20080204 Electrochem 4

20080204 Electrochem 4 - Intro Chemistry II 030.102 -...

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Intro Chemistry II – 030.102 - Chapter 12 Spring, 2008 1 12.5. Corrosion and Its Prevention Corrosion a serious problem. Focus on electrochemical reactions of iron. Can picture as “short-circuited” galvanic cell. Anode reaction – oxidation of iron: Fe( s ) Fe 2+ ( aq ) + 2 e E ° = +0.409 V Cathode reaction in water in absence of oxygen: 2 H 2 O( l ) + 2 e 2 OH ( aq ) + H 2 ( g ) E ° = –0.828 V Overall reaction: Fe( s ) + 2 H 2 O( l ) Fe 2+ ( aq ) + 2 OH ( aq ) + H 2 ( g ) Cell voltage: Δ E ° = +0.409 – 0.828 = –0.399 V Reaction is not spontaneous since Δ E ° negative (less positive in alkaline water). Remember: Δ G ° = – n F Δ E ° Corrosion and Its Prevention Corrosion of iron more extensive in presence of oxygen and water. Cathode reaction now (1/2) O 2 ( g ) + 2 H 3 O + ( aq ) + 2 e 3 H 2 O( l ) E ° = +1.229 V Overall reaction: Fe( s ) + (1/2) O 2 ( g ) + 2 H 3 O + ( aq ) Fe 2+ ( aq ) + 3 H 2 O( l ) Cell voltage: Δ E ° = +0.409 + 1.229 = +1.638 V Reaction is spontaneous since Δ E ° positive. Fe 2+ ions are further oxidized to +3 oxidation state to form rust: 2 Fe 2+ ( aq ) + (1/2) O 2 ( g ) + (6+ x ) H 2 O( l ) Fe 2 O 3 x H 2 O + 4 H 3 O + ( aq ) H 3 O + ions formed allow corrosion cycle to continue. Corrosion can occur when paint chipped off. Other factors speeding corrosion: salt (electrolyte), acidity.
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Intro Chemistry II – 030.102 - Chapter 12 Spring, 2008 2 Corrosion and Its Prevention Inhibit corrosion: Paint – but can chip. Passivation – protective oxide layer (Al 2 O 3 on Al), stainless steel (alloy of Fe and Cr, passivated by Cr) Sacrificial anode – Mg 2+ harder to reduce than Fe 2+ : Fe 2+ ( aq ) + 2 e Fe( s ) E ° = –0.41 V Mg 2+ ( aq ) + 2 e Mg( s ) E ° = –2.39 V So Mg more easily oxidized than Fe. Hence, Mg can be used as sacrificial anode, and is preferentially consumed. Method is widely used (ships, bridges, buried pipe, water heaters). 12.6. Electrometallurgy
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This note was uploaded on 04/14/2008 for the course CHEMISTRY 102 taught by Professor Dagdigian during the Spring '08 term at Johns Hopkins.

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20080204 Electrochem 4 - Intro Chemistry II 030.102 -...

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