2.7 Electrochemistry Flashcards
What is the definition of electrolysis?
Electrolysis is the decomposition of a substance due to the passage of electricity through it.
What are the definitions of the following terms: electrolyte; electrodes; inert electrodes; anode; cathode; anions; cations?
Electrolyte- a molten ionic liquid or aqueous solution of an ionic compound dissolved in water. Both these liquids are able to conduct electricity through the movement of positive and negative ions between the electrodes.
Electrodes- conducting rods that carry electricity into and out of electrolyte.
Inert electrodes- inert means they do not take part in any chemical reactions during the electrolysis.
Anode- the positive electrode : connected to the positive terminal of the battery.
Cathode- the negative electrode: connected to the negative terminal of the battery.
Anions- negatively charged ions, so called because they are attracted and move towards the positive electrode, anode, during electrolysis.
Cations- positively charged ions, so called because they are attracted and move towards the negative electrode, cathode, during electrolysis.
What are the electrodes made from?
Carbon graphite or platinum metal.
What are the observations and products formed at both the anode and cathode during the electrolysis of lead (II) bromide?
Anode:
Observation- bubbles of red-brown gas produced.
Name of product- bromine gas (Br2)
Cathode:
Observation- silvery grey globule of lead metal.
Name of product- molten lead metal.
Why are the electrodes made from graphite?
It conducts electricity and is inert so will not react with the electrolyte or products during electrolysis. It also has a very high melting point.
What are the particles responsible for conduction in the electrolytes?
Mobile positive and negatively charged ions carry the current by moving between electrodes.
Why do salts conduct when molten but not when solid?
The ions are free to move between the electrodes to conduct. They are trapped in fixed positions in the solid but free to move when it is molten.
What are the observations and products formed at both the anode and cathode during the electrolysis of lithium chloride and sodium iodide?
LITHIUM CHLORIDE:
Anode:
Observation- bubbles of pale green gas (test with damp blue litmus paper - blue —> red —> bleaches white).
Name of product- Chlorine gas (Cl2).
Cathode:
Observations- silvery grey globule of metal.
Name of product- lithium metal.
SODIUM IODIDE:
Anode:
Observations- bubbles of purple vapour.
Name of product- iodine gas/vapour (I2)
Cathode:
Observations- silvery grey globules of sodium metal.
Name of product- molten sodium metal.
Describe how aluminium is extracted from its ore.
Aluminium is extracted from alumina (aluminium oxide) using electrolysis. The alumina is obtained by purifying bauxite. The aluminium oxide is dissolved in molten cryolite to reduce the melting point to about 950 degrees celcius from apporx. 2050 degrees celcius. The cryolite also increases the conductivity and so the cost of the process is reduced. A crust of solid aluminium oxide also forms on the surface of the electrolysis cell and prevents heat loss, further reducing costs.
What are the half equations at the cathode and anode (for the extraction of aluminium)?
Cathode:
Al3+ + 3e- —> Al
Anode:
2O2- —> O2 + 4e-
What happens to the aluminium and oxide ions during electrolysis?
Aluminium ions go to the cathode where they are reduced to aluminium atoms.
The oxide ions go to the anodes and are oxidised to form oxygen gas.
What happens to the molten aluminium during the extraction of aluminium?
The molten aluminium sinks to the bottom of the cell as it is denser and is tapped off.
What eventually happens to the carbon electrodes during the extraction of aluminium and what is the equation for this?
The carbons anodes burn away gradually and have to be replaced periodically.
Equation: C (s) + O2 (g) —> CO2 (g)
What is the benefit of use of cryolite and the crust of aluminium oxide during the extraction of aluminium?
The extraction of aluminium is expensive because of the cost of electricity is high. The use of cryolite reduces the operating temperature to 950 degrees celcius, saving money. The crust of aluminium oxide keeps heat in, also saving money.
What are the benefits and importance of recycling aluminium?
Benefits: the expense of recycling aluminium is only a fraction of the cost of producing new aluminium from bauxite. A lot of energy in the form of heat and electricity is needed to extract aluminium from bauxite and much less is needed to recycle aluminium we have already have and use it again.
Importance:
1. It is much cheaper than producing aluminium from bauxite, using about 5% of the energy required to create aluminium from its ore.
2. It saves our natural resources of Bauxite.
3. It saves waste and the cost of landfill.
