Electrochem Lecture Notes

Electrochem Lecture Notes - Two major electrochemical types...

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1 Electrochemistry Two major electrochemical types: Voltaic (galvanic) cells: Using spontaneous chemical reactions to do work (i.e. provide electricity). Electrolytic Cells: Using electricity to force nonspontaneous reactions. Purification of Metals Electroplating Electrochemistry is also tied to Gibbs Free Energy and Chemical Equilibrium Oxidation-Reduction: The Heart of Electrochemistry General Idea: Since redox reactions involve the transfer of electrons from one component of a reaction to another, intercepting that transfer (flow) of electrons, allows useful “work” to be done (i.e. electrical circuits can be powered). Don’t Forget: Leo Goes Ger L oses E lectrons O xidized G ains E lectrons R educed Expanded Definition of Redox Reactions Oxidation: Oxidation: -Lose electrons -Gain oxygen (special case) -Lose hydrogen (special case) Reduction: Reduction: -Gain electrons -Lose oxygen (special case) -Gain hydrogen (special case) Ex. Ex. Cu (s) + 2AgNO 3(aq) Cu(NO 3 ) 2(aq) + 2Ag (s) In this example copper loses two electrons and each silver atom gains an electron Cu 0 Cu 2+ + 2e - and is considered oxidized This is the oxidation half-reaction. It only shows the component oxidized and does not include any spectator ions. 2Ag + + 2e - 2Ag and is considered reduced This is the reduction half-reaction. It only shows the component reduced and does not include any spectator ions. Net Ionic: Cu (s) + 2Ag + (aq) Cu 2+ (aq) + 2Ag (s) Characteristics: -Electrons are lost/gained by particles in the reaction -There must be a change in “oxidation number”. -You must know the rules for assigning oxidation numbers -Oxidation numbers are for “book keeping purposes” and may not represent actual charges -Oxidations and reductions must occur in pairs (i.e. electron loss must be accompanied by electron gain) -The oxidizing “agent” does the oxidizing and is itself reduced (agent refers to entire species) -The reducing “agent” does the reducing and is itself oxidized. -In a disproportionation reaction, an element in one oxidation state is simultaneously oxidized and reduced.
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2 Common Redox Reactions Reactions Single Replacement (A + BC B + AC): Cu (s) + 2AgNO 3(aq) Cu(NO 3 ) 2(aq) + 2Ag (s) Combustion (hydrocarbon + O 2 CO 2 + H 2 O): O): CH 4(g) + 2O 2(g) CO 2(g) + 2H 2 O (g) Redox Redox Redox Reactions in acid/base aqueous medium: Reactions in acid/base aqueous medium: Reactions in acid/base aqueous medium: 14H + (aq) + Cr 2 O 7 2- (aq) + 6Fe 2+ (aq) 2Cr 3+ (aq) + 7H 2 O (l) + 6Fe 3+ (aq) Synthesis (A + B AB): AB): Mg (s) + Cl 2(g) MgCl 2(s) Decomposition (AB A + B): 2H 2 O (l) 2H 2(g) + O 2(g) Not all of these take place in aqueous solutions but in each case electrons are lost and gained somewhere in the reaction equation. Example: Write the oxidation and reduction half reactions for cupric chloride decomposing to form copper metal and chlorine gas.
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