electrolysis 5.5
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what is electrolysis?
the breaking down of a compound using electricity
what are the electrodes role?
in electrolysis the substance to be broken down is connected into a circuit with a cell or battery then there are two electrodes which connect the substance into the circuit
what is the positive electrode called?
anode
is the anode positively or negatively charged?
positively charged
what is the negative electrode called?
cathode
is the cathode positively or negatively charged?
negatively charged
what must electrodes be made of?
made from a solid that conducts electricity but is inert (unreactive) cheap electrodes - graphite expensive electrodes - platinum
what is the substance that is going to be broken down called?
substance broken down is called the electrolyte and it needs to conduct electricity
what compounds conduct electricity?
only ionic compounds that are molten or dissolved so the ions are free to move
what ions are attracted to the cathode?
the cathode is negatively charged therefore, the positively charged ions are attracted cations
what ions are attracted to the anode?
the anode is positively charged therefore the negatively charged ions are attracted anions
what does molten electrolysis produce?
compound > elements
example: what will happen in the electrolysis of molten magnesium chloride (MgCl2)?
at the anode: non-metal ions lose electrons to form molecules Cl- ions are attracted to the anode and they lose electrons to form neutral Cl atoms 2Cl- > Cl2 + 2e- Cl- have been oxidised as they have lost electrons some bubbles of pale green gas would be seen at the cathode: metal ions gain electrons to form atoms Mg2+ ions are attracted to the cathode and they gain electrons to form neutral Mg atoms Mg2+ + 2e- > Mg Mg2+ ions have been reduced because they have gained electrons some grey solid would form on the surface of the cathode
what happens in dissolving electrolysis with water molecules?
water molecules can break apart (dissociate) to form H+ and OH- ions H2O(l) > H+(aq) + OH-(aq)
how to work out which ion reacts at anode in dissolving electrolysis:
if a halide ion (Cl-/Br-/I-) is present, that element is produces as normal otherwise , O2 and H2O are produced - 4OH- > O2 + 2H2O + 4e-
how to work out which ion reacts at cathode in dissolving electrolysis?
if the metal is less reactive than hydrogen, that element is produced otherwise, H2 is produced 2H+ + 2e- > H2