ELECTROCHEMISTRY - Wits Structural Chemistry

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4/1/2010 Preventing the corrosion of iron Covering of iron with e.g. paint or tin - Prevents O 2 and H 2 O from reaching surface. Galvanised iron coated with layer of zinc - Protects Fe by electrochemical means even after coating is damaged Easier to oxidise Zn (anode) than Fe (cathode) Zn rather corrodes - Coating damaged corrosion Cathodic protection Zn = sacrificial anode E o red/V Reduction Half-reaction +0.77 Fe 3+ (aq) + e - Fe 2+ (aq) Cathodic protection is used for e.g. the protection of iron pipelines or storage tanks -0.44 Fe 2+ (aq) + 2e - Fe(s) -0.76 Zn 2+ (aq) + 2e - Zn(s) -2.36 Mg 2+ (aq) + 2e - Mg(s) Mg is the sacrificial anode for the Fe tank. ELECTROLYSIS Electrolysis of molten NaCl Use electrical energy to cause non-spontaneous redox reaction to occur - Takes place in electrolytic cells - Need an external DC source e.g. a battery = electron pump Reduction occurs at the cathode (negative) and oxidation occurs at the anode (positive) 12
4/1/2010 Electrolysis of aqueous solutions Example: Electrolysis of aqueous NaF solution Need high temp’s to produce molten salts. Can also produce ions by dissolving salts in water at room temp. 2 possible cathode reactions: Na + (aq) + e - Na(s) 2H 2 O(l) + 2e - H 2 (g) + 2OH - (aq) E o red = -2.71 V E o red = -0.83 V Problem: Electrolysis of aqueous salts complicated by the presence of water. Favoured reduction reaction more +ve or less –ve E o red Cathode: 2H 2 O(l) + 2e - H 2 (g) + 2OH - (aq) 2 possible anode reactions: 2F - (aq) F 2 (g) + 2e - 2H 2 O(l) O 2 (g) + 4H + (aq) + 4e - Favoured oxidation reaction more -ve or less +ve E o red 2H 2 O(l) O 2 (g) + 4H + (aq) + 4e - E o red = +2.78 V E o red = +1.23 V In fact it is easier still to oxidise OH - (aq) that is produced at the cathode: Anode: 4OH - (aq) O 2 (g) + 2H 2 O(l) + 4e - E o red = +0.40 V Example: Electrolysis of aqueous NaCl solution Cathode reactions as in previous example. 2 possible anode reactions: 2Cl - (aq) Cl 2 (g) + 2e - 2H 2 O(l) O 2 (g) + 4H + (aq) + 4e - Expect favoured reaction to be: 2H 2 O(l) O 2 (g) + 4H + (aq) + 4e - E o red = +1.36 V E o red = +1.23 V Thermodynamics HOWEVER, in experiments Cl - is oxidised. Due to kinetics of electrode process! Activation energy is lower for Cl - oxidation, so it is kinetically favoured. Electrolysis with active electrodes Example: Ni electroplating Inert electrodes do not undergo reaction but only act as a surface where oxidation and reduction occurs. Active electrodes participate in the electrolysis process E.g. electroplating Find the favoured reaction to be: Cathode (steel strip): Ni 2+ (aq) + 2e - Ni(s) Anode (nickel strip): Ni(s) Ni 2+ (aq) + 2e - Pic of cell Overall reaction: “transferring Ni atoms” from the anode to form a thin layer on the steel cathode. 13
Page 1 and 2: 4/1/2010 Electrochemistry Relating
Page 3 and 4: 4/1/2010 EXAMPLE: Mn 2+ (aq) + NaBi
Page 5 and 6: 4/1/2010 Standard Reduction Potenti
Page 7 and 8: 4/1/2010 Example: Consider the reac
Page 9 and 10: 4/1/2010 Alkaline battery Most comm
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ELECTROCHEMISTRY - Wits Structural Chemistry

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