Redox and Electrode Potentials
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How do we balance redox half-equations?
• Determine oxidation states • Balance elements changing oxidation states • Balance oxidation state changes with electrons • Balance O with H2O • Balance H with H+
What does a half-cell comprise?
• Electrode (either solid in redox half-eqtn. or inert Pt) • Aqueous solution from redox eqtn.
Why is equilibrium established at a phase boundary in a half-cell?
Electrons are transferred between the two species where metals and their ions make contact
What is standard electrode potential?
A half-cell’s tendency to gain/lose electrons compared to that of the hydrogen half cell under standard conditions
What are the conditions for measuring standard electrode potential?
100kPa, 1.0 mild-3, 298K
What solutions and electrode are used in a hydrogen half cell?
1.00M HCl solution H2 gas input Pt electrode
How do we measure standard electrode potential?
• Connect hydrogen half-cell to the unknown via a voltmeter and a KNO3 salt bridge • This completes the circuit • Measure the recorded voltage
What does the direction of cell potential represent?
The cell’s tendency to gain or lose electrons
What does a more negative E0 value imply?
• The half cell more readily loses electrons • This means that oxidation occurs • In turn equilibrium is shifted to the left
What does a more positive E0 value imply?
• The half cell more readily gains electrons • This means that reduction occurs • In turn equilibrium is shifted to the right
In which direction are half-cell equations written?
Reduction is the forward process
How can we combine half equations using electrode potentials?
• The shifts of equilibrium indicate the overall direction of reaction • Follow the arrows from start to finish
Does an oxidising agent have a more positive or negative E0?
• They cause other systems to be oxidised so they are reduced • They gain electrons so equilibrium shifts forward • They have the more positive E0 value
What limitations affect electrode potential predictions?
• Though a reaction may be feasible thermodynamically its’ activation energy may be too high for it to progress readily • Concentration also affects the position of equilibrium
How might we practically control the cell potential?
By modifying the concentration of one of the solutions to shift equilibrium for one or the other
Why do we use a Pt electrode when determining E0 for a half-cell comprised of 2 aqueous solutions?
The Pt electrode is an inert surface for electron exchange between the two ions
Why is a KNO3 salt bridge used for measuring cell potential?
• It completes the circuit by allowing movement of ions • This allows oxidation/reduction to occur • It is also fairly inert chemically so doesn’t interfere with the experiment notably
How does a salt bridge complete an electrochemical cell?
• Electrons move from -ve to positive • Ions move to their oppositely charged electrode to maintain neutrality • Otherwise one side becomes very +ve and another -ve
What are primary electrochemical cells?
Non-rechargeable cells
Why are primary cells non rechargeable?
The redox reactions that occur between the chemicals they contain are non-reversible
When do primary cells run flat?
• When the reactant chemicals are used up • So cell emf drops to zero
What are secondary electrochemical cells?
These are rechargeable cells
Why are secondary cells rechargeable?
When an external current applies the redox reactions are reversed and the original reactant are restored
Why must we take care when handling lithium ion cells during disposal?
Lithium is a highly reactive metal in oxygen