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 only is not redox
Always Redox
Often Redox
Predicting Redox Reactions
• Use Table J to predict if a given redox reaction will occur.
• Any metalmetal will donatedonate its electrons to the ion of any metal below it.
• Any nonmetalnonmetal will stealsteal electrons 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.
Which are spontaneous?
• Li + AlCl3 • Cs + CuCl2 • I2 + NaCl • Cl2 + KBr • Fe + CaBr2 • Mg + Sr(NO3)2 • F2 + MgCl2
YesYesYesYesNoNoYesYesNoNo
NoNo
YesYes
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):
1.1. Galvanic or VoltaicGalvanic or Voltaic (NYS – ElectrochemicalElectrochemical)
• Use a spontaneous reaction to produce a flow of electrons (electricity). Exothermic.
2.2. ElectrolyticElectrolytic
• Use a flow of electrons (electricity) to force a nonspontaneous reaction to occur. Endothermic.
Vocabulary
• Redox• Half-reaction• Oxidation• Reduction• Cell• Half-Cell• Electrode
• Anode• Cathode• Galvanic• Voltaic• Electrochemical• Electrolytic• Salt bridge
Electrochemical Cells
• Use a spontaneous single replacement redox reaction to produce a flow of electrons.
• Electrons flow from oxidized substance to reduced substance.
• Called: Galvanic cells, voltaic cells, or electrochemical cells (NYS)
Electrochemical Cells
• Redox reaction is arranged so the electrons are forced to flow through a wire.
• When the electrons travel through a wire, we can make them do work, like light a bulb or ring a buzzer.
• So the oxidation & reduction reactions have to be separated physically.
OJ clock
Al / CuCl2 Lab
• Was a redox reaction.
• Did NOT force electrons to travel through a wire. Got NONO useful work out of system.
• Have to be clever in how we arrange things.
2Al + 3Cu+2 2Al+3 + 3Cu
Got no useful work because half-reactions weren’t separated.
Half-Cell
• Where each of the half-reactions takes place.
• Need 2 half-cells2 half-cells to have a complete redox reaction.
• Need to be connected by a wirewire for the electrons to flow through.
• Need to be connected by a salt bridgesalt bridge to maintain electrical neutrality.
Schematic of Galvanic Cell
Parts of a Voltaic Cell
• 2 half-cells: oxidation & reductionoxidation & reduction
• Each half-cell consists of a container of an aqueous solutionaqueous solution & an electrode electrode or surface at which the electron transfer takes place.
• Wire Wire connecting electrodes.connecting electrodes.
• Salt bridge Salt bridge connects solutions.connects solutions.
How much work can you get out of this reaction?
• You can measure the voltage by making the electrons travel through a voltmeter.
• The galvanic cell is a battery. Of course, 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 OxAn Ox Ate a FatFat Red CatRed Cat
• Anode – OxidationAnode – Oxidation
• The anode = location for the oxidation half-reaction.
• Reduction – CathodeReduction – Cathode
• The cathode = location for the reduction half-reaction.
Anode / Cathode
• How do you know which electrode is which?
• Use Table JUse Table J to predict which electrode is the anode and which electrode is the cathode.
Anode
• Anode = Oxidation = Electron Donor
• The anode is the metal that’s higher The anode is the metal that’s higher in Table J.in Table J.
Cathode
• Cathode = Reduction = Electron Acceptor
• The cathode is the metal that’s lower in The cathode is the metal that’s lower in Table J.Table J.
Zn is above Cu, Zn is anode
Notation for Cells
ZnZn+2Cu+2Cu
Direction of Electron Flow(wire)
Anode to Cathode
Direction of Positive Ion Flow(salt bridge)
Anode to Cathode
Positive & Negative Electrode
• Negative electrodeNegative electrode is where electrons originate – here it’s the Zn electrode.
• Positive electrodePositive electrode 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 may be Cu(NO3)3 or CuSO4.
• Zn electrode: solution may be Zn(NO3)2 or ZnSO4.
Salt Bridge
• Allows for migration of ions between half-cells.
• Necessary to maintain electrical neutrality.
• Reaction will not proceed without salt bridge.
A(s) + BX(aq) B(s) + AX(aq)
• Single replacement rxn occurs during operation of galvanic cell.
• One electrode will gain mass (B) and one electrode will dissolve (A).
• The concentration of metal ions will increase in one solution (making AX) & decrease in one solution (using up BX).
Half-Reactions
Zn Zn+2 + 2e-
Cu+2 + 2e- Cu_________________________
Zn + Cu+2 Zn+2 + Cu
Which electrode is dissolving?Which species is getting more concentrated?
ZnZn
ZnZn+2+2
Zn + Cu+2 Zn+2 + Cu
• Which electrode is gaining mass?
• Which species is getting more dilute?
Cu
Cu+2
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 is where electrons Negative electrode is where electrons originate. Positive electrode attracts originate. Positive electrode attracts electrons.electrons.
Electron flow
Al = anode
Pb = Pb = cathodecathode
wire
Salt bridge
Al+3 & NO3-1 Pb+2 & NO3
-1
Positive ion flow
-
What are half-reactions?
Al Al+3 + 3e-
Pb+2 + 2e- Pb
Al metal is the electrode – it’s dissolving.Al metal is the electrode – it’s dissolving.AlAl+3+3 ions go into the solution. ions go into the solution.
PbPb+2+2 ions are in the solution. They ions are in the solution. They pick up 2 electrons at the surface of pick up 2 electrons at the surface of the Pb electrode & plate out.the Pb electrode & plate out.
Overall Rxn
2(Al Al+3 + 3e-)
3(Pb+2 + 2e- Pb)_____________________________
2Al + 3Pb+2 2Al+3 + 3Pb
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]?
Al
PbIncreasing
Decreasing
Application: Batteries
Dry Cell
Mercury battery
Application: Corrosion
Corrosion Prevention
What’s wrong with this picture?