Download
1 / 42
420 likes | 431 Views
Learn the basics of redox reactions, predicting reactions, and practical applications of electrochemistry. Explore concepts such as galvanic cells, half-reactions, and electrode behavior.
E N D
Utilizes relationship between chemical potential energy & electrical energy
Redox Reactions • Need battery to start car • Prevent corrosion • Bleach is an oxidizing agent • Na, Al, Cl prepared or purified by redox reactions • Breathing • O2 H2O and CO2
Redox Reactions • Synthesis • Decomposition • Single Replacement • Double Replacement are not redox rxn! Redox
Predicting Redox Reactions • Use Table J to predict if a given redox reaction will occur • Any metal will donate its electronsto the ion of any metal below it • Any nonmetal will stealelectrons from the ion of any nonmetal below it
Predicting Single Replacement Redox Reactions Element + Compound New Element + New Compound If the element is above the swapable ion, the reaction is spontaneous If the element is below the swapable ion, the reaction is not spontaneous
Predicting Redox Reactions A + BX B + AX A & B are metals If metal A is above metal B in Table J, the reaction is spontaneous X + AY Y + AX • X & Y are nonmetals If nonmetal X is above nonmetal Y in Table J, the reaction is spontaneous
Yes Yes No Yes No No Yes Spontaneous or not? • Li + AlCl3 • Cs + CuCl2 • I2 + NaCl • Cl2 + KBr • Fe + CaBr2 • Mg + Sr(NO3)2 • F2 + MgCl2
Started with: Zn(NO3)2 & Cu and AgNO3 & Cu Which beaker had the Zn ions & which had the Ag ions?
Overview of Electrochemistry • TWO kinds of cells (kind of opposites): 1. Galvanic or Voltaic (NYS – Electrochemical) Use a spontaneous rxn to produce a flow of electrons (electricity) = Exothermic 2. Electrolytic Use a flow of electrons (electricity) to force a nonspontaneous rxn to occur =Endothermic
Anode Cathode Galvanic Voltaic Electrochemical Electrolytic Salt bridge Vocabulary • Redox • Half-reaction • Oxidation • Reduction • Cell • Half-Cell • Electrode
Electrochemical Cells • Use spontaneous SR redox reaction to produce 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 are forced to flow through a 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-reactions takes place • two ½ cells needed for complete redox rxn • need to be connected by awirefor the electrons to flow through • need to be connected by asalt bridgeto maintain electrical neutrality
Parts of a Galvanic/Voltaic Cell • 2 half-cells: oxidation & reduction • Each half-cell consists of: • container with aqueous solution & electrode (surface at which the electron transfer takes place) • Wire connects electrodes • Salt bridge connects solutions
How much work can you get out of this reaction? • You can measure the voltage by allowing electrons to travel through a voltmeter • The galvanic cell is a battery • it’s not a very 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 do you know which electrode is which? • Use Table J to predict which electrode is the anode and which electrode is the cathode
Anode • Anode = Oxidation = Electron Donor • anode is metal that’s higher on Table J
Cathode • Cathode = Reduction = Electron Acceptor • cathode is metal that’s lower on Table J
ZnZn+2Cu+2Cu Notation for Cells
Direction of Positive Ion Flow (salt bridge): Direction of Electron Flow(through wire): Anode → Cathode Anode →Cathode
Positive & Negative Electrode • Negative electrode (anode) is where electrons originate • here it’s the Zn electrode • Positive electrode (cathode) is electrode that attracts electrons • here it’s the Cu electrode
Aqueous Solution • Solution containing ions of the same element as the electrode • Cu electrode: • Solution: Cu(NO3)3 or CuSO4 • Zn electrode: • Solution: Zn(NO3)2 or ZnSO4
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)
_________________________ Zn + Cu+2 Zn+2 + Cu Zn Zn+2 Half-Reactions Zn Zn+2 + 2e- Cu+2 + 2e- Cu Which electrode is dissolving? Which species is increasing its mass?
Cu Cu+2 Zn + Cu+2 Zn+2 + Cu • Which electrode is gaining mass? • Which species is getting more dilute?
When the reaction reaches equilibrium • The voltage goes to 0!
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 Positive ion flow Pb = cathode Al = anode Salt bridge (-) Pb+2 & NO3-1 Al+3 & NO3-1
Al metal is the electrode: it’s dissolving • Al+3 ions go into the solution What are half-reactions? Al Al+3 + 3e- Pb+2 + 2e- Pb • Pb+2 ions are in the solution • Ions pick up 2 electrons & platetogetheron surface of Pb electrode as Pb0
_____________________________ 2Al + 3Pb+2 2Al+3 + 3Pb Overall Rxn 2(Al Al+3 + 3e-) + 3(Pb+2 + 2e- Pb) 2Al + 3Pb+2 + 6e- 2Al+3 + 3Pb + 6e-
Al Pb Increasing Decreasing 2Al + 3Pb+2 2Al+3 + 3Pb • Which electrode is losing mass? • Which electrode is gaining mass? • What’s happening to the [Al+3]? • What’s happening to the [Pb+2]?
