Electrochemical cells: utilize relationship between chemical potential energy & electrical energy Redox Reactions battery to start car prevent corrosion cleaning with bleach (oxidizing agent) Na, Al, Cl prepared or purified by redox reactions breathing O 2 H 2 O and CO 2 Redox Reactions synthesis rxns decomposition rxns SR rxns DR rxns are NOT redox rxns! redox rxns Predicting Redox Reactions Table J: used to predict if given redox reaction will occur metalsdonatemetals donate electrons to ion of metals below itself non-metaltakesnon-metal takes electrons from ion of non-metal below itself Predicting SR Redox Reactions Element + Compound New Element + New Compound If element above swapable ion, reaction is spontaneous If element below swapable ion, reaction is NOT spontaneous Predicting Redox Reactions A + BX B + AX If metal A above metal B (Table J): reaction is spontaneous X + AY Y + AX If non-metal X above non-metal Y (Table J): reaction is spontaneous Spontaneous or not? Li + AlCl 3 Cs + CuCl 2 I 2 + NaCl Cl 2 + KBr Fe + CaBr 2 Mg + Sr(NO 3 ) 2 F 2 + MgCl 2 Yes Yes No Yes No No Yes Started with: Zn(NO 3 ) 2 & Cu AgNO 3 & Cu Which beaker had Zn ions & which had Ag ions? A Overview of Electrochemistry TWO kinds of cells: 1.galvanic or Voltaicelectrochemical 1.galvanic or Voltaic (NYS electrochemical) use spontaneous rxn to produce flow of electrons (electricity) Exothermic 2.electrolytic use flow of electrons (electricity) to force non-spontaneous rxn to occur endothermic Vocabulary galvanic cell Voltaic cell electrochemical cell Board of Regents considers all be the same type of cell Electrochemical Cells use spontaneous SR redox rxn: produces flow of electrons electrons flow from oxidized substance to reduced substance names: galvanic cells, Voltaic cells, or electrochemical cells (NYS) Electrochemical Cells Redox rxn arranged so electrons forced to flow through wire when electrons travel through a wire, can make them do work - light a bulb, ring a buzzer oxidation & reduction reactions must be separated physically Half-Cell place where each half-reaction takes place cells: 2 needed for complete redox rxn wireconnected by wire so electrons flow from 1 st to 2 nd cell salt bridgeconnected by salt bridge to maintain electrical neutrality Schematic of Galvanic/Voltaic Cell Parts of a Galvanic/Voltaic Cell 2 half-cells: one for oxidation rxn one for reduction rxn each consists of: aqueous solution container with aqueous solution & electrode electrode surface where electron transfer takes place wire connects electrodes salt bridge connects solutions How much work can you get out of this reaction? can measure voltage by allowing electrons to travel through voltmeter galvanic cell is a battery not easy battery to transport or use in real-life applications Electrode: surfaces at which oxidation or reduction half-reaction occur anode: oxidation surface decreases in mass cathode: reduction surface increases in mass An OxRed Cat An Ox ate a Red Cat Anode OxidationAnode Oxidation anode is location for oxidation half-rxn Reduction CathodeReduction Cathode cathode is location for reduction half-rxn Anode / Cathode how know which electrode is which? Table J:Table J: use to predict which electrode is anode and which electrode is cathode Anode Anode = Oxidation = Electron Donor anode composed of metal higher on Table J Cathode Cathode = Reduction = Electron Acceptor cathode composed of metal lower on Table J Zn above Cu: Zn is anode; Cu is cathode cathode Direction of Electron Flow (through wire): Anode Cathode Direction of Positive Ion Flow (salt bridge): Anode Cathode Negative electrode Negative electrode (anode): where electrons originate: here it s Zn electrode Zn electrode decreases in mass Zn +2 ions increase in concen aq solns contain ions of same element as electrode Positive electrode Positive electrode (cathode): where electrons attracted: here its Cu electrode Cu electrode increases in mass Cu +2 ions decrease in concen aq solns contain ions of same element as electrode Salt Bridge allows migration of ions between half-cells necessary to maintain electrical neutrality reaction can not proceed without salt bridge Half-Reactions ox: Zn Zn e - red: Cu e - Cu _________________________ Zn + Cu +2 Zn +2 + Cu Which electrode is dissolving? Which electrode is gaining mass? Which species is increasing its concen? Which species is getting more dilute?Zn Zn +2 Cu Cu +2 When the reaction reaches equilibrium voltage is 0! electrons no longer flow Construct Galvanic Cell with Al & Pb Use Table J to identify anode & cathode Draw Cell: put in electrodes & solutions Label: anode, cathode, positive electrode, negative electrode, direction of electron flow in wire, direction of positive ion flow in salt bridge[remember: negative electrode: where electrons originate positive electrode: attracts electrons] Electron flow Al: anode Pb:cathode wire Salt bridge Al +3 & NO 3 -1 Pb +2 & NO 3 -1 Positive ion flow (-) Oxidation: Al Al e - Reduction: Pb e - Pb Overall Rxn (Al Al e - ) + (Pb e - Pb) _____________________________ 2Al + 3Pb +2 2Al Pb 2Al + 3Pb e - 2Al Pb + 6e - 2 3 Application: Batteries Dry Cell Mercury battery Corrosion Corrosion Prevention Whats wrong with this picture?