Utilizes relationship between chemical potential energy electrical energy
Utilizes 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 • Decomposition • Single Replacement • DR rxns NOT redox rxn! Redox rxns
Predicting Redox Reactions • Table J: predict if given redox reaction will occur • metals donate electrons to ion of metals below itself • nonmetal steals electrons from ion of nonmetal below itself
Predicting Single Replacement 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 nonmetal X above nonmetal Y (Table J): reaction is spontaneous
Spontaneous or not? • Li + Al. Cl 3 Yes • Cs + Cu. Cl 2 Yes • I 2 + Na. Cl No • Cl 2 + KBr Yes • Fe + Ca. Br 2 No • Mg + Sr(NO 3)2 No • F 2 + Mg. Cl 2 Yes
Started with: 1. Zn(NO 3)2 & Cu 2. Ag. NO 3 & Cu Which beaker had Zn ions & which had Ag ions?
Overview of Electrochemistry • TWO kinds of cells: 1. Galvanic or Voltaic (NYS – Electrochemical) Electrochemical Use spontaneous rxn to produce flow of electrons (electricity) = Exothermic 2. Electrolytic Use flow of electrons (electricity) to force nonspontaneous rxn to occur = Endothermic
Vocabulary • • Redox Half-reaction Oxidation Reduction Cell Half-Cell Electrode • • Anode Cathode Galvanic Voltaic Electrochemical Electrolytic Salt bridge
Electrochemical Cells • 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 – ½ cells: • 2 needed for complete redox rxn • connected by wire so electrons flow through • connected 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: • container with aqueous solution & 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 Surface at which oxidation or reduction half-reaction occurs: Anode & Cathode
An Ox Ate a Red Cat • Anode – Oxidation – the anode = location for the oxidation half-reaction • Reduction – Cathode – the cathode = location for the reduction half-reaction
Anode / Cathode • How know which electrode is which? • Table J: predict which electrode and which electrode is cathode
Anode • Anode = Oxidation = Electron Donor – anode is metal higher on Table J
Cathode • Cathode = Reduction = Electron Acceptor – cathode is metal lower on Table J
Zn is above Cu, Zn is anode
Direction of Electron Flow (through wire): Anode → Cathode Direction of Positive Ion Flow (salt bridge): Anode → Cathode
Positive & Negative Electrode • Negative electrode (anode): where electrons originate – here it’s Zn electrode • Positive electrode (cathode): electrode that attracts electrons – here it’s the Cu electrode
Aqueous Solution • Solution containing ions of same element as electrode • Cu electrode: – Solution: Cu(NO 3)3 or Cu. SO 4 • Zn electrode: – Solution: Zn(NO 3)2 or Zn. SO 4
Salt Bridge • migration of ions between half-cells • necessary to maintain electrical neutrality • reaction can not proceed without salt bridge
A(s) + BX(aq) B(s) + AX(aq) • SR rxn occurs during operation of galvanic/voltaic cell • One electrode gains mass (B) and one electrode dissolves (A) • concen of metal ions ↑ in one soln (making AX) & ↓ in other soln (using up BX)
Half-Reactions Zn Zn+2 + 2 e. Cu+2 + 2 e- Cu _____________ Zn + Cu+2 Zn+2 + Cu Which electrode is dissolving? Zn Which species is increasing its mass? Zn+2
Zn + Cu+2 Zn+2 + Cu • Which electrode is gaining mass? Cu • Which species is getting more dilute? 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, direction of electron flow in wire, direction of positive ion flow in salt bridge, positive electrode, negative electrode • Negative electrode: where electrons originate Positive electrode: attracts electrons
Electron flow wire Al = anode (-) Al+3 Positive ion flow Salt bridge & NO 3 -1 Pb = cathode Pb+2 & NO 3 -1
What are half-reactions? Al Al+3 + 3 e • Al metal is electrode that’s dissolving • Al+3 ions go into solution Pb+2 + 2 e- Pb • Pb+2 ions are in the solution • Ions pick up 2 electrons & plate together on surface of Pb electrode as Pb 0
Overall Rxn 2(Al Al+3 + 3 e-) + 3(Pb+2 + 2 e- Pb) _______________ 2 Al + 3 Pb+2 + 6 e- 2 Al+3 + 3 Pb + 6 e 2 Al + 3 Pb+2 2 Al+3 + 3 Pb
2 Al + 3 Pb+2 2 Al+3 + 3 Pb • • Al Which electrode is losing mass? Which electrode is gaining mass? Pb What’s happening to the [Al+3]? Increasing What’s happening to the [Pb+2]? Decreasing
Application: Batteries
Dry Cell
Mercury battery
Application: Corrosion
Corrosion Prevention
What’s wrong with this picture?
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