Unit 16 Electrochemistry Oxidation & Reduction. Oxidation verses Reduction Gain oxygen atoms 2 Mg + O 2 2 MgO Lose electrons (e - ) Mg (s) Mg + 2.

Unit 16Unit 16

ElectrochemistryOxidation & Reduction

Oxidation verses ReductionOxidation verses ReductionGain oxygen atoms

2 Mg + O2 2 MgO

Lose electrons (e-)Mg(s) Mg+2 + 2 e-

Lose hydrogenCH3OH CH2O + H2

Lose oxygen atoms2 KClO3 2 KCl + 3

O2

Gain electrons (e-)Cu+2 + 2 e- Cu (s)

Gain hydrogenCO + 2 H2 CH3OH

l l l l l l l l l l l l l l l l l l -4 -3 -2 -1 0 1 2 3 4 -4 -3 -2 -1 0 1 2 3 4

reduction reduction

oxidation oxidationWhen oxidation occurs, there is also When oxidation occurs, there is also reduction. They go “hand in hand”.reduction. They go “hand in hand”.

Not all reactions are oxidation Not all reactions are oxidation reduction reactions.reduction reactions.

Using Appendix 6 chart:Using Appendix 6 chart:

To determine the strength as an oxidizing agent, To determine the strength as an oxidizing agent, look at the left side of appendix 6. look at the left side of appendix 6. The chemical that is higher on the list (has a The chemical that is higher on the list (has a greater magnitude + voltage) is the better greater magnitude + voltage) is the better oxidizing agent.oxidizing agent.

Which is better? BrWhich is better? Br22 or Sn or Sn++22

To determine the strength as a reducing agent, To determine the strength as a reducing agent, look at the right side of appendix 6. look at the right side of appendix 6. The chemical that is lower on the list (has the The chemical that is lower on the list (has the greater magnitude – voltage) is the better greater magnitude – voltage) is the better reducing agent.reducing agent.

Which is better? PbWhich is better? Pb(s)(s) or H or H22OO22

The chemical that is The chemical that is oxidizedoxidized acts as the acts as the reducing agent.reducing agent.

The chemical that is The chemical that is reducedreduced acts as the acts as the oxidizing agent.oxidizing agent.

In the following, indicate which chemical is beingIn the following, indicate which chemical is beingoxidized, reduced, acts as the oxidizing agent, oxidized, reduced, acts as the oxidizing agent, and acts as the reducing agent.and acts as the reducing agent.

ZnZn(s)(s) + Cu + Cu++22(aq)(aq) Zn Zn++22

(aq)(aq) + Cu + Cu(s)(s)

ElectricityElectricity is the movement of electrons to is the movement of electrons to provide energy. Two factors affect the provide energy. Two factors affect the amount of energy produced.amount of energy produced.

1. 1. VoltageVoltage is the push behind the e is the push behind the e--. . Voltage is measured in volts Voltage is measured in volts

2. 2. Current Current which is how many ewhich is how many e-- pass a pass a point each second. Current is measured in point each second. Current is measured in Amperes. Amperes. 1 Amp = 6.0 x 101 Amp = 6.0 x 101818 e e--/s/s

An electrochemical An electrochemical CellCell (Galvanic or voltaic (Galvanic or voltaic cell) is a chemical device to make electricity cell) is a chemical device to make electricity from chemical reactions. It employs from chemical reactions. It employs oxidation and reduction. It will have a oxidation and reduction. It will have a positive (+) voltage while it works. positive (+) voltage while it works.

Cell PotentialCell Potential (E (Eoocellcell) or ) or Electromotive force Electromotive force

(emf) (emf) is the pull or “driving force” on the is the pull or “driving force” on the ee--. It is measured in volts . It is measured in volts

(E(Eoo ) Standard reduction potential is the ) Standard reduction potential is the potential for the half – reaction at 1potential for the half – reaction at 1MM

the standard being measured off the standard being measured off 2 H2 H++ + 2 e + 2 e-- H H22 , Pt electrode = 0.00 v , Pt electrode = 0.00 v

Remember that these are listed as Remember that these are listed as Reduction Potentials. Reduction Potentials.

When there is reduction, there is also oxidation. One of When there is reduction, there is also oxidation. One of the two half – reactions will need to be reversed the two half – reactions will need to be reversed

(so that it is written as an oxidation)(so that it is written as an oxidation)

To calculate the voltage of an electrochemical To calculate the voltage of an electrochemical cell:cell:

1. Obtain the two half – reactions from the reduction 1. Obtain the two half – reactions from the reduction potential chart.potential chart.

2. Determine which half – reaction is the oxidation, 2. Determine which half – reaction is the oxidation, reverse the reaction. When the reaction is reversed, the reverse the reaction. When the reaction is reversed, the sign on the voltage is changed.sign on the voltage is changed.

3. Balance the e3. Balance the e-- in the two half – reactions by in the two half – reactions by multiplying the coefficients by some whole number. multiplying the coefficients by some whole number. Remember that voltage is a property of the kind of Remember that voltage is a property of the kind of chemical, not the amount. Do not multiply the voltage.chemical, not the amount. Do not multiply the voltage.

4. Add the two half – reactions to determine the net 4. Add the two half – reactions to determine the net voltage for the cell.voltage for the cell.

A salt bridge or porous disk connects the half cells and allows e- to flow, completing the circuit.

We use a porous cup as the barrier between the 2 half cells

Calculate the voltage of a Galvanic cell Calculate the voltage of a Galvanic cell made with Mg and Cu used to run the made with Mg and Cu used to run the clock.clock.

MgMg++22 + 2e + 2e-- Mg Mg(s) (s) voltsvoltsCuCu++22 + 2 e + 2 e-- Cu Cu(s)(s) voltsvolts

Line NotationLine Notation : : A way to denote a galvanic cellA way to denote a galvanic cellAnode: the oxidation is written firstCathode: the reduction is written secondOne line separates the phases of a half

cellTwo lines separate the anode and cathode

Mg l Mg+2 ll Cu+2 l Cu

Corrosion is the process of returning metals to their natural Corrosion is the process of returning metals to their natural state. The oxidation of metals.state. The oxidation of metals.

Example: 4 FeExample: 4 Fe(s)(s) + 3 O + 3 O2 (g)2 (g) 2 Fe 2 Fe22OO3 (s)3 (s)

All metals corrode easily, except Au, Ag, CuAll metals corrode easily, except Au, Ag, Cu

Preventing corrosion:Preventing corrosion:Coating with Cd or SbCoating with Cd or SbGalvanizing (coating) with ZnGalvanizing (coating) with ZnStainless steel is an alloy of Fe with Cd + NiStainless steel is an alloy of Fe with Cd + NiSacrificial metals which corrode first are Mg or Zn Sacrificial metals which corrode first are Mg or Zn

A battery is a group of galvanic cells connected in series. (add potentials Eo

cell)

Wet cell – contain fluid medium for e- flowDry cell - contain a paste medium for e- flow

Primary cell – one time use, not rechargeableSecondary cell - rechargeable

• Dry Cell (Batteries) – do not contain a liquid electrolyte

– Acid version• Anode reaction - oxidation

Zn Zn2+ + 2e +0.76 v• Cathode reaction – reduction2NH4

+ + 2MnO2 + 2e Mn2O3 + 2NH3 + 2H2O +0.74 v

– Other types•Alkaline – Zn + 2OH- ZnO + H2O + 2e- anode 2MnO2 + H2O + 2 e- Mn2O3 + 2OH- cathode•Silver cell – Zn anode, Ag2O cathode • Mercury cell – Zn anode, HgO cathode •Cadmium – anode, NiO2 – cathode •(secondary – rechargable)

Mercury

cell

Overall reaction

Pb + PbO2 + 2H2SO4 2PbSO4 + 2H2O +2.02 v

Wet cell - Lead storage battery – rechargeableAnode Pb + HSO4

- PbSO4 + H+ + 2 e- +0.34 vCathode PbO2 + HSO4

- + 3 H+ + 2 e- PbSO4 + 2 H2O +1.68 v

An An Electrolytic cellElectrolytic cell is a cell with a is a cell with a negative voltage, so there is no reaction. negative voltage, so there is no reaction. An outside source of energy is used as a An outside source of energy is used as a way to drive the reaction.way to drive the reaction.

This type of cell is often used as a This type of cell is often used as a controlled way to deposit thin layers of controlled way to deposit thin layers of metals, metals, electroplatingelectroplating, on another , on another metal. metal.

Cr+3 + 3 e- Cr Cu+2 + 2 e- Cu

An electrolytic cell can be used to electroplate one metal on another metal.

PbAnode

CuCathode

Power source

Balance the reaction and determine the minimum voltage that needs to be supplied to drive the reaction.

Electrochemical Review1. a. Zn +2Ag+1 Zn+2 + 2 Ag +1.56 v c. Fe + Sn+2 Fe+2 + Sn

+0.30 v

b. Cu + 2 Ag+1 Cu+2 + 2 Ag +0.46 v d. Hg (L) + 2 H+1 Hg+2 + H2 -0.78 v

e. Cell d does not work

2. oxidation take place at the anodereduction take place at the cathode

Cu is at the anodeAu is at the cathode

+1.16 v3 Cu + 2 Au+3 3 Cu+2 + 2 Au

Au continuously get larger

3. Cl2 (g) +2 Br-1 2 Cl-1 + Br2 (g) +0.30 volts

Pollution CrosswordPollution CrosswordDown3. Potable21. Photochemical25. Organic37. Zeolite

Across6. Inorganic8. Aeration18. Precipitation29. Detergent40. Sulfate