Commercial electrolytic cells Flashcards
what are the electrolytic cells
downs cell, membrane cell, hall heroult cell
features of a downs cell
molten electrolyte, products are continually removed, use density differences to collect sodium, calcium chloride used as a flux, iron used as an anode, electrodes seperated by iron mesh screen
if iron is used as the anode, how come it isnt oxidised
it recieves a continual supply of electrons from the power supply so dont lose electrons, instead they facilitate the transfer of electrons
what happens to the sodium liquid produced in a downs cell
the molten sodium is deposited at the cathode, since it is less dense than the electrolyte, it floats to the surface where is overflows to a seperate container
what does the iron mesh screen allow for in a downs cell
it keeps the produycts apart so they dont react
flux
a substance added to a melt to lower its melting point, making it more fluid at a given temperature
why are aqueous electrolytes used instead of molten electrolytes where possible
as it takes energy to maintain an electrolyte in a molten state, so a cell that uses an aqueous electrolyte is more cost effective
membrane cell
aqueous electrolyte, two compartments are seperated by a semi-permeable membrane, made from a polymer that only allows positive ions to pass through it
membrane cell function to produce sodium from sodium chloride
the semi-permeable membrane prevents Cl- and OH- from coming into contact and chlorine and hydrogen ghas coming into contact, Na+ can pass through
features of a hall heroult cell
alumina soluble in molten cryolite to lower temperature, reduction in temperature also aided by small amount og CaF3, LiF and AlF3, reactive carbon anode
what occurs in a hall heroult cell
a direct current is applied across each cell which moves the ions of alumina (Al3+ and O2-) in opposite directions, the oxide ions are then oxidised to form O2 which reacts with the carbon anode to form CO2, aluminium ions are reduced by electrones in the power supply to produce Al(l)
alumina formula
AlO3
cryolite formula
NaAlF6
why does carbon react in a hall heroult cell
carbon is typically inert under standard conditions, but the high temperature and energy and presence of O2- ions makes carbon electrodes reactive
why does there need to be a specific amount of alumina added to the hall heroult cell
if too little alumina is added, maximum yields and productivity rates of aluminium production become unfavourable, if too much alumina is added, it falls to the bottom of the cell instead of dissolving (due to being denser than molten aluminium) this interferes with the flow of the current
electroplating
the process of coating an object in a metal
what occurs in electroplating
the article to be plated is used as the cathode and the metal being plated onto the article is used as the anode. the electrolyte solution contains a salt of the metal being plated. as the plating proceeds, the anode gradually disappears and maintains the metal ions concentration
why is electrorefiening used
the pure copper from impure copper, impure copper is used as the anode, pure copper is used as the cathode
what occurs in electrorefining
at the anode, electrones are drawn away from the impure copper, copper impurities whcih are stronger reductants than copper are first oxidised, followed by copper which enter the solution of ions. impurities that are weaker reductants than copper wont be oxidised and fall to the bottom of the tank from the anode. at the cathode, copper ions are stonger oxidants so gain electrons to produce pure copper
