A-Level Chemistry AQA Notes

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Electrode Potentials

Redox reactions can be used in electrochemical cells to generate electricity (a flow of charge).
An electrochemical cell is a device capable of generating a potential difference from redox reactions
Electrochemical cells consist of two half-cells. At one oxidation occurs, at the other reduction. Electrons flow between the two cells, driving the redox reaction.

Zn → Zn^2+ + 2e^-
Cu^2+ + 2e^- → Cu

An equilibrium is reached: Cu^2+(aq) + 2e^- ⇌ Cu (s)

Each one of these beakers is a half-cell. A solution in a standard half-cell will have a concentration of 1.00 mol dm^-3
An electrode is a solid surface which allows the transfer of electrons to and from it.
In half cells involving gases, the gas is bubbled through the liquid and an electrode provides a surface for the reaction to occur on.
In half cells with both oxidation states present as aqueous ions, equimolar solutions of both ions are present with a platinum electrode.
The standard electrode potential, E^⦵ is the voltage measured under standard conditions when the half-cell is connected to a standard hydrogen electrode.
Standard conditions includes 298K, 100kPa and 1.00 mol dm^-3.
E^⦵ gives the position of equilibrium, telling us a half-cell’s tendency to accept or release electrons
The voltage measured is also known as the electromotive force of the cell (EMF).

An electrochemical series is a list of standard electrode potentials of all the possible half-cells.

The more negative the electrode potential, the more the oxidation (backwards) reaction is favoured.
E^⦵cell = E^⦵positive terminal - E^⦵negative terminal

Electrochemical Cells

Electrochemical cells can be used as a commercial source of electrical energy
In a rechargeable battery, when the chemicals have reacted fully, a potential difference can be applied to the cell in the opposite direction, which will regenerate the original chemicals
Lithium-ion batteries are rechargeable:

Some cells are non-rechargeable and disposed of when the chemicals have fully reacted.
In fuel cells the chemicals are stored externally and are fed into the cell when electricity is required.
An alkaline hydrogen fuel cell:

Advantages	Disadvantages
1. They are more efficient than burning fossil fuels 2. They release water, which isn’t harmful 3. They do not need to be recharged – they keep producing electricity for as long as they have fuel	1. Energy is needed to build the fuel cells and produce hydrogen – this energy comes from fossil fuels 2. Hydrogen is highly flammable so needs to be carefully handled

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