Electrolytic Cells & Electrolytic Cells & Electrolysis ReactionsElectrolysis Reactions

Outside electrical source provides electronsOutside electrical source provides electrons

that force a that force a non-spontaneous redox reactionnon-spontaneous redox reaction..

Electricity “splits” a compound into it’s neutralElectricity “splits” a compound into it’s neutral

elementselements

Ex: HEx: H22O(l) + electricity O(l) + electricity → H→ H22(g) + O(g) + O22(g)(g)

Electrolysis Set Up Electrolysis Set Up Ex: NaCl(l)Ex: NaCl(l)

• Single Cell filled with Single Cell filled with electrolyte with +/- ionselectrolyte with +/- ions

• Attach battery to two electrodes.Attach battery to two electrodes.• Adds e- onto one electrode - Making it NEGATIVEAdds e- onto one electrode - Making it NEGATIVE• Pulls e- off one electrode - Making it POSITIVEPulls e- off one electrode - Making it POSITIVE

• Electrodes are made of an inert substance Electrodes are made of an inert substance (like platinum or graphite) that conducts. (like platinum or graphite) that conducts.

Which Way do the Ions Move?Which Way do the Ions Move?

To electrode ofTo electrode of

opposite chargeopposite charge

At neg. electrodeAt neg. electrode

electrons are gainedelectrons are gained by ion by ion

(reduction at CATHODE)(reduction at CATHODE)

At positive electrodeAt positive electrode

electrons are lostelectrons are lost by ion by ion

(oxidation at ANODE)(oxidation at ANODE)

What is Oxidized/Reduced?What is Oxidized/Reduced?

RememberRemember

AN OXAN OX

RED CATRED CATAnode is where oxidation happensAnode is where oxidation happens

Cathode is where reduction happensCathode is where reduction happens

Half Reactions & Net EquationHalf Reactions & Net Equation

Rxn at Anode:Rxn at Anode: (Ox) (Ox)ClCl-- Cl + 1e- Cl + 1e-Or more correctlyOr more correctly DIATOMIC!!!!!!DIATOMIC!!!!!!2Cl2Cl-- Cl Cl22 + 2e- + 2e-

Rxn at Cathode:Rxn at Cathode: (Red) (Red)NaNa+ + + 1e-+ 1e- Na Na(Multiply by 2 to balance electrons)(Multiply by 2 to balance electrons)

NET:NET: 2Na2Na++ + 2Cl + 2Cl-- 2Na + Cl 2Na + Cl22

Electrolysis of Molten NaCl (l)Electrolysis of Molten NaCl (l)

Determining Voltage Needed Determining Voltage Needed (Honors)(Honors)

Use the Voltage Table to determine the totalUse the Voltage Table to determine the total

voltage “needed” to run the Electrolytic cell.voltage “needed” to run the Electrolytic cell.

Total voltage should be a NEGATIVE numberTotal voltage should be a NEGATIVE number

Electrolysis of PbBrElectrolysis of PbBr22(l)(l)

What is oxidized?What is oxidized?

What is reduced?What is reduced?

What are the ox/redWhat are the ox/redhalf reactions?half reactions?

What is the netWhat is the netequation?equation?Negative

ElectrodePositive Electrode

Electrolysis of PbClElectrolysis of PbCl22(l)(l)

Oxidized: ClOxidized: Cl-- Reduced: PbReduced: Pb+2+2

Half ReactionsHalf ReactionsOx:Ox: ClCl-1-1 Cl + 1e-Cl + 1e-

2Cl2Cl-1-1 ClCl22 + 2e- + 2e-

Red:Red: PbPb+2+2 + 2e- + 2e- PbPb

Net:Net: PbPb+2+2 + 2Cl + 2Cl-1-1 Pb + ClPb + Cl22

Electrolysis of NaCl (aq)Electrolysis of NaCl (aq)

Electrolysis of WaterElectrolysis of Water

At Positive Electrode:At Positive Electrode:Ox: Ox: OO-2-2 O + 2e- O + 2e-

but there is a diatomic!but there is a diatomic!2O2O-2-2 OO22 + 4e- + 4e-

At Negative ElectrodeAt Negative ElectrodeRed:Red: HH+1+1 + 1e- + 1e- HH

but there is a diatomic!but there is a diatomic!2H2H+1+1 + 2e- + 2e- HH22

Net:Net: 2H2H22O O 2H2H22 + O + O22

Electrolysis of Water (Animation)Electrolysis of Water (Animation)http://youtu.be/2t13S-KpGeE

Electrolysis of Water (Simple)Electrolysis of Water (Simple)http://youtu.be/HQ9Fhd7P_HA

ElectroplatingElectroplating

Electrolysis reaction Electrolysis reaction used to coat aused to coat asubstance with a thinsubstance with a thinlayer of metal.layer of metal.

Often coating is a lessOften coating is a lessreactive metal that isreactive metal that isnot easily oxidized ornot easily oxidized orcorroded.corroded.

ElectroplatingElectroplating

Negative ElectrodeNegative Electrode Is the OBJECT TO BE Is the OBJECT TO BE

PLATED PLATED so the positive metal ions so the positive metal ions

would go towards it and be would go towards it and be REDUCED.REDUCED.

It is the CATHODEIt is the CATHODE

Red:Red: Ag Ag++ + 1e- + 1e- AgAg00

ElectroplatingElectroplating

Positive ElectrodePositive ElectrodeMade of plating metalMade of plating metal It dissolves into solution as It dissolves into solution as

metal strip gets OXIDIZED.metal strip gets OXIDIZED. It is the ANODEIt is the ANODEThis replenishes the ions for This replenishes the ions for

plating.plating.

Ox:Ox: Ag Ag00 Ag+ + 1e- Ag+ + 1e-

Electroplating Problems Electroplating Problems (Honors)(Honors)

Coulomb = measure of electrical chargeCoulomb = measure of electrical charge

1 mole e- = 96,500 coulombs1 mole e- = 96,500 coulombs

# coulombs = # amps x seconds# coulombs = # amps x seconds

Electroplating Problems Electroplating Problems (Honors)(Honors)

Reduction:Reduction:

Happens on object to be platedHappens on object to be plated

Look at Look at ReductionReduction half reaction half reaction

Look at mole relationshipsLook at mole relationships

between electrons and metal atoms.between electrons and metal atoms.

Ex: AgEx: Ag++ + 1e- + 1e- AgAg00

Electroplating Problems Electroplating Problems (Honors)(Honors)

You can now answer questions regarding You can now answer questions regarding the amount of a substance in moles or the amount of a substance in moles or grams that can be electroplated over a grams that can be electroplated over a certain amount of time.certain amount of time.

Electroplating Problems Electroplating Problems (Honors)(Honors)

If 10 amps are run through a CuSOIf 10 amps are run through a CuSO4 4 solution for solution for 5 minutes, calculate the grams of Cu that will plate5 minutes, calculate the grams of Cu that will plateonto the spoon.onto the spoon.

We Know:We Know:1 mole e- = 96,500 coulombs1 mole e- = 96,500 coulombs# coulombs = # amps x seconds# coulombs = # amps x seconds

Red:Red: Cu Cu+2+2 + 2e- + 2e- CuCu00

2 moles electrons make 1 mole of Cu2 moles electrons make 1 mole of Cu00

1 mole Cu = 63.5 grams1 mole Cu = 63.5 grams

So….Let’s start hereSo….Let’s start here

# coulombs # coulombs = 10 amps x 300 seconds= 10 amps x 300 seconds

= 3000 coulombs= 3000 coulombs

3000 coul. x 3000 coul. x 1 mole e-1 mole e- x x 1 mole Cu1 mole Cu x x 63.5g Cu63.5g Cu = .987 grams = .987 grams

96,500 coul 2 mole e- 1 mole Cu96,500 coul 2 mole e- 1 mole Cu

Mole ratio from Reduction half reactionCuCu+2+2 + 2e- + 2e- CuCu00

You Try OneYou Try One

How long will it take to deposit 20 grams ofHow long will it take to deposit 20 grams of

silver from a solution of AgCl onto a coppersilver from a solution of AgCl onto a copper

tray if a current of 5 amps is used?tray if a current of 5 amps is used?

Answer = 3, 574 sec Answer = 3, 574 sec

or 59.5 minutes or about 1 houror 59.5 minutes or about 1 hour

You Try OneYou Try One

How many amps are needed to deposit .504g.How many amps are needed to deposit .504g.

of Iron in 40 minutes by passing a currentof Iron in 40 minutes by passing a current

through a solution of Iron II Sulfate?through a solution of Iron II Sulfate?

Answer: .72 ampsAnswer: .72 amps