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

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