Half equations are always written as reduction processes, with the particle on the left gaining electrons to form the particle on the right
Oxidation states
Half equations always have two different oxidation states
Standard electrode potential (E standard)
A measure of the ability of a species to accept electrons, also known as reduction potential
More positive standard electrode potential
Greater ability to gain electrons
Relative ability to gain electrons
Chlorine (Cl2) most able
Copper (Cu2+) middle
Vanadium (V2+) least able
How the half equation is balanced does not affect the standard electrode potential
Balancing half equations
The important thing is to get the electrons to cancel, not to change the standard electrode potential
Standard electrode potentials give a measure of the ability of a species to gain electrons
Combining two half equations/cells
The more positive standard electrode potential will proceed in the forward direction, forcing the less positive one to go in reverse
Cell voltage (E cell)
Calculated by taking the more positive standard electrode potential and subtracting the less positive one
Determining if chlorine can oxidise Fe2+ to Fe3+
1. Compare standard electrode potentials
2. Cl2 is more positive, so it can oxidise Fe2+ to Fe3+
3. Calculate cell voltage as 0.59V
Determining if iodine can oxidise Fe2+ to Fe3+
1. Compare standard electrode potentials
2. Fe3+/Fe2+ is more positive, so it can oxidise I- to I2
3. Iodine is not a powerful enough oxidising agent
Copper 2+ copper half cell
Copper metal electrode dipping into 1 M aqueous Cu2+ solution
V2+ V half cell
Vanadium metal electrode dipping into 1 M aqueous V2+ solution
Electrochemical cell formation
1. Connect the two half cells with a platinum wire and a salt bridge
2. The more positive standard electrode potential half-reaction will occur in the forward direction, forcing the less positive half-reaction to occur in the reverse direction
More positive standard electrode potential
Substance can gain electrons more readily
Reaction in the electrochemical cell
1. Cu2+ ions are reduced to Cu metal at the cathode
2. V metal is oxidized to V2+ ions at the anode
Anode
The electrode with the less positive (more negative) standard electrode potential
Oxidation reaction occurs here
Cathode
The electrode with the more positive standard electrode potential
Reduction reaction occurs here
Electrons flow from the anode to the cathode through the external circuit
Salt bridge
Allows the flow of ions to maintain charge balance between the two half-cells
Standard hydrogen electrode
Half-cell with a standard electrode potential of 0 V, used as a reference to measure the standard electrode potentials of other half-cells
Measuring standard electrode potentials
1. Connect the unknown half-cell to the standard hydrogen electrode
2. The voltage measured is the standard electrode potential of the unknown half-cell