Electrochem Lecture Notes - Two major electrochemical types...

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1ElectrochemistryTwo major electrochemical types:Voltaic (galvanic) cells: Voltaic (galvanic) cells: Voltaic (galvanic) cells: Voltaic (galvanic) cells: Using spontaneous chemical reactions to do work (i.e. provide electricity).Electrolytic Cells: Electrolytic Cells: Electrolytic Cells: Electrolytic Cells: Using electricity to force nonspontaneousreactions.Purification of MetalsElectroplatingElectrochemistry is also tied to Gibbs Free Energy and Chemical EquilibriumOxidation-Reduction: The Heart of ElectrochemistryGeneral 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 GerLoses Electrons OxidizedGains Electrons ReducedExpanded Definition of Redox ReactionsOxidation:Oxidation:Oxidation:Oxidation:-Lose electrons-Gain oxygen (special case)-Lose hydrogen (special case)Reduction:Reduction:Reduction:Reduction:-Gain electrons-Lose oxygen (special case)-Gain hydrogen (special case)Ex.Ex.Ex.Ex. Cu(s)+ 2AgNO3(aq)Cu(NO3)2(aq)+ 2Ag(s)In this example copper loses two electrons and each silver atom gains an electronCu0Cu2++ 2e-and is considered oxidizedThis is the oxidation half-reaction. It only shows the component oxidized and does not include any spectator ions.2Ag++ 2e-2Ag and is considered reducedThis is the reduction half-reaction. It only shows the component reduced and does not include any spectator ions.Net Ionic: Cu(s)+ 2Ag+(aq)Cu2+(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.
2Common Common RedoxRedoxReactionsReactionsSingle Replacement (A + BC Single Replacement (A + BC Single Replacement (A + BC Single Replacement (A + BC B + AC):B + AC):B + AC):B + AC):Cu(s)+ 2AgNO3(aq)Cu(NO3)2(aq)+ 2Ag(s)Combustion (hydrocarbon + OCombustion (hydrocarbon + OCombustion (hydrocarbon + OCombustion (hydrocarbon + O2COCOCOCO2+ H+ H+ H+ H2O):O):O):O):CH4(g)+ 2O2(g)CO2(g)+ 2H2O(g)RedoxRedoxRedoxRedox Reactions in acid/base aqueous medium:Reactions in acid/base aqueous medium:Reactions in acid/base aqueous medium:Reactions in acid/base aqueous medium:14H+(aq)+ Cr2O72-(aq)+ 6Fe2+(aq)2Cr3+(aq)+ 7H2O(l)+ 6Fe3+(aq)Synthesis (A + B Synthesis (A + B Synthesis (A + B Synthesis (A + B AB):AB):AB):AB):Mg(s)+ Cl2(g)MgCl2(s)Decomposition (AB Decomposition (AB Decomposition (AB Decomposition (AB A + B):A + B):A + B):A + B):2H2O(l)2H2(g)+ O2(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.CuCl2(aq)Cu(s)+ Cl2(g)Answer:Reduction:Cu2+(aq)+ 2e-Cu(s)Cu goes from +2 to 0Oxidation:2Cl-(aq)Cl2(g)+ 2e-Cl goes from (-1 to 0)x2Balancing Balancing RedoxRedoxReactions in Acidic and Reactions in Acidic and Basic MediumBasic MediumMany redox reactions take place in either acidic or basic solutions.

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