5.4.2 - electrolysis of aqueous compounds

Question 1

what happens to water molecules and what does this mean for electrolysis

Answer 1

they can break apart to form H+ and OH- ions
H2O(l) -> H+(aq) + OH-(aq)

extra ions complicate electrolysis - which ions react and which don’t?

Question 2

what are the rules for finding out which ion reacts at the anode

Answer 2

if a halide ion (Cl-/Br-/I-) is present, that element is produced as per normal
eg. 2Br- -> Br2 + 2e-

otherwise, O2 and H2O are produced, according to the following half equation:
4OH- -> O2 + 2H2O + 4e-

Question 3

what are the rules for finding out which ion reacts at the anode

Answer 3

if the metal is less reactive than hydrogen, that element is produced as per normal
(how to remember - ANYTHING IN JEWELLERY WILL GO TO CATHODE AND BE DISCHARGED OVER H+, eg. copper, silver, gold, platinum)
eg. Cu2+ + 2e- -> Cu

otherwise, H2 is produced according to the following half equation:
2H+ + 2e- -> H2

Question 4

what happens to ions that did not react at anode or cathode

Answer 4

they remain leftover in the solution as a third product

Question 5

what would happen at the anode in molten electrolysis of NaCl

Answer 5

chlorine (Cl-) is a halide ion, so reacts at the anode:
2Cl- -> Cl2 + 2e-
bubbles of pale green gas would be observed

Question 6

what would happen at the cathode in molten electrolysis of NaCl

Answer 6

sodium (Na) is more reactive than hydrogen so we get hydrogen instead:
2H+ + 2e- -> H2
bubbles of colourless gas would be observed

Question 7

what would be leftover in solution in molten electrolysis of NaCl

Answer 7

the Na+ (from NaCl) and OH- ions (from H2O) didn’t react
sodium hydroxide (NaOH) is leftover