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ELECTROCHEMISTRY. The study of the interchange of chemical and electrical energy http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/electroChem/voltaicCell20.html. OIL RIG. o xidation i s l oss, r eduction i s g ain (of electrons)

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Electrochemistry
ELECTROCHEMISTRY

The study of the interchange of chemical and electrical energy

http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/electroChem/voltaicCell20.html


Oil rig
OIL RIG

oxidation is loss, reduction is gain

(of electrons)

Oxidation--the loss of electrons;

increase in charge

Reduction-- the gain of electrons;

reduction of charge


Electrochemistry

Oxidizing agent -the species that is reduced and thus

CAUSES oxidation

Reducing agent- the species that is oxidized and thus CAUSESreduction


Two types of processes

Galvanic (voltaic cells) spontaneous chemical reactions (battery)

Electrolytic cells

non-spontaneous and require

external e-source (DC power source)

Two types of processes


Parts of the voltaic cell
Parts of the voltaic cell

Anode- AnOx

the electrode where oxidation occurs

After a period of time, the anode may appear to become smaller as it falls into solution.

Cathode- RedCat

the electrode where reduction occurs

After a period of time it may appear larger, due to ions from solution plating onto it.

Ca+hode is always positive in galvanic cell.


Inert electrodes
Inert Electrodes

  • used when a gas is involved OR ion

    to ion involved such as:

  • Fe3+ being reduced to Fe2+ rather

    than Fe0

  • made of Pt or graphite


Salt bridge
Salt Bridge

A device used to maintain electrical

neutrality (balance the charge) in a galvanic cell.

This may be filled with agar which

contains a neutral salt or it may be

replaced with a porous cup.Salt is often

KNO3 because it will not form a precipitate.


Electron flow
Electron Flow

always from anode to cathode

(through the wire)

FATCAT

From the Anode To the CAThode


Standard cell notation line notation
Standard cell notation (line notation)

“Ion sandwich” in alphabetical order

Anode metal | anode ion ||

cathode ion | Cathode metal

Zn | Zn2+ (1.0M) || Cu2+ (1.0M) | Cu


If we place mno 4 and fe 2 in the same container
If we place MnO4- and Fe2+ in the same container:

The electrons are transferred directly when the reactants collide. No useful work is obtained from the chemical energy involved which is instead released as heat!


Electrochemistry

We can harness this energy if we separate the oxidizing agent from the reducing agent, thus requiring the e- transfer to occur through a wire!

We can harness the energy that way to run a motor, light a bulb, etc.


Electrochemistry

Sustained electron flow cannot occur in this picture. Why not? As soon as electrons flow, a separation of charge occurs which stops the flow of electrons.


Salt bridge1
Salt Bridge not?

It’s job is to balance the charge using an electrolyte [usually in a U-shaped tube filled with agar that has the salt dissolved into it before it gels].


Porous disk or cup
Porous Disk or Cup not?

… also allows both cells to remain neutral by allowing ions to flow.


Cell potential
Cell Potential not?

Ecell, Emf, or Ecell —

a measure of the electromotive force or the “pull” of the electrons as they travel from the anode to the cathode


Standard reduction potentials
Standard Reduction Potentials not?

Each half-reaction has a cell potential.

Each potential is measured against a

standard which is the standard hydrogen

electrode [consists of a piece of inert

platinum that is bathed by hydrogen gas

at 1 atm].


Standard conditions
Standard Conditions not?

1 atm for gases

1.0M for solutions

25C for all (298 K)

Ecell, Emf, or Ecell become Ecello , Emfo , or Ecello when measurements are taken at standard conditions.


Electrochemistry
Notice that 1.0 M solutions of each salt are used… not?Notice an overall voltage of 1.10 V for the process…


Reading the reduction potential chart
Reading the reduction potential chart not?

Elements that have the most positive

reduction potentials are easily reduced

(in general, non-metals).

Elements that have the least positive

reduction potentials are easily oxidized

(in general, metals).


Electrochemistry

The table can also be used to tell the strength of various oxidizing and reducing agents.

It can also be used as an activity series.

Metals having less positive reduction potentials are more active and will replace metals with more positive potentials.


Electrochemistry

The oxidizing and reducing agents.MORE POSITIVE reduction

potential gets to indeed be reduced

IF you are trying to set up a cell

that can act as a battery.


Electrochemistry

Write both equations oxidizing and reducing agents.AS IS from the

chart with their voltages.

Reverse the equation that will be oxidized

and change the sign of the voltage

[this is now Eoxidation].

Balance the two half reactions.

**do not multiply voltage values**


Electrochemistry

Add the two half reactions and the oxidizing and reducing agents.

voltages together.

Ecell = Eoxidation + Ereduction

° means standard conditions:

1atm, 1M, 25C


Salt bridge2
Salt Bridge oxidizing and reducing agents.

Bridge between cells

whose purpose is

to provide ions to

balance the charge.

Usually made of a salt

filled agar (KNO3) or a

porous cup.



Dependence of cell potential on concentration
Dependence of Cell Potential on Concentration the salt move to the cathode!

Voltaic cells at NONstandard conditions --

LeChatlier’s principle can be applied.

An increase in the concentration of a

reactant will favor the forward reaction

and the cell potential will increase.

The converse is also true!


When cell is not at standard conditions use nernst equation
When cell is not at standard conditions, use the salt move to the cathode!Nernst Equation

E = Eo – RT ln Q

nF

R = Gas constant 8.315 J/K mol

F = Faraday constant

Q = reaction quotient

[productscoefficient]/[reactantscoefficient]

E = Energy produced by reaction

T = Temperature in Kelvins

n = # of electrons exchanged in BALANCED

redox equation


Rearranged another useful form
Rearranged, another useful form the salt move to the cathode!

NERNST EQUATION:

E = E° - 0.0592 log Q@ 25°C(298K)

n


Electrochemistry

As E declines with reactants converting the salt move to the cathode!

to products, E eventually reaches zero.

Zero potential means reaction is at

equilibrium [dead battery].

Also, Q = K AND G = 0 as well.



Electrochemistry

Because the right compartment contains 1.0 M Ag left.+ and the left compartment contains 0.10 M Ag+, there will be a driving force to transfer electrons from left to right.


Silver will be deposited on the right electrode thus lowering the concentration
Silver will be deposited on the right electrode, thus lowering the concentration

of Ag+ in the right

compartment. In the

left compartment the

silver electrode

dissolves [producing

Ag+ ions] to raise

the concentration of

Ag+ in solution.


Concentration cells
Concentration cells lowering the concentration

e-

e-

Ag

Ag

  • Electrons will move from 0.1 M Ag+ to 1 M Ag+

  • Pull Ag+ from 1M side—plating metal

  • Taking e- from Ag electrode will produce more Ag+ on 0.1 M side

0.1 M Ag+

1.0 M Ag+


Free energy
Free Energy lowering the concentration

G = -nFE

G = Gibb’s Free Energy

[Reaction is spontaneous if ΔGis negative]

n = number of mole of electrons

F = 96,500 coulomb/mol e-

E = cell potential 1 volt = Joule/coulomb


Summary of gibb s free energy and cells
Summary of Gibb’s Free Energy and Cells lowering the concentration

  • -Eo implies NONspontaneous

  • +Eo implies spontaneous (would be a good battery!)

  • E = 0, equilibrium reached (dead

    battery)

  • the larger the voltage, the more spontaneous

    the reaction

  • G will be negative in spontaneous reactions

  • K>1 are favored


Two important equations
Two important equations lowering the concentration

G = - nFE [“minus nunfe”]

G = - RTlnK [“ratlink”]

G = Gibbs free energy n = number of moles of electrons.

F = Faraday constant 9.6485309 x 104 J/V (1 mol of

electrons carries 96,500C )

E = cell potential

R = 8.31 J/molK

T = Kelvin temperature

K = equilibrium constant [products]coeff/[reactants]coeff


Favored conditions
Favored conditions lowering the concentration

Ecell > 0 G < 0 K>1


Electrochemistry
ELECTROLYSIS AND ELECTROLYTIC CELLS lowering the concentration[NON spontaneous cells]Purpose: To produce purified forms of elements


Electrolytic cells
Electrolytic cells lowering the concentration

Nonspontaneous

Usually one cell

Two of the same electrodes (graphite is a good choice)

Must have a power source

Use energy to create a chemical change


Electrochemistry

Voltaic cells are spontaneous lowering the concentration

Separated into two cells

Is a battery

Anode is negative

Cathode is positive

AnOx

Red Cat

Fat Cat

Electrolytic cells are forced to work by using a power source

Single container

NEEDS a battery

Anode is positive

Cathode is negative

AnOx

Red Cat

Fat Cat

EPA (electrolytic positive anode)


Electrochemistry

If there is no water present and lowering the concentration

you have a pure molten ionic

compound, then…

The cation will be reduced (gain

electrons/go down in charge).

The anion will be oxidized (lose

electrons/go up in charge).


Electrochemistry

Diagram an electrolytic cell of a molten solution of KI. Be sure to include anode, cathode, and overall cell reactions. Clearly state any observations and label electron flow.


Electrochemistry

  • To predict products in aqueous solutions: sure to include anode, cathode, and overall cell reactions. Clearly state any observations and label electron flow.

  • No IA or IIA metal will ever be oxidized

  • No polyatomic ion will ever oxidize

    Instead– use water

    Memorize these equations:

    Oxidation2H2O  O2 + 4H+ + 4e-

    Reduction2H2O + 2e-  H2 + 2OH-


Electrochemistry

If water is present and you sure to include anode, cathode, and overall cell reactions. Clearly state any observations and label electron flow.

have an aqueous solution of the

ionic compound, then…

You’ll need to figure out if the ions

are reacting, or the water is reacting.

You can always look at a reduction

potential table to figure it out.


Electrochemistry

Draw a diagram of an electrolytic cell containing an aqueous solution of KI. Be sure to label anode, cathode, electron flow and show all half-reactions and the overall reaction. Clearly state any observations at each electrode.


Faraday s law
Faraday’s Law solution of KI. Be sure to label anode, cathode, electron flow and show all half-reactions and the overall reaction. Clearly state any observations at each electrode.

  • The amount of substance being oxidized or reduced at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the cell.

  • Use dimensional analysis

    Conversions to memorize:

  • 1 volt = 1 Joule/Coulomb

  • 1 Amp = 1 Coulomb/second (current)

  • Faraday = 96,500 Coulombs/mole of e-

  • mole of e- come from balanced redox equation


Electrochemistry

A steady current of 1.00 ampere is passed through an electrolytic cell containing a 1 molar solution of AgNO3 and having a silver anode and a platinum cathode until 1.54 grams of silver is deposited.

  • How long does the current flow to obtain this deposit?

  • What weight of chromium would be deposited in a second cell containing 1-molar chromium III nitrate and having a chromium anode and a platinum cathode by the same current in the same time as was used in the silver cell?

  • If both electrodes were platinum in this second cell, what volume of O2 gas measured at STP would be released at the anode while the chromium is being deposited at the cathode? The current and the time are the same as in (a).


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