Chapter 23: Electrode Potentials
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What is a half-cell?
• A half cell contains the species present in a half-equation
What are the 3 types of half-cells, and what does each contain?
• Metal/ metal ion: a metal is dipped into a solution of its metal ions • Ion/ ion: metal ions in different oxidation states are in the solution, and there is a platinum electrode • Non-metal/ ion: glass tube connected to a gas supply of the non-metal, its ions are in solution, and there is a platinum electrode
How are the species in half-cells represented? Show using an example.
• Cu2+ (aq)|Cu (s)
What do cells do, and how are they made?
• They turn chemical energy into electrical energy • By connecting 2 half-cells
How do cells work?
• The electrode that is more likely to release electrons (the stronger reducing agent) is the negative electrode • Oxidation takes place at the negative electrode • It transfers electrons to the positive electrode • Reduction takes place at the positive electrode
What limitation is there in a set up with just 2 half-cells? What is done to prevent this?
• Eventually there would be a build up of positive ions in the negative electrode’s solution and a build up of negative ions in the positive electrode’s solution • A salt bridge is added to slowly release anions and cations to stop too much charge building up in the solutions as they would react with the ions
How would you make a salt bridge?
• Soak filter paper in a solution of concentrated potassium nitrate (or another salt that would not react with either solution)
How are half-equations written for the reactions taking place in a cell?
• They are written as a reversible reaction with the forward reaction showing reduction
How can the tendency of a metal to lose and gain electrons be measured?
• Using standard electrode potential, E^⦵
What is standard electrode potential? Required.
• The e.m.f (energy transfer per unit of charge) of a half cell compared to a hydrogen half-cell
What do negative standard electrode potential values show?
• That the metal is a strong reducing agent
How are standard electrode potentials determined?
• Hydrogen half-cells are taken as having a standard electrode potential of 0V • A hydrogen half-cell is then connected to another half-cell (so that you can find its standard electrode potential) • The electrode potential under standard conditions is then measured using a voltmeter
What are standard conditions when measuring electrode potentials?
• 298K • 100kPa • 1 moldm^-3 (if it’s an ion/ion half cell, both ions have a concentration of 1)
What is the equation for standard cell potential?
• E⦵cell = E⦵(positive electrode) - E⦵(negative electrode)
How can electrode potentials be used to predict the feasibility of a reaction?
• In the half-equation with a more negative electrode potential oxidation will take place • Equilibrium will therefore shift left • In the half-equation with a more positive electrode potential, reduction will take place • Equilibrium will therefore shift right
How can electrode potentials be used to predict the feasibility of a displacement reaction? Use the example of copper reacting with sulfuric acid. (Copper’s electrode potential is positive.)
• In order for copper to react with sulfuric acid, copper would have to displace protons • However, copper’s electrode potential is more positive than hydrogen’s • Therefore, copper ions would not be oxidised and the protons would not be reduced, and so they would not react
What limitations are there to using electrode potentials to predict the feasibility of reactions?
• If the reaction has a very large activation energy, this could mean the rate of reaction is very slow • Changing the concentration might lead to different electrode potential values (the standard electrode potentials would no longer apply)
How would a change in concentration affect the feasibility of a reaction (according to its electrode potential)? Use zinc ions as an example.
• Increasing the concentration of zinc ions to above 1 moldm^-3 causes equilibrium to shift to the right • This removes electrons, and makes the electrode potential less negative • Decreasing the concentration of zinc ions to below 1 moldm^-3 causes equilibrium to shift to the left • This adds electrons, and makes the electrode potential more negative
What 3 categories can cells come under?
• Primary • Secondary • Fuel
What are primary cells?
• Non-rechargeable cells
What are secondary cells? How do they work?
• Rechargeable cells • The reactions that happen inside of them can be reversed during recharging • This regenerates the reactants so the cell can be used again
What do fuel cells do? What makes them different from primary and secondary cells?
• They convert a supply of oxygen and fuel into voltage • They can continuously operate without being charged provided that they have a supply of oxygen and fuel
What do both types of hydrogen fuel cells have in common?
• Water is always the product