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Chapter 16 Krissy Kellock Analytical Chemistry 221.

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1 Chapter 16 Krissy Kellock Analytical Chemistry 221

2 Redox Reactions - -reaction in which electrons are transferred from one reactant to another – –Ce 4+ + Fe 2+ ↔ Ce 3+ + Fe 3+ Ce is the oxidizing agent or oxidant because it accepts electrons from iron and Fe is the reductant because it donated electrons to Ce. A redox reaction can be split into 2 half reactions Ce 4+ + e- ↔ Ce 3+ Fe 2+ ↔ Fe 3+ + e-

3 Redox Reactions The 2 half reactions must be balanced just like any other reaction. The number of atoms of each element and the net charge on each side of the equation must be in balance so for the oxidation of Fe2+ by MnO4- the half reactions would be: – –MnO H + + 5e- ↔ Mn H 2 O – –5Fe 2+ ↔ 5Fe e- The net charge for the first half reaction is (-1 – 5 + 8) = +2 which is the same on the right side. For the second half reaction it must by multiplied by 5 so that the number of electrons lost by Fe equals the number gained by MnO4. The balanced net equation for the reaction would be: – –MnO Fe H + ↔ Mn Fe H 2 O

4 Problem 16-7 a) 2Fe 3+ + Sn 2+  2Fe 2+ + Sn 4+ b) Cr + 3Ag +  Cr Ag c) 2NO Cu + 4H +  2NO 2 + 2H 2 O + Cu 2+ d) 2MnO H 2 SO 3  2Mn SO H + + 3H 2 0 e) Ti 3+ + Fe(CN) H 2 O  TiO 2+ + Fe(CN) H + f) H 2 O 2 + 2Ce 4+  O 2 + 2Ce H + g) 2Ag + 2I- + Sn 4+  2AgI + Sn 2+ h) UO Zn + 4H +  U 4+ + Zn H 2 O i) 5HNO 2 + 2MnO H +  5NO Mn H 2 O j) H 2 NNH 2 + IO H + + 2Cl  N 2 + ICl H 2 O

5 Redox Reactions in Electrochemical Cells Types- – –reaction is performed by direct contact between the oxidant and the reductant in a suitable container. – –The reactants do not come in contact with one another, salt bridge employed. b b - a salt bridge prevents the mixing of the contents of the 2 electrolyte solutions making up electrochemical cells

6 Source: /lecture_set10/lecture10.html

7 - an array consisting of 2 or 3 electrodes, each of which is in contact with an electrolyte solution. Typically, the electrolytes are in electrical contact through a salt bridge. An external metal conductor connects the electrodes. A cathode is an electrode where reduction occurs An anode is the electrode where oxidation occurs -Electrochemical Cells are either galvanic or electrolytic galvanic cells store and supply electrical energy (ex. Batteries) electrolytic cells require an external source of electrical energy for operation Electrochemical Cell

8 Source: /lecture_set10/lecture10.html

9 Source: /lecture_set10/lecture10.html

10 Electrode Potentials the potential difference that develops between the electrodes of the cell is a measure of the tendency for the reaction to proceed from a nonequilibrium state to the condition of equilibrium. Gibb’s Free Energy – –ΔG = -nFE cell

11 Electrode Potentials If the reactants and products are in their standard states the resulting cell potential is called the standard cell potential and is related to Gibb’s Free Energy Equation: b b ΔG˚ = -nFE˚ = -RT ln Keq

12 The Nerst Equation E = E˚ - ( / n) log [C] c [D] d [A] a [B] b

13 Problem E = – log 1/[Zn2+] [ZnY 2- ]= 3.2x10 16  [ZnY 2- ] [Y 4- ][Zn 2+ ][Y 4- ]3.2x10 16 E = – log [Y 4- ]3.2x10 16 [ZnY 2- ] when [Y 4- ] = [ZnY 2- ] = 1.00 E = E˚ ZnY2- b b E = E˚ ZnY2- = b b – log (1.00) (3.2x10 16 /1.00) = V


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