Electro chemistry

Prepared by

Odyssa Natividad RM. Molo

Effect of Concentration on Cell emf

• As a voltaic cell is discharged, the Reactant/s

of the rxn are consumed & the Product/s are

generated so the conc of these substances

change. The emf progressively drops until

E=0, at which point, we say the cell is “dead”.

At that point the conc of the R & P cease to

change; they are at eqlbm.

• The emf generated under nonstandard

conditions can be calculated using an eqn

first derived by Walther Nernst, a German

chemist who established many of the

theoretical foundations of electrochemistry.

Nernst Equation

Practice Exercise

• Calculate the emf at 298K generated

by the cell below:

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

where [Cu2+] = 5.0M & [Zn2+] = 0.050M

2) Al(s) + I2(s) Al3+(aq) + I-(aq) where [Al3+] = 4.0 x 10-3M & [I-]=0.010M

3) Zn(s) + Cd2+(aq) Zn2+(aq) + Cd(s)

where [Cd2+] = 0.0750M & [Zn2+] =

0.950M

Cell emf & Chemical Equilibrium

• From Nernst eqn: As R are converted to

P, the value of Q inc, so the value of E

dec. The cell emf eventually reaches E

= 0, where G = 0; system is at eqlbm;

Q = K.

• Rearranging Nernst eqn (@ T = 298K):

Concentration Cells

• Cell based solely on the emf generated because of a difference in a concentration.

• more dilute soln = anode; more conc = cathode.

• Applications: pH meter; regulation of heartbeat in mammals.

Electrolysis

• In contrast to spontaneous

redox reactions, which result in

the conversion of chemical

energy into electrical energy,

electrolysis is the process in

which electrical energy is used

to cause a nonspontaneous

chemical reaction to occur.

Electrolytic Cells

• Is an apparatus for carrying out

electrolysis

• Same principle with galvanic

cells

• Example: Electrolysis of molten NaCl

Electrolysis of water

Electrolysis of an aqueous NaCl soln

Electrolysis of molten NaCl

Electrolysis of molten NaCl

• Molten NaCl is electrolyzed

to form Na metal & Cl gas.

• Cation: Na+ Anion: Cl-

• Electrode: anode (C); cathode (Fe)

• Battery serves as electron pump

• Redox rxn:

• Major source of pure Na metal & Cl gas.

• Theoretical E° value is -4V which means a

minimum of 4V must be supplied by battery to

carry out the rxn.

Electrolysis of Water

Electrolysis of Water

• Water is electrolyzed

to form H & O gas.

• Electrodes: Pt immersed in H2O

• When connected to battery, nothing happens

since not enough ions (1 x10-7 M @ 25°C) in

pure water to carry much of an electric current,

hence 0.1M H2SO4 soln is used.

• Redox rxn:

• No net H2SO4 is consumed. Why?

• What is the minimum voltage needed?

Electrolysis of aqueous NaCl soln

Electrolysis of aqueous NaCl solution

Comparison

• Electrolysis of molten NaCl

• Electrolysis of water

• Electrolysis of an aqueous NaCl

soln

Electrolysis of Mixture of Ions

More Example & Practice

Analysis of Electrolysis

• Cations are likely to be reduced at the

cathode and anions are likely to be oxidized

at the anode.

• In aqueous solutions, water itself may be

oxidized and/or reduced.

• The outcome depends on the nature of other

species present.

• Electrolysis has many applications

in industry, mainly in the extraction

and purification of metals.

Electrochemical vs Electrolytic Cell Electrochemical electrolytic

Electrodes Cathode Site of

reduction

(+ charge)

Site of

reduction

(- charge)

Anode Site of

oxidation

(- charge)

Site of

oxidation

(+ charge)

Ions Cations Migrate to

cathode

Migrate to

cathode

Anions Migrate to

anode

Migrate to

anode

Electron flow anode to

cathode

anode to

cathode

Voltage + voltage

Produces a

voltage

- voltage

Requires a

voltage

source

Spontaneity Spontaneous Non

spontaneous

Quantitative Aspects of Electrolysis

END PART 2