Unit 11 ElectrochemistryUnit 11 Electrochemistry

What is electrochemistry?What is electrochemistry?The study of the relationship

between chemical change and electrical work. ◦Investigated using redox rxns.

1. Electrolytic- use of electricity to cause a chemical reactionElectroplating

2. Galvanic or Voltaic the production of electricity by a chemical reaction.

Battery

Oxidation and Reduction Oxidation and Reduction Rxns. Rxns. Type of chemical reaction in

which one substance transfers an electron to another substance. ◦Called Redox reactions

How do redox rxns. work?How do redox rxns. work?Oxidation is the loss of electrons

◦Referred as “reducing agent”◦Ionic charge increases; more (+)

Reduction is the gain of electrons ◦Referred as “oxdizing agent”◦Ionic charge decreases; more (-)

◦“LEO” the lion says “GER”◦Zn(s) + 2H+ (aq) Zn+2(aq) +

H2(g)

How to keep track of How to keep track of electrons: Oxidation Stateselectrons: Oxidation StatesEvaluating oxidation state (OS) of

each atom in a formula helps us…◦Determine if electrons are being

transferred ◦Determine which atoms gains or loses

electronsOS is generally not the electric

charge, instead it’s used to us if the environment is “electron rich” or “electron poor”.

Rules for Assigning OS Rules for Assigning OS numbersnumbers1. Total of O.S. = charge shown2. group 1 metal = +1

group 2 metal = +23. F = –14. H = +15. O = –2

Let’s Practice…Let’s Practice…What is the OS of …

◦sulfur (S) in SF6

◦Mn in MnO41–

◦Fe in Fe3+

Now for a challenge…◦O in KO2.

Balancing redox rxns.Balancing redox rxns.Reduction and oxidation are linked

forever. Yet, it is most helpful and useful to look at them separately.

Conisder a rxn. between Zn and Cu+2

oxidation half-rxn: Zn (s) → Zn2+ (aq) + 2 e–

reduction half-rxn: Cu2+ (aq) + 2 e– → Cu (s)

overall redox rxn: Zn (s) + Cu2+ (aq) → Cu (s) + Zn2+ (aq)

Balancing redox rxns. in acid Balancing redox rxns. in acid solutionsolutionIn a acid solution, you can add H20 and H+

to balance the overall rxn.Here are the rules… 1. Split the rxn. into its two ½-reactions, oxidation &

reduction 2. Balance each ½-rxn.:

◦ balance the atoms other that O and H

◦ add H2O where needed to balance the O

◦ add H1+ where needed to balance the H◦ add e– to the more positive side to make it the same

overall charge is the same on both sides 3. Multiply each half-rxn by a whole number to make the

e– lost = e– gained. 4. Combine the half-rxns and simplify.

Let’s Practice…Let’s Practice…

Voltaic CellsVoltaic CellsWhat would happen if you could

control how an electron was transferred?◦If you could do that, you have

yourself a battery. A.K.A a voltaic cell – developed by Alessandro Volta.

Voltaic CellVoltaic Cell

Voltaic CellVoltaic CellIn a spontaneous (voltaic) cell, electrons flow from the anode to the cathode through the wire, allowing us to divert some of this electron current to do useful work. ◦Anions move thru salt bridge to

anode◦Cations move thru salt bridge to

cathode

How to represent a Voltaic How to represent a Voltaic CellCellYou can represent a voltaic cell

using a simple diagram like this one:

Zn (s) | Zn2+ (aq) || Cu2+ (aq) | Cu (s)

Cell NotationCell Notationshorthand description of Voltaic

cellelectrode | electrolyte || electrolyte

| electrodeoxidation half-cell on left, reduction

half-cell on the rightsingle | = phase barrier

◦if multiple electrolytes in same phase, a comma is used rather than |

◦often use an inert electrodedouble line || = salt bridge

Is there a way to predict the Is there a way to predict the way electrons will flow?way electrons will flow?Absolutely! Some metals are

more active than others. For example:

A closer look at batteries…and that A closer look at batteries…and that

stinkin’ bunny that keepsstinkin’ bunny that keeps goinggoing andand goinggoing and going and going and going and going and going and going and going and going

Alkaline BatteryAlkaline BatteryThe reactions in an alkaline cell

like the one in your calculator are: Zn (s) + 2 OH1– (aq) → Zn(OH)2 (s) + 2 e–

2 MnO2 (s) + H2O + 2 e– → Mn2O3 (s) + 2 OH1– (aq)

Zn (s) + 2 MnO2 (s) + 2 H2O → Zn(OH)2 (s) + 2 MnO(OH) (s)

Alkaline batteries are primary cells, which cannot be recharged.

When most of the reactants have been converted to products, the

cell is “dead.”

Hydrogen fuel-cellHydrogen fuel-cellFuel cells “burn” fuel to produce

electric current instead of heat, and are much more efficient than ordinary combustion.

The overall rxn. :

2 H2 (g) + O2 (g) → 2 H2O(g).

Lead-acid BatteryLead-acid BatteryIs a secondary (rechargable)

battery.

Anode (ox): Pb (s) + SO4-2 (aq)

PbSO4 (s) + 2e-

Cathode (red): PbO2(s) + 4H+(aq) +SO4-2(aq) + 2e- PbSO4 (s)+ 2H2O (l)

Overall: PbO2 (s) + 2H2O (l) PbO2 (s) + Pb (s) + 2H2SO4(aq)

Corrosion Corrosion The corrosion of steel looks like

this:

anode: 2 Fe (s) → 2 Fe2+ (aq) + 4 e–

E° = + 0.440 v cathode: O2 (g) + 2 H2O (l) + 4 e– → 4 OH1–

(aq) E° = + 0.401 v

overall: 2 Fe (s) + O2 (g) + 2 H2O (l) → 2 Fe2+ (aq) + 4 OH1–

(aq) E° = + 0.841 v

Corrosion PreventionCorrosion PreventionBlock moisture or air from

contacting iron.Paint or plate with non-reactive Cu

Offer up a more active metal like Zn◦Galvanized nails – thin coat of Zn;

better electron source◦Shipping boats, underground pipes,

attach pieces of Zn

Exceptions Exceptions Gold and Platinum are called

noble metals (resistant to losing electrons)