Physical - 1.11 Electrode Potentials & Electrochemical Cells Flashcards
What is an electrochemical cell?
An electrochemical cell converts chemical energy into electrical energy through redox reactions. It is made up of two half-cells, with each containing an electrode submerged in a solution of ions.
What are the two components of half-cells?
1) An external circuit
2) A salt bridge
What is an external citcuit?
This is a wire that allows the flow of electrons from one electrode to the other, creating an electric current. This current can be measured with a voltmeter.
What is a salt bridge?
This bridge allows ions to transfer between the half-cells, maintaining charge balance. It’s often a filter paper soaked in a neutral electrolyte like potassium nitrate.
Where does the oxidation reaction occur?
Oxidation happens at the anode (the positive electrode)
What does the reduction reaction occur?
Reduction happens at the cathode (the negative electrode)
What are the two main types of half-cell?
1) A metal/metal ion half-cell
2) Ion-ion half cell
What is a metal/metal ion half-cell?
It consists of a metal electrode in contant with a solution of its own ions. E.g. a zinc electrode immersed in a solution of Zn2+ ions. The metal can either be oxidised to its ions or the ions can be reuced to the metal, depending on the metal’s reactivity.
What is an Ion-ion half-cell?
Consists of two ions of the same element in different oxidation states, in contact with an inert platinum electrode. E.g. Fe2+ and Fe3+ ions in solution with a platinum electrode. The platinum electrode provides an inert surface for the electron transfer between the ions of different oxidation states.
What does the electrode potential indicate about the metal?
Metals that oxidise more readily have more negative electrode potentials. Metals that oxidise less readily have less negative or even positive electrode potentials.
What are the key features of the shorthand notation?
1) The half-cell with the more negative potential is listed on the left.
2) A single vertical line indicates a phase boundary between the metal electrode and the aqueous ion.
3) A double vertical line denotes a salt bridge.
4) The oxidised species are written next to the salt bridge (on the right).
What type of electrode acts as a universal reference?
The standard hydrogen electrode acts as a universal reference for measuring electrode potentials.
How is the standard cell potential (E⦵cell) for an electrochemical cell calculated?
1) Write the reduction half-equation for both species.
2) Determine which reaction occurs in the oxidation direction (more negative E⦵) and which occurs in the reduction direction (more positive E⦵).
3) Combine the half-reactions to give the overall redox reaction.
4) Calculate E⦵cell using the equation: E cell = E reduced - E oxidised.
What are the three types of commercial cells?
1) Non-rechargeable cells
2) Rechargeable cells
3) Fuel cells
What are non-rechargeable cells?
In these cells, the chemicals are used up over time and the emf drops. Once one or more of the chemicals have been completely used up, the cell is flat and the emf is 0 volts. These cells cannot be recharge and have to be disposed of after their single use.
What is a zinc-carbon cell?
The standard, cheap non-rechargeable cell but has a fairly short life.
What is an alkaline cell?
This is a non-rechargeable cell, higher cost but has a longer life.
What are rechargeable cells?
In rechargeable cells the reactions are reversible, they are reversed by applying an external current and regenerate the chemicals.
What is a lithium ion cell?
A rechargeable cell used in phones, tablets, cameras, laptops etc.
What is a lead-acid cell?
A rechargeable cell made of six cells and used in cars.
What is a nickel-cadmium cell?
A type of rechargeable cell.
What are fuel cells?
Fuel cells are different to other cells. They have a continuous supply of the chemicals into the cell and so neither run out of chemicals nor need re-charging.
What is a common fuel cell?
The hydrogen-oxygen fuel cell.
What are the benefits of using cells?
Portable source of electrical energy
What are the risks of using cells?
Waste issues
What are the benefits of using non-rechargeable cells?
Cheap
What are the risks of using non-rechargeable cells?
Waste issues
What are the benefits of using re-chargeable cells?
- Less waste
- Cheaper in the long run
- Lower environmental impact
What are the risks of using re-chargeable cells?
Some waste issues (at end of useful life)
What are the advantages of using hydrogen fuel cells?
- Only waste product is water
- Do not need re-charging
- Very efficient
What are the risks of using hydrogen fuel cell?
- Need a constant supply of fuels
- Hydrogen is flammable and explosive
- Hydrogen is usually made using fossil fuels - High cost of fuel cells.
What is an electrochemical series?
A list of electrode potentials in order of decreasing or increasing potential.
What is the acronym for identifying which species is being oxidised and which is being reduced based on the electrode potential?
NO PROBLEM
Negative is Oxidised
Positive is Reduced
