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 –O 2  H 2 O and CO 2

Redox Reactions Synthesis Decomposition Single Replacement Double Replacement only is not redox Redox

Predicting Redox Reactions Use Table J to predict if a given redox reaction will occur. metaldonateAny metal will donate its electrons to the ion of any metal below it. nonmetalstealAny nonmetal will steal 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 + 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 and AgNO 3 & 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 VoltaicElectrochemical 1.Galvanic or Voltaic (NYS – Electrochemical) Use a spontaneous reaction to produce a flow of electrons (electricity). Exothermic. 2.Electrolytic 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 / CuCl 2 Lab Was a redox reaction. NODid NOT force electrons to travel through a wire. Got NO useful work out of system. Have to be clever in how we arrange things.

2Al + 3Cu +2  2Al Cu Got no useful work because half- reactions weren’t separated.

Half-Cell Where each of the half-reactions takes place. 2 half-cellsNeed 2 half-cells to have a complete redox reaction. wireNeed to be connected by a wire for the electrons to flow through. salt bridgeNeed to be connected by a salt bridge to maintain electrical neutrality.

Schematic of Galvanic Cell

Parts of a Voltaic Cell oxidation & reduction2 half-cells: oxidation & reduction aqueous solutionelectrodeEach half-cell consists of a container of an aqueous solution & an electrode or surface at which the electron transfer takes place. Wire connecting electrodes.Wire connecting electrodes. Salt bridge connects solutions.Salt bridge 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 OxRed Cat An Ox Ate a Red 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 in Table J.The anode is the metal that’s higher in Table J.

Cathode Cathode = Reduction = Electron Acceptor The cathode is the metal that’s lower in Table J.The cathode is the metal that’s lower in Table J.

Zn is above Cu, Zn is anode

Notation for Cells Zn  Zn +2  Cu +2  Cu

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(NO 3 ) 3 or CuSO 4. Zn electrode: solution may be Zn(NO 3 ) 2 or ZnSO 4.

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 e - Cu e -  Cu _________________________ Zn + Cu +2  Zn +2 + Cu Which electrode is dissolving? Which species is getting more concentrated? Zn Zn +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 originate. Positive electrode attracts electrons.Negative electrode is 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  - 

What are half-reactions? Al  Al e - Pb e -  Pb Al metal is the electrode – it’s dissolving. Al +3 ions go into the solution. Pb +2 ions are in the solution. They pick up 2 electrons at the surface of the Pb electrode & plate out.

Overall Rxn 2(Al  Al e - ) 3(Pb e -  Pb) _____________________________ 2Al + 3Pb +2  2Al Pb

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 Pb Increasing Decreasing

Application: Batteries

Dry Cell

Mercury battery

Fuel Cell Converts chemical to electrical energy –Use oxygen or other oxidizing agent Constant supply of fuel (hydrogen or other hydrocarbon)

Application: Corrosion

Corrosion Prevention

What’s wrong with this picture?

Daniell Cell Invented in 1836 by John Frederic Daniell Improved battery technology (voltaic pile –problem with hydrogen bubbling) Definition of the volt