Electrolysis

ELECTROLYSI S CHAPTER 11

ELECTROLYSIS The chemical change that takes place when an electric current passes through a molten or aqueous electrolyte.

ELECTROLYTE A compound which when molten or dissolved in aqueous solution conducts an electric current and is decomposed in the process.

ELECTRODE A metal or graphite rod by which the current enters or leaves an electrolyte. ANODE ( +) The positive electrode from which electrons leave the electrolyte. CATHODE (-) The negative electrode from which electrons enter the electrolyte.

ANION A negatively charged ion which is attracted to the positively charged electrode, the anode.

CATION A positively charged ion that is attracted to the negatively charged electrode, the cathode.

Electrolysis of molten lead (II) bromide Electrolyte: Ions present:

PbBr2 (l) Pb 2+ ,

Br -

Using graphite electrodes: At the cathode:

Pb 2+ + 2e

At the anode:

2Br -

Pb Br2 + 2e

Electrolysis of a binary electrolyte . Step 1. Break the ionic compound into its ions. X m+ and Y n X m+ is call the Cation Y m- is call the Anion. Step 2 When the switch is on , the Cation will migrate to Cathode and accept M electrons from the battery. (Reduction occurs) Anion will migrate to Anode to donate N electrons (oxidation occurs)

Eg : Electrolysis of molten NaCl Cathode: Na++ e Na or 2Na++ e 2Na Anode: Cl-  Cl+ e Or 2Cl-  2Cl+ 2e Hence overall 2Na++ 2Cl-  2Na + Cl2

Selective discharge of ions ❐ Nature of electrode. ❐ Concentration - increase of concentration of an ion tends to promote its discharge. ❐ Position of metal or radical in the ECS - if all other factors remain constant, all ions will be discharged in preference to those above it.

Electrolysis of dilute sulphuric acid Electrolyte:

H2SO4 (aq), equivalent to acidified water

Ions present: H + , SO4 2-

from acid

H + , OH Using platinum electrodes:

from water

At the cathode:

H + ions migrate to it. 2H + + 2e H2

At the anode:

SO4 2- and OH - ions migrate to it.

OH - ions preferentially discharged because of its lower position in the ECS. 4 OH - (aq)

2H2O (l) + O2 (g) + 4e

Electrolysis of dilute sulphuric acid

Note  concentration of acid increases  products are equivalent to those obtained by electrolysis of water  volume of oxygen gas to hydrogen gas obtained is 1 : 2

Electrolysis of concentrated hydrochloric acid Electrolyte:

HCl (aq)

Ions present:

H + , Cl -

from HCl

H + , OH -

from H2O

Using graphite electrodes: At the cathode: H + ions migrate to it. 2H + + 2e H2 At the anode:

Cl - and OH - ions migrate to it.

Cl - ions preferentially discharged due to higher concentration. 2Cl - - 2e

Cl2

Electrolysis of dilute sodium chloride solution Electrolyte: Ions present:

NaCl (aq) Na + , Cl -

from NaCl

H + , OH -

from H2O

Using graphite electrodes: At the cathode:

Na + and H + ions migrate to it. H+ ions preferentially discharged due to its lower position in the ECS. 2H + + 2e H2

At the graphite anode:

Cl - and OH - ions migrate to it.

OH - preferentially discharged due to its lower position in ECS.

4 OH - (aq)

2H2O (l) + O2 (g) + 4e

Electrolysis of concentrated sodium chloride solution Electrolyte:

NaCl (aq)

Ions present:

Na + , Cl -

from NaCl

H + , OH -

from H2O

Using graphite electrodes: At the cathode:

Na + and H + ions migrate to it. H+ ions preferentially discharged due to its lower position in the ECS. 2H + + 2e H2

If mercury cathode is used: •

Na + and H to it.

+

ions migrate

• Na + is preferentially discharged. The sodium + forms a dilute amalgam with Na + e Na

mercury. The amalgam is led into iron tanks containing water where it decomposes to produce sodium hydroxide, hydrogen and mercury. 2Na/Hg (l) + 2H2O(l)

2NaOH(aq) + H2(g) + 2Hg(l)

In the industrial process unreacted sodium chloride crystallised out leaving behind a concentrated solution of sodium hydroxide. The concentrated sodium hydroxide solution is then evaporated to dryness to yield sodium hydroxide pellets. These pellets are used to make soap by reaction with vegetable oil.

At the graphite anode:

Cl - and OH - ions migrate to it. Cl - ions preferentially discharged due to higher concentration.

2Cl - - 2e

Cl2

• Chlorine is observed as a greenish-yellow gas around the anode. • Chlorine is used in the manufacture of bleach, sodium chlorate (I), by reaction with

lectrolysis of copper (II) sulphate solutio Electrolyte:

dilute CuSO4 (aq)

Ions present:

Cu 2+ , SO4 2-

from CuSO4

H+

from water

, OH -

Using graphite electrodes: At the cathode: H + and Cu 2+ ions migrate to it. Cu 2+ preferentially discharged due to lower position in ECS. Cu 2+ (aq) + 2e

Cu (s)

Copper is deposited on the electrode as brown copper metal.

At the graphite anode: • SO4 2 - and OH - ions migrate to it. • OH - preferentially discharged due to its lower position in ECS. • • •

OH - (aq) OH (g) + OH (g) O (g) + O (g)

•

4OH - (aq)

OH (g) + e H2O (l) + O (g) O2 (g) 2H2O (l) + O2 (g) + 4e

Blue colour of solution slowly fades due to removal of Cu 2+ ions. As H + and SO4 2- ions remain in the solution, the resulting solution is sulphuric

ectrolysis of copper (II) sulphate solution Ions present:

Cu 2+ , SO4 2-

from CuSO4

H+

from water

, OH -

Using copper electrodes: At the cathode:

H + and Cu 2+ ions migrate to it.

Cu 2+ preferentially discharged due to lower position in ECS. Cu 2+ (aq) + 2e

Cu (s)

Copper is deposited on the electrode as brown copper metal.

At the copper anode: • SO4 2 - and OH - ions migrate to it. • since copper is able to lose electrons more easily than SO4 2 and OH - ions, copper anode “dissolves” to form Cu 2+ ions. •

Cu (s)

Cu 2+ (aq) + 2e

For every Cu 2+ ion discharged at cathode, one Cu 2+ ion is formed at the anode. There is no change in concentration of Cu 2+ in solution; solution remains blue throughout. Mass of cathode increases, mass of anode decreases.