Electrochemistry Flashcards
Electrolysis
Electrolysis is the decomposition of a liquid electrolyte using a direct current of electricity
PANIC
Positive Anode Negative Is Cathode
Electrolyte
Ionic compound that is molten or dissolved in water
Cathode
Negative electrode
Anode
Positive electrode
Cation
Positive ion
Anion
Negative ion
Inert electrode
Metal strips or graphite rods placed into the liquid and connected to a supply of electricity
Give one use of electrolysis in industry
Extracting aluminium from its ore
How does electrolysis work?
- All electrolytes conduct electricity because they have free ions that can move and carry charge
- ions are attracted to the oppositely charged electrode
- Positive cations at the negative electrode (cathode) gain electrons to become atoms
- When electrons are gained this is reduction
- the negative anions At the positive electrode (anode) lose electrons to become atoms, which may combine to form diatomic molecules for elements e.g chlorine
- When electrons are lost this is oxidation
What are electrodes made from ?
Carbon
Why graphite is used for electrodes
- it is a good conductors of electricity
- It is unreactive
Describe what is observed at the cathode with molten lead (ii) bromide
- Positive ion Pb2+ attracted to negative cathode
- Half equation: Pb2+ + 2e- —> Pb
Observations: silvery grey liquid formed, which sinks to the bottom
Describe what is observed at the anode with molten lead (ii) bromide
Negative ion Br- attracted to positive anode
Half equation - 2Br- —> Br2 + 2e-
Observations - red-brown pungent has evolved
Describe what is observed at the cathode with molten lithium chloride
Positive ion Li+ attracted to negative cathode
Half equation - Li+ + e- —> Li
Observations - molten silvery grey liquid formed
Describe what is observed at the anode with molten lithium chloride
Negative ion Cl- attracted to anode
Half equation - 2Cl- —> Cl2 + 2e-
Observations - pale green pungent gas evolved. Oxidation reaction
Describe the products of electrolysis of dilute sulfuric acid and half equations for the reactions at the anode and cathode
Cathode: Positive ion H+ attracted to cathode
2H+ + 2e- —> H2
Observations - Colourless odourless gas evolved
Anode: Negative ions OH- and SO4 2- attracted to anode
4OH- —> O2 + 2H2O + 4e-
Colourless, odourless gas evolved
State what is observed at the anode during electrolysis of dilute sulfuric acid
The negative ions OH- and SO4 2- are attracted to the anode. The PH- ion is preferentially discharged instead of the SO4 2- ion
It takes four electrons to be removed at the anode to produce one O2 molecule
Why is the volume of hydrogen gas twice the volume of oxygen gas when using electrolysis of dilute sulfuric acid (2)
- because two electrons given up at the cathode produce one H2 molecule, whereas it takes four electrons to be removed at the anode to produce one O2 molecule
- Four electrons lost and gained will produce two H2 molecules for every one O2 molecule
Name aluminium ore
Bauxite
Reaction at the anode and cathode of extracting aluminium
Cathode
Al3+ + 3e- —> Al
Anode
2O2- —> O2 + 4e-
Reaction at cathode is reduction because aluminium ions are gaining electrons
Reaction at anode is oxidation because oxide ions are losing electrons
Why does carbon have to be replaced periodically ?
It wears away as it reacts with oxygen
C + O2 —> CO2
Describe the industrial extraction of aluminium (3)
- Bauxite is purified by mixing it with sodium hydroxide solution
- The alumina (aluminium oxide) in the ore dissolves in molten cryolite to reduce the operating temperature and increase its conductivity
- Crust of aluminium oxide keeps the heat in. Operating temperature is between 900 and 1000 degrees
Disadvantage of electrolysis of aluminium
- Extraction of aluminium is expensive because the cost of electricity is high and a high temperature is needed to keep the aluminium oxide molten
Advantages of recycling aluminium (3)
- Aluminium oxide crust keeps some heat in, saving money
- the expense of recycling aluminium is only a fraction of the cost of producing new aluminium from bauxite
- This is why it’s important to recycle materials such as aluminium, it saves resources, saves energy, prevents waste going to landfill and costs less
Why is cryolite used ?
The use of cryolite increases the conductivity and reduces the operating temperature, saving money
Draw the working out f the products of the electrolysis of dilute sulfuric acid solution
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Definition of electrolyte
The electrolyte is the liquid or solution that conducts electrify and is decomposed by it
Inert electrodes definition
Electrodes that do not take part in the reactions
What are common electrolytes
Molten ionic compounds and aqueous ionic compounds
What happens if a molten metal oxide is electrolysed
Oxygen gas is produced at the anode
Observations include a colourless, odourless gas being evolved
Observations of sodium and iodine at the cathode/anode
Silvery grey liquid formed
Purple, pungent gas
