Electrolysis - 5th
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what is electrolysis?
• the breaking down of a compound using electricity • the substance to be broken down is connected into a circuit with a cell or a battery
what connects the substance to the circuit?
• two electrodes • the positive electrode is called the anode • the negative electrode is called the cathode
what must the electrodes be made out of?
• the electrodes must be made from a solid that conducts electricity but is inert (unreactive) • usually, one of graphite (cheap) or platinum (expensive) are used
what is the electrolyte?
• the substance to be broken down, and it needs to conduct electricity
what are the only compounds that conduct electricity?
• the ionic compounds that are molten or dissolved so the ions are free to move
why wouldn’t solid ionic compounds conduct electricity?
• because the ions cannot move • covalent compounds won’t work because the molecules aren’t even charged
what are the ions in the compound called?
• positive ions are called cations - they are attracted to the cathode • negative ions are called anions - they are attracted to the anode
what does electrolysis of a molten compound produce?
• its elements • compound -> elements
how is electrolysis thought of?
• in two halves • the anode half and the cathode half
what happens at the anode?
• non-metal ions lose electrons to form molecules • the Cl- ions are attracted to the anode and they lose electrons to form neutral Cl atoms • each Cl- ion has a 1- charge, they lose one electron each to become neutral
what are non-metals made of?
• diatomic molecules • Cl atoms pair up to form Cl2 molecules
how do we write out what happens at the anode?
2Cl- -> Cl2 + 2e-
what happens to the Cl- ions?
• they are oxidised - have lost electrons
what will likely be seen at the anode?
• a pale green gas (Cl2) • bubbles of pale green gas
what happens at the cathode?
• metal ions gain electrons to form atoms • the Mg2+ ions are attracted to the cathode and they gain electrons to form neutral Mg atoms • as each Mg2+ ion has a 2+ charge, they gain two electrons to become neutral
why don’t atoms pair up at the cathode?
• because metals aren’t made up of atons
how do we write out what happens at the cathode?
• as a half-equation, showing the Mg2+ ions gaining electrons and becoming atoms • Mg2+ + 2e- -> Mg
what happens to the Mg2+ ions?
• they have been reduced • they have gained electrons
what would be seen at the cathode?
• Mg is a grey solid, so some grey solid would form on the surface of the cathode
describe what happens in the electrolysis of molten Al2O3. (word equation)
• overall: aluminium oxide -> aluminium + oxygen
what happens at the anode in this reaction?
• 2O2 -> O2 + 4e- • O2- ions are oxidised because they lost electrons • bubbles of colourless gas would be observed at the anode
what happens at the cathode in this reaction?
• Al3+ + 3e- -> Al • Al3+ ions are reduced because they gain electrons • a shiny grey solid would be observed at the cathode
what can water molecules break apart to form?
• to form H+ and OH- ions • H2O (l) -> H+ (aq) + OH- (aq)
what do the extra ions do?
• complicate electrolysis