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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)