Chapter 19 Notes

Chapter 19 Notes - Chapter 19 Electrochemistry & its...

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Unformatted text preview: Chapter 19 Electrochemistry & its Applications Electrochemistry Electrochemistry is the study and use of e- flow in chemical reactions. Redox reactions generate (and use) e- Those e- can be harnessed (batteries). Corrosion is an electrochemical reaction. Applied e- flow: Can drive reactant-favored reactions toward products. Rechargeable batteries, electrolysis and electroplating. Oxidation Number Refresher Pure element = 0. Monatomic ion = charge of ion. (ox. numbers in a species) = overall charge. Element ox. no. Exceptions? F 1 None Cl, Br, I 1 Interhalogens H +1 Metal hydrides = -1 O 2 Metal peroxides = -1 Halogen oxides Redox Reactions Oxidation & reduction (Redox) always occur together. 2 HCl(aq) + Mg(s) H 2 (g) + MgCl 2 (aq) Reduction = gain of e- = decrease in ox. no. Oxidation = loss of e- = increase in ox. no. +2 e--2 e- +1 -1 +2 -1 Redox Reactions H + is reduced, Mg is oxidized. Give oxidation numbers for each atom. Identify the oxidizing and reducing agents: 6 Fe 2+ + Cr 2 O 7 2- + 14 H 3 O + 6 Fe 3+ + 2 Cr 3+ + 21 H 2 O Species Ox. number Explanation Fe 2+ +2 charge on ion Cr 2 O 7 2- O = -2; Cr = +6 O is usually -2; 2(Cr) + 7(-2) = -2 H 3 O + O = -2; H = +1 O is usually -2; H is usually +1 Fe 3+ +3 charge on ion Cr 3+ +3 charge on ion H 2 O O = -2; H = +1 O is usually -2, H is usually +1 Fe 2+ Fe 3+ oxidation Cr(+6) Cr 3+ reduction Fe 2+ = reducing agent Cr 2 O 7 2- = oxidizing agent Redox Reactions must be +6 Redox reactions split into half reactions: Using Half-Reactions Half-reactions may include different numbers of e- : Al(s) Al 3+ (aq) + 3 e- Zn 2+ (aq) + 2 e- Zn(s) e- must balance in the full reaction. 2[ Al(s) Al 3+ (aq) + 3 e- ] 3[ Zn 2+ (aq) + 2 e- Zn(s) ] 2 Al(s) + 3 Zn 2+ (aq) 2 Al 3+ (aq) + 3 Zn(s) Using Half-Reactions Redox in acidic or basic solutions are harder (H 2 O, H 3 O + or OH- are often omitted). Balance: H 3 AsO 4 + I 2 HAsO 2 + IO 3- which occurs in aqueous acidic solution. Balancing Redox Equations H 3 AsO 4 + I 2 HAsO 2 + IO 3- (acidic solution) (i) What is oxidized? Reduced? I 2 (I = 0) IO 3- (I = +5) oxidation H 3 AsO 4 (As = +5) HAsO 2 (As = +3) reduction (iii) Balance atoms (except H and O). H 3 AsO 4 HAsO 2 I 2 2 IO 3- (ii) Write unbalanced half-reactions: H 3 AsO 4 HAsO 2 I 2 IO 3- Balancing Equations in Acidic Solution (iv) Balance O (add H 2 O as needed). H 3 AsO 4 HAsO 2 + 2 H 2 O I 2 + 6 H 2 O 2 IO 3- (vi) Balance charges (add e- ). H 3 AsO 4 + 2 H + + 2 e- HAsO 2 +2 H 2 O I 2 + 6 H 2 O 2 IO 3- + 12 H + + 10 e- zero charge 2(-1) + 12(+1)} 10(-1) 0 = 10 + -10 (v) Balance H (add H + as needed). H 3 AsO 4 + 2 H + HAsO 2 +2 H 2 O I 2 + 6 H 2 O 2 IO 3- + 12 H + Balancing Equations in Acidic Solution (viii) Make H 3 O + (H 2 O + H + ). Add H 2 O if needed....
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This note was uploaded on 01/27/2012 for the course CH 1213 taught by Professor Xia during the Spring '07 term at Mississippi State.

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Chapter 19 Notes - Chapter 19 Electrochemistry & its...

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