IB Chemistry Electrolysis

Electrolytic Cells

Causes reduction to happen at the anode and oxidation to happen at the cathode.

Electrical => Chemical

non-spontaneous reaction

Requires:

-Anode and Cathode

-Solutions

-Salt Bridge

A Voltage Supply

Galvanic Cells

Reduction happens at the cathode and oxidation happens at the anode creating chemical energy.

Chemical => Electrical

Spontaneous reaction.

Requires:

-Anode and Cathode

-Solutions

-Salt Bridge

Anode
The material which experiences oxidation in a galvanic cell and experiences reduction in an electrolytic cell.
Cathode
The material which experiences reduction in a glavanic cell and reduction in an electrolytic cell.
Oxidation

The loss of electrons or an increase in oxidation state by a molecule, atom or ion.

 

Loss of      

Electrons is

Oxidation   

 

Reduction

The gain of electrons or a decrease in oxidation state by a molecule, atom, or ion.

 

Gain of      

Electrons is

Reduction   

Redox Titrations

A chemical analysis used to determine the concentration of a substance.

The titrant is a powerful oxidiaing agent that is added to a reducing agent.

The color change indicates the end-point of the reaction.

Potassium permanganate (KMnO4)

as a titrant in a redox titration.

One of the most useful reactants is potassium permanganate (KMnO4)

MnO4 has a deep purple color

but!!

Mn2+ is colorless.

? end-point is colorless

Potassium Dichromate (KCr2O72-)

as a titrant in a redox titration

 

Cr2O72-is a bright orange color

but!!

Cr3+ is green.

? end-point is green

Iodine (I2)

as a titrant in a redox titration

 

 

I2(aq) is a colored

 

but!!

 

I(aq) is colorless

 

? end-point is colorless

 

Electrolysis and Aqueous Solutions

 

electrode reactions are more difficult to predict

oxidation or reduction of water may occur

ie.(an/red) 2H2O(l) +  2e   =>   H2(g) + 2OH(aq)

 

(cat/ox) 2H2O(l)=> O2(g) + 4H+(aq) + 4e  

 

Inert Electrodes

 

Electrodes made from a solid electrical conductor that will not react with the aqueous potassium nitrate solution.

They are dipped into the cell and then connected to a source of DC electric current.

Electrolysis and Molten Ionic Compounds

Although solid ionic compounds are comprimised of ions, they do not conduct electricity since the ions are fixed/bound in a crystal structure.

Molten ionic compounds are excellent conductors of electricity. Electrolysis occurs when electric current passes through molten ionic compounds.

Electric Charge

A body an electric charge by either gaining or losing electrons.

Electric charge can either be measured in Faradays (F) or coulombs (C)

– a Faraday is a charge of 6.03•1023 e

– a coulomb is a charge of 6.24•1018 e

? 1 F = 9.65•104 C

Electric Current

The flow of electric charge.

If the charge is flowing at the rate of one coulomb per second, the electric charge is defined as being equal to ome ampere (A)

electric current (in A) = charge (in C)

                                       time (in sec)

Q = I • t

Nernst Equation

When all ionic concentrations inside and electrochemical cell are at 1.00 mol/L the cell potential is equal to the standard potential.

For other ionic concentrations, the cell potential is not equal to the standard potential.

The nerst equation is used for calculating non-standard cell potentials

Ecell = Eocell –  0.0592 logQ

              n


Nernst Equation


Ecell = Eocell –  0.0592 logQ

              n

Ecell= non standard cell potential (in volts)

 Eocell = standard (all concentrations = 1mol/L) cell potential (in volts)

n = the number of moles of electrons transferred

Q = the reaction quotient for the reaction

 

Cell Potentials

and

Thermodynamics

standard cell potentials (Eo) can be used to calculate free energy changes and equilibrium constants.

?Go = -nFEocell

?Go=standard free energy change(in J) (at 25o C and 101.3 kPa)

n= the number of moles of electrons transferred

F= the faraday constant (96500 C/mol)

Eocell= standard cell potential (at 25o C and 101.3 kPa)

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