ELECTROLYSIS

LEARNING OBJECTIVES

• Understand what electrolysis is and when it occurs

• Identify the components of an electrolysis cell (electrodes, electrolyte)

• Describe the movement of ions and electrons in electrolysis

• Write equations for electrolysis reactions at anode and cathode

• Predict products of electrolysis of different electrolytes

• Understand the applications of electrolysis in industry

KEY DEFINITIONS

• Electrolysis: Chemical decomposition of a compound using electrical current

  
  

• Electrolyte: A compound that conducts electricity when molten or dissolved in water (contains mobile ions)

  
  

• Electrode: A conductor through which electrical current enters or leaves the electrolyte

  
  

• Anode: The positive electrode where oxidation occurs (electrons are lost)

  
  

• Cathode: The negative electrode where reduction occurs (electrons are gained)

  
  

• Oxidation: Loss of electrons

  
  

• Reduction: Gain of electrons

  
  

• Ion: A charged particle formed when an atom loses or gains electrons

WHAT IS NEEDED FOR ELECTROLYSIS

• Electrolyte: A molten or dissolved ionic compound (contains mobile ions)

• Two electrodes: Usually made of carbon (graphite) or metal

• Power source: Connected to create potential difference (battery or power supply)

• Closed circuit: Current must flow through the electrolyte

Key point : The electrolyte must conduct electricity for electrolysis to occur

THE ELECTROLYSIS PROCESS

Step 1: Ions in the electrolyte are free to move

• Positive ions (cations) are attracted to negative electrode (cathode)

• Negative ions (anions) are attracted to positive electrode (anode)

  
  

Step 2: Electrons flow through external circuit

• Electrons flow from power supply to cathode

• Electrons flow from anode back to power supply

  
  

Step 3: Reactions occur at the electrodes

• At cathode (negative): Cations gain electrons (reduction)

• At anode (positive): Anions lose electrons (oxidation)

Step 4: Products are formed

• Neutral atoms or molecules are produced at each electrode

• These products depend on which ions are present

REACTIONS AT THE ELECTRODES

AT THE CATHODE (negative electrode, reduction) :

• Cations move here and gain electrons

• General equation: X⁺ + e⁻ → X (or X₂⁺ + 2e⁻ → X)

• Example: Cu₂⁺ + 2e⁻ → Cu (copper is deposited)

• Example: 2H⁺ + 2e⁻ → H₂ (hydrogen gas is produced)

AT THE ANODE (positive electrode, oxidation) :

• Anions move here and lose electrons

• General equation: X⁻ → X + e⁻ (or X₂⁻ → X + 2e⁻)

• Example: 2Cl⁻ → Cl₂ + 2e⁻ (chlorine gas is produced)

• Example: 4OH⁻ → O₂ + 2H₂O + 4e⁻ (oxygen gas is produced)

Remember :

• Cathode = Reduction = Gain of electrons

• Anode = Oxidation = Loss of electrons

• Cations go to cathode, anions go to anode

ELECTROLYSIS EXAMPLES

ELECTROLYSIS OF MOLTEN LEAD BROMIDE (PbBr₂)

• Electrolyte: Molten PbBr₂ (contains Pb₂⁺ and  B⁻ ions)

• At cathode: Pb₂⁺ + 2e⁻ → Pb (lead metal is deposited)

• At anode: 2Br- → Br₂ + 2e⁻ (bromine liquid is produced)

• Overall: 2PbBr₂ → 2Pb + 2Br₂

ELECTROLYSIS OF COPPER SULFATE SOLUTION (with copper electrodes)

• Electrolyte: CuSO₄ solution (contains Cu²⁺, SO₄²⁻, H⁺, OH⁻ ions)

• At cathode: Cu₂ + 2e- → Cu (copper is deposited on cathode)

• At anode: Cu → Cu₂⁺ + 2e- (copper electrode dissolves)

• Effect: Copper is transferred from anode to cathode (purification)

ELECTROLYSIS OF COPPER SULFATE SOLUTION (with inert electrodes)

• Electrolyte: CuSO₄ solution

• At cathode: Cu₂⁺ + 2e⁻ → Cu (copper is deposited)

• At anode: 4OH⁻ → O2 + 2H₂O + 4e⁻ (oxygen gas is produced)

• Overall: 2CuSO₄ + 2H₂O → 2Cu + O₂ + 2H₂SO₄

ELECTROLYSIS OF DILUTE SODIUM CHLORIDE (brine)

• Electrolyte: Dilute NaCl solution (contains Na⁺, Cl⁻, H⁺, OH⁻ ions)

• At cathode: 2H⁺ + 2e⁻ → H₂ (hydrogen gas is produced)

• At anode: 2Cl⁻ →  Cl₂ + 2e⁻ (chlorine gas is produced)

• In solution: Na⁺ and OH⁻ remain, forming NaOH

• Overall: 2NaCl + 2H₂O → Cl₂ + H₂ + 2NaOH (industrially important)

ELECTROLYSIS OF WATER

• Electrolyte: Dilute sulfuric acid or NaOH solution (provides ions for conductivity)

• At cathode: 2H⁺ + 2e⁻ → H₂ (hydrogen gas)

• At anode: 4OH⁻ → O2 + 2H₂O + 4e- (oxygen gas)

• Ratio: 2 volumes of H₂ : 1 volume of O₂

INDUSTRIAL APPLICATIONS OF ELECTROLYSIS

Chlor-alkali process (production of chlorine, hydrogen, sodium hydroxide)

• Uses: electrolysis of brine (concentrated NaCl solution)

• Products: Cl₂ (bleach),H₂ (fuel), NaOH (alkali)

  
  

Electrorefining of copper

• Purifies impure copper using electrolysis

• Anode: impure copper

• Cathode: pure copper

• Electrolyte: copper sulfate solution

  
  

Electroplating

• Coating objects with thin layer of metal

• Object to be plated is cathode

• Metal to plate with is anode

• Example: gold plating, silver plating, nickel plating

  
  

Extraction of reactive metals

• Electrolysis of molten ionic compounds

• Example: Aluminum extracted by electrolysis of molten Al2O3

• Metals too reactive to be extracted by other methods

FACTORS AFFECTING ELECTROLYSIS

• Voltage: Higher voltage increases rate of electrolysis

• Current: Higher current produces more product in same time

• Concentration of electrolyte: Higher concentration increases conductivity

• Temperature: Higher temperature increases ion movement

• Nature of electrodes: Inert vs. reactive electrodes affect products

• Distance between electrodes: Closer electrodes require less voltage

QUICK REFERENCE: ELECTROLYSIS SUMMARY