C6 - Electrolysis Flashcards
When can ionic compounds carry charge and what are they known as
- When they are molten or aqueous
- This is because their ions can move and conduct electricity
- This substance is now known as the electrolytes
What are the electrodes made of
- A conducting metal or graphite
- These are inert (unreactive) so they don’t interfere with the electrolysis
What are the two different electrodes
- Negative electrode (cathode)
- Positive electrode (anode)
What happens at the positive electrode
- Negative ions travel to the anode
- They discharge by losing electrons (oxidation) and froming an element
What happens at the negative electrode
- Positive ions go to the cathode
- They discharge by gaining electrons (reduction) and forming an element
How is aluminium extracted from aluminium oxide using electrolysis
- Aluminium oxide is mixed with cryolite to lower the melting point which saves money
- Electrodes are made of graphite
- Mixture is then heated until it is molten
- Aluminium is produced at cathode and is then removed
- Oxygen is produced at the anode
Why is carbon(graphite) used as the electrodes in aluminium oxide electrolysis
- Cheap
- Good conductor
Why does the anode need to be replaced often
- The anode is made from graphite (carbon)
- The carbon reacts with the oxygen made here during electrolysis to form CO2 (due to high temps)
- This slowly wears the anode away so needs to be repaced frequently
Half equation at the cathode in the electrolysis of aluminium oxide
Al3+ + 3e- —> Al
The aluminium is reduced (gains electrons)
Half equation at the anode in the electrolysis of aluminium oxide
2O2- —> O₂ + 4e-
Oxygen is oxidised (loses electrons)
Why is electrolysis expenisve
- lots of energy required to melt the ionic compound
- lots of energy is needed to produced the electric current
What reaction happens to water molecules during electrolysis
- They ionise forming hydroxide ions and hydrogen ions
- H₂O ⇌ H+ + OH-
Rule for products at the cathode of the electrolysis of an aqueous solution
- Hydrogen is produced at the cathode if the metal is more reactive than hydrogen
- This is because the more reactive ion stays in the compound
Rule for products at the anode of the electrolysis of an aqueous solution
- If the solution contains halide ions, the halogen will be produced at the electrode
Required pratical for the electrolysis of copper (II) chloride solution
- Set up beaker with 50cm3 of the solution
- Add a petri dish with 2 holes and put graphite electrodes in them
- Attach them to a power pakc and make sure they don’t touch
- Once it is turned on, the cathode will be coated in copper
- The anode will have bubbles of chlorine around it and this can be proved as it turns damp blue litmus paper white
Required pratical for the electrolysis of sodium chloride solution
- Same set up as before
- At the anode, there are bubbles of chlorine as it is a halogen
- At the cathode, there are bubbles of hydrogen
Possible half equations at the cathode for the electrolysis of aqueous solutions
- 2H+ (aq) + 2e- —> H₂ (g)
Possible half equations at the anode for the electrolysis of aqueous solutions
- 4OH- (aq) —> O₂(g) + 2H₂O(l) +4e-
- (If chlorine is used) - 2Cl- (aq) —> Cl₂(g) +2e-
another name for positive ions and negative ions
Positive ions - cations
Negative ions - anions
Full balanced equation for the breakdown of aluminium oxide by electrolysis
- Aluminium oxide —> aluminium + oxygen
- 2Al₂ O3 (l) —> 4Al (l) + 3O₂
Explain the purpose of cryolite
- Al₂O3 must be molten to be electrolysed
- Cryolite reduces the melting point of the mixture
- This means less energy is required to melt it so reduces the costs
