electrolysis 5.5

Question 1

what is electrolysis?

Answer 1

the breaking down of a compound using electricity

Question 2

what are the electrodes role?

Answer 2

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

Question 3

what is the positive electrode called?

Answer 3

anode

Question 4

is the anode positively or negatively charged?

Answer 4

positively charged

Question 5

what is the negative electrode called?

Answer 5

cathode

Question 6

is the cathode positively or negatively charged?

Answer 6

negatively charged

Question 7

what must electrodes be made of?

Answer 7

made from a solid that conducts electricity but is inert (unreactive)
cheap electrodes - graphite
expensive electrodes - platinum

Question 8

what is the substance that is going to be broken down called?

Answer 8

substance broken down is called the electrolyte and it needs to conduct electricity

Question 9

what compounds conduct electricity?

Answer 9

only ionic compounds that are molten or dissolved so the ions are free to move

Question 10

what ions are attracted to the cathode?

Answer 10

the cathode is negatively charged
therefore, the positively charged ions are attracted

cations

Question 11

what ions are attracted to the anode?

Answer 11

the anode is positively charged
therefore the negatively charged ions are attracted

anions

Question 12

what does molten electrolysis produce?

Answer 12

compound > elements

Question 13

example:
what will happen in the electrolysis of molten magnesium chloride (MgCl2)?

Answer 13

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

Question 14

what happens in dissolving electrolysis with water molecules?

Answer 14

water molecules can break apart (dissociate) to form H+ and OH- ions

H2O(l) > H+(aq) + OH-(aq)

Question 15

how to work out which ion reacts at anode in dissolving electrolysis:

Answer 15

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-

Question 16

how to work out which ion reacts at cathode in dissolving electrolysis?

Answer 16

if the metal is less reactive than hydrogen, that element is produced

otherwise, H2 is produced
2H+ + 2e- > H2