Electrochemistry

Electrochemistry

Any redox reaction involves the transfer of electrons, which can be used to generate an electric current in a galvanic cell. In fact, all half-reactions can be written with an electrode potential to the side.

Electrochemistry 1

Galvanic Cells vs Electrolytic cell

The main difference between a galvanic cell and an electrolytic cell is the fact that in an electrolytic cell electricity is passed through the solution in order to make a reaction proceed even if this is not energetically favoured while in a galvanic cell the electricity is produced since the reaction is favourable.

The salt bridge is highly important since this:

                Provides electrical continuity

                Prevents mixing of solutions

                Keeps solutions neutral

Electrochemistry 2

Writing a galvanic cell notation (Cell notation)

LHS // RHS

Anode (being oxidised) // Cathode (being reduced)

The substance being oxidised/oxidised substance//The substance being reduced/reduced substance

Fe/Fe2+ // Cu2+/Cu

Cell notation follows the electron flow, from the substance being oxidised to the product of the half-cell, to the substance being reduced to the product of the reduction.

Electrode Potentials

In a galvanic cell, the electrode potential is the voltage produced between the two cells. Each half-reaction has its own potential called an electrode potential which is dependant on the:

               Substances involved

               Concentration of the ions in solution

               Pressure of any gases

               Temperature

The electrode potential is always written as a reduction, such as:

Cu2+(aq) +2e –> Cu(s) Eo = 0.34V

Zn2+(aq) +2e –> Zn(s) Eo = -0.76

With these half-reactions showing the value for a 1Molar solution at 25 oC and 1 atm.

The Potential difference in a galvanic cell being found by the following equation:

Eocell = Eocathode  – Eoanode

It is important to note that a positive value shows a galvanic cell while a negative value shows an electrochemical cell.

Experimental determination of standard electrode potential

The electrode potential can be found by connecting a half cell with a standard hydrogen electrode under standard conditions. The value obtained can then be used to find the value of the half cell. If standard conditions and a 1 Molar solution are used the value would simply be the value obtained on the voltameter.

Electrochemistry 3

Using Eo values

To determine if a given redox reaction takes place

A reaction will only occur if the cell potential is positive.  Some examples are the displacement reactions for the halogens.

                                                       I2 + 2e –> 2I                      Eo = 0.54V

                                                   Br2 + 2e –> 2Br                         Eo = 1.07V

                                                       Cl2 + 2e –> 2Cl                  Eo = 1.36V

I2 + 2Br –> 2I + Br2

Eocell = Eocathode  – Eoanode

Eocell = 0.54 – 1.07

Eocell = -0.53V (reaction not possible)

2I + Br2–> I2 + 2Br

Eocell = Eocathode  – Eoanode

Eocell = 1.07 – 0.54

Eocell = 0.53V (reaction possible)

To determine the order of reactivity of elements/ions

The more positive the electrode potential is, the higher the reducing power.

The more negative the electrode potential is, the higher the oxidising power.

The Nernst Equation

Electrochemistry 4

Electrochemistry 5

This equation can be used to determine the standard electrode potential from a non-standard half cell.

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