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.

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

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) E^o = 0.34V

Zn²⁺_(aq) +2e^– –> Zn_(s) E^o = -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:

E^o_cell = E^o_cathode  – E^o_anode

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.

Using E^o 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^–                       E^o = 0.54V

                                                   Br₂ + 2e^– –> 2Br^–                         E^o = 1.07V

                                                       Cl₂ + 2e^– –> 2Cl^–                  E^o = 1.36V

I₂ + 2Br^– –> 2I^– + Br₂^–

E^o_cell = E^o_cathode  – E^o_anode

E^o_cell = 0.54 – 1.07

E^o_cell = -0.53V (reaction not possible)

2I^– + Br₂^– –> I₂ + 2Br^–

E^o_cell = E^o_cathode  – E^o_anode

E^o_cell = 1.07 – 0.54

E^o_cell = 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

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