C3 Topic 3 - Electrolytic Processes

Electrolysis is decomposing compounds using electricity.

Direct current. Needs an electrolyte, can be molten or aqueos ionic substance (electrically charged group of atoms) - free ions to conduct. Electrolyte gets electricty from electrodes. Electrolyte contains compound, which isbroken down into component parts, which are released as molecules/atoms - often a gas.

For an electrical circuit to work, there must be a flow of electrons. Electroms are taken away from the ions at the positive anode (the negative ions - anions) and given to the cations at the cathode. As they gain or lose the electrons, the ions become neutrally charged and become atoms/molecules and are released.

E.g. molten sodium chloride (NaCl) - Sodium (used in street lamps and cooler in nucler reactants) produced at the cathode (negative) as postive ions go there and gain electrons. Chlorine produced at anode (positive) as negative ions go there, loose electrons, pair up.

Electrolysis always has an oxidation and reduction reaction.

Oxidation Reduction Cathode: Na+ + e- = Na

Is Is Anode: 2Cl- = Cl2 + 2e-

Loss Gain

Electrolytes in solution can behave differently to molten ones because: molten ionic salts contain the ions for that salt only, but when aqeous, there are H+ and OH- ions too.

At anode, the simplest ion is discharged, e.g. single element - Cl- not OH-, or with powers.

At cathode, a metal will be discahrged if its less reactive than hydrogen, if not then hydrogen is discharged.

e.g. Electrolysis of Concentrated Sodium Chloride Solution. Na+, Cl-, H+, OH-

Cathode  - two hydrogen ions get electron each and one hydrogen molecule is discharged

Anode - two chloride ions lose electron each and one chlorine molecule is discharged

NaOH Sodium Hydroxide is left in the electrolyte.

Electrolyte

Half equations

Product at cathode

Product at anode

Copper chloride solution CuCl₂

Cathode: Cu²⁺ + 2e^- = Cu

Anode: 2Cl^- = Cl₂ + 2e^-

Copper _(s)

Chloride _(g)

Copper sulphate solution, CuSO₄

Cathode: Cu²⁺ + 2e^- = Cu

Anode: 4OH^- = 2H₂O + O₂ + 4e^-

Copper _(s)

Oxygen _(g)

Sodium sulphate solution Na₂SO₄

Cathode: 2H⁺ + 2e^- = H₂

Anode: 4OH^- = 2H₂O + O₂ + 4e^-

Hydrogen _(g)

Oxygen _(g)

Sodium chloride solution NaCl

Cathode: 2H⁺ + 2e^- = H₂

Anode: 2Cl^- + Cl₂ + 2e^-

Hydrogen _(g)

Chlorine _(g)

Electrolysis is used to obtain pure copper, because when it is obtained by reduction from its ore its not pure enough for use in electric conductors.

The electrolyte is copper II sulfate solution. The cathode starts as a thin peice of pure ion and more pure copper adds to it during the reaction. The anode is a big lump of pure copper which will dissolve.

The electrical supply pulls copper atoms off the anodle, causing them to go back into the solution as Copper 2+ ions. The offering electrons at the cathode near to the Copper 2+ ions turn them back into copper atoms. The impurities are dropped at the anode as a sludge, whilst the pure copper atoms bond to the cathode.

Pure copper is deposited on the negative cathode. Copper ions are reduced to copper atoms by gaining electrons. Cu2+ + 2e- = Cu(s)

Copper dissolves from the impure positive anode. Copper atoms are oxidised into copper ions by losing electrons. Cu(s) = Cu2+(aq) + 2e-

The mass of the anode and cathode will change - movement of copper ions. To measure, weigh anode + cathode before. Put in copper sulfate solution, connect power pack for 5 mins. Remove, dry, weigh anode+cathode. Anode should be less, cathode more.

Electroplating is applying a metal coating to an object using electricity - electrolysis. 

Cathode - object to be plated. Anode - bar of the metal used for plating.

Electrolyte - silver nitrate e.g.  Silver ions in solution migrate to cathode and metal deposited on cup. Anode keeps the silver ions in the solution topped up.

Jewelerry and ornaments are electroplated with gold/silver to improve appearance - shiny.

Household objects e.g. cutlery/cooking utensils are electroplated with unreactive metals to prevent corrossion e.g. nickel or chromium