C3.4 Electrolysis Flashcards
Electrolysis
Process where electric current is passed through a compound causing a chemical change
Also splits ionic compounds
Electrolyte - compound in its liquid state or in solution that can conduct electricity and contains mobile ions
2 electrodes made form metal or graphite to conduct electricity to the electrolyte:
Negative cathode
Positive anode
Electrical supply such as a power pack or battery
Negative electrode
Cathode
Positive electrode
Anode
What happens to positive ions during electrolysis
Gain electrons at cathode and become atoms
What happens to negative ions during electrolysis
Lose electrons at anode and become atoms
What might happen if atoms formed in electrolysis are non metal atoms
Covalent bonds will form between them, making molecules
Positive ions electrolysis name
Cations
Negative ions electrolysis name
Anions
Binary ionic compound
Contains only 2 elements
Discharged
When ions become atoms or molecules at an electrode
Inert electrodes
Don’t change during electrolysis
Made from unreactive metals such as copper and platinum, or graphite
Used for many ionic compounds in their liquid state or aqueous solution
What happens during the electrolysis of water
H2O(l) ⇌ H+(aq) + OH-(aq)
Hydrogen ions discharged at cathode, in gas state
Hydroxide ions discharged at the anode, forming water and oxygen in its gas state
Electroplating
Type of electolysis that uses non inert electrodes
They change during the electrolysis
Anode loses mass
Cathode gains mass
What is cathode in electroplating
The object to coat
What is anode in electroplating
Piece of the metal to coat with
What is electrolyte in electroplating
Solution containing ions of the coating metal
How metals are purified with electrolysis
Both electrodes made from the same metal
Anode: impure metal
Cathode: very pure metal
Cathode gains metal atoms and increases in mass
Anode loses metal atoms and reduces in mass
Impurities collect under the anode
Molten electrolysis
Compound is split up
Positive ion attracted to negative electrode
Negative ion attracted to positive electrode
Ions are discharged at the electrodes - become atoms
Bunsen burner needed
Might not be used as high amount of energy needed in order to get the compound in the molten state.
Aqueous electrolysis
At the Cathode (-)
Least reactive ion discharged (often hydrogen) Link to reactivity series
At the Anode (+)
If halide present, it will be discharged and a halogen is formed
If no halide, then oxygen given off
ONLY 1 ION CAN BE DISCHARGED AT AN ELECTRODE
Reactivity series
Potassium. Sodium. Calcium. Magnesium. Aluminium. (Carbon). Zinc. Iron. Tin. Lead. (Hydrogen). Copper. Silver. Gold
Platinum
Most reactive. Least reactive
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Gold
Platinum
Which electrode does reduction and oxidisation take place?
Reduction - cathode (red-cat)
Oxidisation - anode
Example electrolysis half equations
Lead bromide:
Anode reaction: 2Br- —> Br2 + 2e- : bromine (non metal) is produced
Cathode reaction: Pb²+ + 2e- —> Pb : lead (metal) is produced
(Molten electrolysis)
Sodium chloride:
Anode Reaction: 2Cl- —> Cl2 + 2e-
Cathode Reaction: 2H+ + 2e- —> H2
(Aqueous electrolysis)
Remember format of half equations for anode and cathode.
NB: THESE QUESTIONS CAN BE VERY UNPREDICTABLE AND ARE ONLY 1 OR 2 MARKS IN A TEST. THERE IS NO CLEAR WAY AND WE WERE NOT CLEARLY TAUGHT.
Electrode half equation for Copper Chloride (CuCl2)
Anode: 2Cl- —> Cl2 + 2e-
Cathode: Cu 2+ + 2e- —> Cu
Electrode half equation for Dilute Sodium Chloride
Anode: 2Cl- -> Cl2 + 2e-
Cathode: 2H+ + 2e- -> H2
Electrode half equation for Concentrated Sodium Chloride (NaCl)
Anode: 2Cl- —> Cl2 + 2e-
Cathode: 2H+ + 2e- —> H2
Electrode half equation for Concentrated Copper (II) Sulfate (CuSO4)
Anode: 4OH- -> O2 + 2H2O + 4e-
Cathode: Cu2+ + 2e- —> Cu
Electrode half equation for Sodium Sulfate (Na2SO4)
Anode: 4OH- —> O2 + 2H2O + 4e-
Cathode: 2H+ + 2e- —> H2
Why ionic compounds in the solid state cannot be electrolysed
Do not have ions that are free to move around as they are fixed within the lattice structure
Ionic compounds only conduct electricity when molten or aqueous where ions are free to move around in solution.
Therefore it cannot conduct electricity.
half-equation for the discharge of hydroxide ions
4OH- -> 2H2O + O2 + 4e-
