Chemical changes - electrolysis Flashcards
what is electrolysis
splits ionic compounds into the different elements that they are made of using electricity
what happens during electrolysis
an electric current is passed through an electrolyte
- the ions move towards the electrodes where they react and the compound decomposes
what ions in the electrolyte will go to the negative electrode
the positive ion
what ions in the electrolyte will go to the positive electrode
the negative ion
what happens when the ions gain or lose electrons in electrolysis
they form the uncharged element and are discharged from the electrolyte
why cant an ionic solid be electrolysed
because the ions are in fixed positions and can’t move around the structure and carry charge
why can molten / aqueous ionic compounds be electrolysed
because the ions can move freely and carry charge, as the forces of attraction are broken
what ions are reduced and what ions are oxidised
- the positive metal ions are reduced to their element at the cathode as they gain electrons
- the negative non - metal ions are oxidised to their element at the anode
what is the positive electrode called
anode
what is the negative electrode called
cathode
what gas is produced at the anode
oxygen
what gas is produced at the cathode when the metal ion is more reactive than hydrogen
hydrogen gas
how is aluminium electrolysed
- high melting point so mixed with cryolite to lower the melting point
what ore is aluminium extracted from
bauxite
what happens during electrolysis of aluminium
- the positive Al3 + ions are attracted to the cathode, each pick up 3 electrons and turn into neutral aluminium atoms, these then sink to the bottom of the electrolysis tank
- negative O2- ions are attracted to the anode where they each lose 2 electrons, neutral atoms will then combine to form O2 molecules
using half equations, what happens at the anode and cathode during extraction of aluminium
at the cathode - reduction
Al3+ + 3e- —- Al
at the anode - oxidation
2O2- —– o2+ 4e
why do the carbon electrodes need to be replaced regularly during the extraction of aluminium
- anode made of carbon, needs to be replaced regularly as aluminium oxide will react to produce carbon dioxide
- will corrode
what are negatively charged ions called
anions
what are positively charged ions called
cations
why is carbon used in electrodes
cheap and good conductor
disadvantages of electrolysis
- lots of energy needed to melt the solid ionic compound and produce the electrical current
- energy costs money
what is an electrolyte
a form of an ion in melted or dissolved in water
what is produced at the cathode if the metal ion is less reactive than hydrogen
a solid layer of the pure metal will be produced
what is produced at the anode if halides ( Cl-, I - Br - ) are present
gases of bromine, chlorine, iodine etc will be formed
what is produced at the anode if there are no halides present
OH - ions are discharged and oxygen will be formed
what happens during the electrolysis of copper sulfate
contains cu2+ , SO4 2+ , H+ and OH- ions
copper is less reactive than hydrogen so at the cathode, copper metal is produced and coats the cathode
cu 2+ + 2e- — Cu
no halide ions present, so at the anode there will be oxygen gas
4OH- + O2 —- 2H2O + 4e-
what happens during the electrolysis of sodium chloride
contains Na + , Cl - , OH - , H+
- sodium more reactive than hydrogen, so hydrogen gas is produced at the cathode
2H+ + 2e —- H2
- chloride ions are present in the solution, so chlorine gas is formed at the anode
2Cl —- Cl2 + 2e-
how can we test for chlorine
use litmus paper, chlorine will bleach it, turning it white
how can we test for hydrogen
light a splint, hydrogen is present if you hear a “squeaky pop” sound
how to test for oxygen
oxygen will light a glowing splint
What type of reaction happens at the anode (positive electrode) during electrolysis
oxidation
What type of reaction happens at the cathode (negative electrode) during electrolysis
reduction
half equation for the electrolysis of Aluminium oxide
Al3+ + 3e- —> Al (negative cathode)
2O2 —> O2 + 4e- (positive electrode)
why must the anode be replaced regularly
because the oxygen molecules produced at the anode react with graphite (carbon) forming co2 gas
why is electrolysis expensive
- melting the compound requires a great deal of energy
- a lot of energy is required to produce the electric current
