Electrochemistry Equations

This page collects all electrolysis equations for 0620, organised by system. Use it alongside how electrolysis works and the electrochemistry topic pages.

Key definitions

• Electrolysis: decomposition of an ionic compound by passing electric current through it when molten or in solution
• Cathode: negative electrode — attracts cations (positive ions); reduction occurs here
• Anode: positive electrode — attracts anions (negative ions); oxidation occurs here
• OILRIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons)

Molten ionic compounds

In molten compounds, only the ions of the compound are present.

Lead(II) bromide (PbBr2)

Electrode	Half-equation	Observation
Cathode	Pb2+ + 2e- -> Pb	Silvery molten metal
Anode	2Br- -> Br2 + 2e-	Brown fumes

Aluminium oxide (Al2O3) — dissolved in cryolite

Electrode	Half-equation	Observation
Cathode	Al3+ + 3e- -> Al	Molten aluminium collects at bottom
Anode	2O2- -> O2 + 4e-	Oxygen gas (reacts with carbon anode)

Carbon anodes burn: C + O2 -> CO2 and must be replaced.

Sodium chloride (molten NaCl)

Electrode	Half-equation	Observation
Cathode	Na+ + e- -> Na	Molten sodium metal
Anode	2Cl- -> Cl2 + 2e-	Green-yellow gas

Zinc chloride (molten ZnCl2)

Electrode	Half-equation	Observation
Cathode	Zn2+ + 2e- -> Zn	Grey metal
Anode	2Cl- -> Cl2 + 2e-	Green-yellow gas

Aqueous solutions

In aqueous solutions, H+ and OH- ions from water are also present, competing with the solute ions for discharge.

Discharge rules for aqueous electrolysis

At the cathode (choosing which cation):

• If the metal is more reactive than hydrogen (above H in reactivity series) -> H2 is produced
• If the metal is less reactive than hydrogen -> the metal is deposited
• Exception: concentrated solutions may deposit the metal even if fairly reactive

At the anode (choosing which anion):

• If a halide ion is present (Cl-, Br-, I-) -> the halogen is produced
• If no halide -> OH- is discharged, giving O2

Concentrated sodium chloride solution (brine)

Electrode	Half-equation	Product
Cathode	2H+ + 2e- -> H2	Hydrogen (Na is too reactive)
Anode	2Cl- -> Cl2 + 2e-	Chlorine (halide present)

NaOH remains in solution. Three useful products: Cl2, H2, NaOH.

Dilute sulfuric acid (H2SO4)

Electrode	Half-equation	Product
Cathode	2H+ + 2e- -> H2	Hydrogen
Anode	4OH- -> 2H2O + O2 + 4e-	Oxygen

This is effectively the electrolysis of water. Ratio: H2 : O2 = 2 : 1 by volume.

Copper(II) sulfate with carbon electrodes

Electrode	Half-equation	Product
Cathode	Cu2+ + 2e- -> Cu	Copper metal (pink-brown deposit)
Anode	4OH- -> 2H2O + O2 + 4e-	Oxygen (no halide present)

Solution fades from blue to colourless as Cu2+ ions are removed.

Copper(II) sulfate with copper electrodes (purification)

Electrode	Half-equation	Observation
Cathode	Cu2+ + 2e- -> Cu	Pure copper deposits; mass increases
Anode	Cu -> Cu2+ + 2e-	Anode dissolves; mass decreases

Blue colour stays constant. Impurities fall as anode sludge. See copper purification.

Concentrated copper(II) chloride solution

Electrode	Half-equation	Product
Cathode	Cu2+ + 2e- -> Cu	Copper metal
Anode	2Cl- -> Cl2 + 2e-	Chlorine (halide present)

Electroplating

Electroplating uses electrolysis to coat an object with a thin layer of metal:

• Object to be plated = cathode
• Plating metal = anode
• Electrolyte = solution of plating metal’s salt

Example (silver plating): cathode is the object, anode is silver, electrolyte is silver nitrate solution.

Hydrogen-oxygen fuel cell

Not strictly electrolysis (reverse process), but included here:

At anode: 2H2 -> 4H+ + 4e-

At cathode: O2 + 4H+ + 4e- -> 2H2O

Overall: 2H2 + O2 -> 2H2O

See hydrogen-oxygen fuel cells.

During the electrolysis of concentrated sodium chloride solution, state what is produced at each electrode. Write half-equations. [4 marks]

• Cathode: hydrogen gas [1]
• Half-equation: 2H+ + 2e- -> H2 [1]
• Anode: chlorine gas [1]
• Half-equation: 2Cl- -> Cl2 + 2e- [1]

Explain why the blue colour fades during electrolysis of copper(II) sulfate solution using carbon electrodes. [2 marks]

• Cu2+ ions are discharged/deposited at the cathode as copper metal: Cu2+ + 2e- -> Cu [1]
• The concentration of Cu2+ ions in solution decreases, so the blue colour fades [1]