LECTURE 30 Electrochemical devices

LECTURE 30 Electrochemical devices - SUMMARY FROM LAST...

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Dr. P. Lucas U of A MSE 110 Corrosion Corrosion is the deterioration of metals by spontaneous redox reactions. Corrosion in the environment is due to the combined effect of water and oxygen. Most metal have E o <1.23 hence the thermodynamically stable equilibrium state of most metals in nature is in the oxidized form. Corrosion can be naturally slowed down by the lack of H 2 O or by formation of a protective layer such as Zn 2 (OH) 2 CO 3 or Al 2 O 3 . Fe oxidizes in two steps, first forming Fe(OH) 2 followed by Fe(OH) 3 . This rust peals off and re-expose the metal to further oxidation. At the anodic site Fe is oxidized and goes in solution as Fe 2+/3+ and at the cathodic site H 2 O/O 2 are reduced to OH - and the rust is deposited. A sacrificial anode is a metals with low redox potential connected to Fe which is oxidized in place of the Fe part. SUMMARY FROM LAST CLASS E o (O2/H2O) = 1.23 V O 2 + 4 H + + 4e - Æ 2 H 2 O E o (O2/OH-) = 0.40 V O 2 + 2 H 2 O + 4e - Æ 4 OH -
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Dr. P. Lucas U of A MSE 110 Batteries: There is two general class of batteries: rechargeable and non- rechargeable. ELECTROCHEMICAL DEVICES Primary batteries : The redox reaction taking place in a primary battery is non-reversible, the reactant can not be regenerated by applying a potential to drive the reverse reaction. Alkaline batteries are a typical example of primary battery. Zn + 2 MnO 2 Æ ZnO + Mn 2 O 3 This reaction can not be reversed, hence the alkaline battery is non-rechargeable
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