : 2(O.N. of H) + 2(O.N. of O) = 0 2(+1) + 2(O.N. of O) = 0 2(O.N. of O) = -2 (O.N. of O) = -1 A closer look at MgO : (O.N. of Mg) + (O.N. of O) = 0 +2 + (O.N. of O) = 0 (O.N. of O) = -2
OXIDATION NUMBERS (continued) Rule 6: The algebraic sum of the oxidation numbers of all atoms in a complete compound equals zero. For example: MgSO 4 (O.N. of Mg = +2 by rule 2, O.N. of O = -2 by rule 5, and O.N. of S = +6 by algebra and rule 6) A closer look at MgSO 4 : (O.N. of Mg) + (O.N. of S) + 4(O.N. of O) = 0 +2 + (O.N. of S) + 4(-2) = 0 +2 + (O.N. of S) + -8 = 0 (O.N. of S) = +6 Because there is only one Mg and one S, the total positive oxidation number is +8. The four O atoms, with an O.N. of -2 each, give a total negative O.N. of -8. Thus, the total positive and the total negative O.N. values add up to zero.
OXIDATION NUMBERS (continued) Rule 7: The algebraic sum of the O.N. of all the atoms in a polyatomic ion is equal to the charge on the ion. For example: HCO 3 - (O.N. of H is +1 by rule 4, O.N. of O is -2 by rule 5, and O.N. of C is +4 by algebra and rule 7) A closer look at HCO - 3 : (O.N. of H) + (O.N. of C) + 3(O.N. of O) = -1 +1 + (O.N. of C) + 3(-2) = -1 +1 + (O.N. of C) + -6 = -1 (O.N. of S) = +4 Because there is only one H and one C, the total positive oxidation number is +5. The three O atoms, with an O.N. of -2 each, give a total negative O.N. of -6. Thus, the total positive and the total negative O.N. values add up to -1, which is the charge on the bicarbonate ion.
Assign Oxidation States To All Atoms: Fe 2 O 3 O: -2; Fe: +3 Na 2 CO 3 Na : +1; O: -2; C: +4 V(OH) 3 O: -2; H: +1; V: +3 K 2 Cr 2 O 7 K: +1; O: -2; Cr: +6 5.1. Oxidation-reduction reactions involve electron transfer 9
Your Turn! What is the oxidation number of F atoms in F 2 O? A. +1 B. +2 C. +3 D. +4 E. none of these 5.1. Oxidation-reduction reactions involve electron transfer 10
Your Turn! What is the oxidation number of P in H 3 PO 3? A. +1 B. +2 C. +3 D. +4 E. none of these 5.1. Oxidation-reduction reactions involve electron transfer 11
Your Turn! What is the oxidation number of O atoms in H 2 O 2? A. -1 B. -2 C. -3 D. -4 E. none of these 5.1. Oxidation-reduction reactions involve electron transfer 12
Problems: 5.3, 5.5, 5.7 5.25, 5.27, 5.29 5.1. Oxidation-reduction reactions involve electron transfer 13
Balancing Redox Reactions: Ion-Electron Method 1. Identify the half-reactions 2. Balance each atom in the half reaction, saving H and O for last 3. Balance O by adding 1 water molecule for each needed O 4. Balance H by adding 1 H + ion for each needed H 5.1. Oxidation-reduction reactions involve electron transfer 14
Balancing Redox Reactions: (Cont.) 5. Balance charges by adding electrons to the more positive side 6. Find the least common multiple of electrons for the two half- reactions. Multiply each reaction by the factor needed to achieve the LCM of electrons 7. Add the half reactions, canceling like substances that appear on both sides 5.1. Oxidation-reduction reactions involve electron transfer 15
Balancing Basic Reactions The simplest way to balance reactions in basic solution is to first balance them as if they were in acidic solution, then convert to basic solution Additional Steps for Basic Solutions 8) To both sides of the equation, add the same number of OH - ions as there are H +. 9) Combine H + and OH - to form H 2 O 10) Cancel H 2 O molecules that are on both sides of the reaction. 5.1. Oxidation-reduction reactions involve electron transfer 16
Balance, Using the Ion-Electron Method MnO 4 - + C 2 O 4 2- MnO 2 + CO 3 2-5.1. Oxidation-reduction reactions involve electron transfer 17
Balance, Using the Ion-Electron Method Mn (s) + Cu 2+ (aq) Cu (s) + MnO 2(s) (basic) 5.1. Oxidation-reduction reactions involve electron transfer 18
Balance, Using the Ion-Electron Method H 2 SO 3(aq) + H 2 CO 3(aq) H 2 C 2 O 4(aq) + SO 4 2- (aq) 5.1. Oxidation-reduction reactions involve electron transfer 19
Balance, Using the Ion-Electron Method ClO - + VO 3(aq) - ClO 3 - (aq) + V(OH) 3(s) (basic) 5.1. Oxidation-reduction reactions involve electron transfer 20
Your Turn! What is the coefficient indicated when the following is balanced? MnO 4 - (aq) + Cr (s) Cr 2 O 7 2- (aq) +? MnO 2(s) A. 1 B. 2 C. 3 D. 4 E. none of these 5.1. Oxidation-reduction reactions involve electron transfer 21
Your Turn! What is the coefficient indicated when the following is balanced?? PbSO 4(s) Pb (s) + PbO 2(s) + H 2 SO 4(aq) A. 1 B. 2 C. 3 D. 4 E. none of these 5.1. Oxidation-reduction reactions involve electron transfer 22
Problems: 5.9, 5.11 5.35, 5.37, 5.41 5.1. Oxidation-reduction reactions involve electron transfer 23
Reactions Of Metals In Non-oxidizing Acids Metals can be replaced in a compound by hydrogen ion when they react with acid In this case, H + is reduced so that the metal can be oxidized Only metals more active than H can undergo this single-replacement type reaction Cu (s) + H + NR Fe (s) + 2H + (aq) Fe2+ (aq) + H 2(g) 5.1. Oxidation-reduction reactions involve electron transfer 24
Oxidizing Acids Can React Most Metals HNO 3 : (conc) NO 3 - + 2H + (aq) + e- NO 2(g) + H 2 O (l) (dil) NO 3(aq) - + 4H + (aq) + 3e- NO (g) + 4 H 2 O (l) (v.dil): NO 3(aq) - + 10H + + 8e - NH 4(aq) + + 3H 2 O (l) H 2 SO 4 : (hot, conc.) SO 4 2- + 4H +(aq) + 3 e _ SO 2(g) + 2H 2 O (l) (hot, conc, with strong reducing agent) SO 4(aq) 2- + 10H + (aq) + 8e- H 2 S (g) + 4H 2 O (l) 5.1. Oxidation-reduction reactions involve electron transfer 25
Single Displacement (Replacement) Reaction A + BC AC + B Metal A can replace metal B if it is a more active metal, or Nonmetal A can replace nonmetal C if it is more active than C. An activity series arranges metals according to their ease of oxidation (Table 5.2) 5.1. Oxidation-reduction reactions involve electron transfer 26
Learning Check: Metal Activity Using the following observations, rank these metals from most reactive to least reactive Cu (s) + HCl (aq) no reaction Zn (s) + 2HCl (aq) ZnCl 2(aq) + H 2(g) Mg (s) + ZnCl 2(aq) MgCl 2(aq) + Zn (s) Mg > Zn > H > Cu 5.1. Oxidation-reduction reactions involve electron transfer 27
Activity Series 5.1. Oxidation-reduction reactions involve electron transfer 28
Learning Check: Predict the Products of the Following: Zn + CuSO 4 Cu (s) + ZnSO 4(aq) Cu + ZnSO 4 NR AgNO 3(aq) + Cu (s) Ag (s) + Cu(NO 3 ) 2 (aq) Solid magnesium and aqueous iron(iii) chloride Mg (s) + FeCl 3(aq) MgCl 2(aq) + Fe (s) Solid nickel and aqueous sodium chloride Ni s) + NaCl (aq) NR 5.1. Oxidation-reduction reactions involve electron transfer 29
Your Turn! Which of the following will react with Cu (s)? A. HNO 3 (conc) B. ZnCl 2(aq) C. AgNO 3(aq) D. None of these 5.1. Oxidation-reduction reactions involve electron transfer 30
Problems: 5.12, 5.5.15, 5.19 5.43, 5.47, 5.49, 5.51 5.1. Oxidation-reduction reactions involve electron transfer 31
Oxygen Reacts With Many Substances The products depend, in part, on how much oxygen is available Combustion of hydrocarbons O 2 plentiful; CH 4 + 2O 2 CO 2 + 2H 2 O O 2 limited: 2CH 4 + 3O 2 CO + 4H 2 O O 2 scant: CH 4 + O 2 C + 2H 2 O Organic compounds containing O also produce carbon dioxide and water C 2 H 5 OH + 3O 2 2CO 2 + 3H 2 O 5.1. Oxidation-reduction reactions involve electron transfer 32
Other Oxygen Reactions Organic compounds containing S produce sulfur dioxide 2C 2 H 5 SH + 9O 2 4CO 2 + 6H 2 O + 2 SO 2 Many metals corrode or tarnish when exposed to oxygen 4Fe + 3O 2 2Fe 2 O 3 4Ag + O 2 2Ag 2 O Most nonmetals react with oxygen directly Plentiful: C + O 2 CO 2 Limited: 2C + O 2 CO 5.1. Oxidation-reduction reactions involve electron transfer 33
Learning Check: Complete the Following Reactions Aluminum metal and oxygen gas forms aluminum oxide solid. 4Al (s) + 3O 2(g) 2Al 2 O 3(s) Solid sulfur (S 8 ) burns in oxygen gas to make gaseous sulfur trioxide S 8 (s) + 12 O 2(g) 8SO 3(g) Copper metal is heated in oxygen to form black copper(ii) oxide solid. 2Cu (s) + O 2(g) 2CuO (s) 5.1. Oxidation-reduction reactions involve electron transfer 34
Your Turn! What is the coefficient on O 2 when octane, C 8 H 18 is combusted with scant oxygen? A. 1 B. 2 C. 3 D. 4 E. none of these 2 C 8 H 18 + 25 O 2 16 CO 2 + 18 H 2 O 5.1. Oxidation-reduction reactions involve electron transfer 35
Problems: 5.21, 5.23 5.53, 5.57, 5.61 5.1. Oxidation-reduction reactions involve electron transfer 36
Your Turn! What is the coefficient on O 2 when iron combusts with plentiful oxygen available? A. 1 B. 2 C. 3 D. 4 E. none of these 5.1. Oxidation-reduction reactions involve electron transfer 37
Ore Analysis A 0.3000 g sample of tin ore was dissolved in acid solution converting all the tin to tin(ii). In a titration, 8.08 ml of 0.0500 M KMnO4 was required to oxidize the tin(ii) to tin (IV). What was the percentage tin in the original sample? 5.1. Oxidation-reduction reactions involve electron transfer 38
Your Turn! A 25.0 g sample of granite contains a vein of copper. What is the % of Cu present if 25.00 ml of concentrated 15 M HNO 3 are reacted? Cu (s) + 2NO 3 - (aq) + 4H + (aq) 2NO 2(g) + 2H 2 O (l) + Cu 2+ a) 12 b) 38 c) 48 d) 95 e) none of these 5.1. Oxidation-reduction reactions involve electron transfer 39
Problems: 5.65, 5.69, 5.71, 5.75, 5.77 5.1. Oxidation-reduction reactions involve electron transfer 40
Problems: 5.81, 5.89, 5.91, 5.95, 5.97 5.1. Oxidation-reduction reactions involve electron transfer 41