Electrolysis Basics Quiz

Electrolysis

• Electrolysis is the process of using electricity to break down or decompose a compound, typically an ionic compound in molten or aqueous solution.
• Electrolysis occurs in an electrolytic cell.

Learning Objectives

• Define electrolysis.
• Describe electrolysis as evidence of the structure of ionic compounds in the solid, molten, and aqueous states.
• Predict the products of the electrolysis of a molten ionic compound.
• Predict the products of the electrolysis of an aqueous electrolyte using the idea of selective discharge of ions.
• Explain the reactivity series.
• Construct ionic equations for the reactions occurring at the electrodes during electrolysis.

Parts of an Electrolytic Cell

• Battery: Acts as an electron pump, drawing electrons from the anode and supplying them to the cathode. The anode becomes positively charged and the cathode negatively charged.
• Electrodes: Conduct electricity. Usually carbon rods or metal plates. The electrode connected to the positive terminal is the anode, and the electrode connected to the negative terminal is the cathode.
• Electrolyte: Conducts electricity. A molten ionic compound or an aqueous solution. Is decomposed to form positive ions (cations) and negative ions (anions).

How Does Electrolysis Work?

• At the anode:
  • Anions move to the anode.
  • Anions give up electrons at the anode.
  • Oxidation occurs at the anode.
• At the cathode:
  • Cations move to the cathode.
  • Cations receive electrons at the cathode.
  • Reduction occurs at the cathode.
• Within the electrolyte:
  • During electrolysis, the flow of ions towards the electrodes constitutes the flow of electric current through the electrolyte.
• In the external circuit:
  • During electrolysis, electrons flow from the negative terminal to the positive terminal of the battery.

What Happens to Ions at the Electrodes?

• Cations receive electrons at the negatively-charged cathode.
• Anions give up electrons at the positively-charged anode.
• When cations/anions gain/lose electrons at the electrodes, they form atoms or molecules; this is called discharge.

Electrolysis as Evidence of the Structure of Ionic Compounds

• Molten: Ionic compounds conduct electricity when liquefied by heat.
• Solid: When solid NaCl is used, no current flows, indicating that ions are held in place in the lattice structure and cannot conduct electricity.

Electrolysis of Molten Ionic Compounds

• A metal and a non-metal are formed as products.
• The molten compound contains no water molecules.
• At the anode (Example: NaCl): Negatively charged Cl ions are attracted to the anode and lose electrons to form chlorine gas. Oxidation occurs.
• At the cathode (Example: NaCl): Positively charged Na ions are attracted to the cathode, gain electrons to form sodium metal. Reduction occurs.

Inert Electrodes

• Inert electrodes (e.g., carbon, platinum) are used to prevent the produced chlorine (in molten NaCl electrolysis) from reacting with the electrode itself.
• They do not react with the products or with the electrolyte.

Electrolysis of Aqueous Solutions

• An aqueous solution of a compound is a mixture of the compound with water.
• More than one type of cation or anion are present in the electrolyte.
• Only one cation and one anion are preferentially discharged during electrolysis, known as selective discharge of ions.

Selective Discharge of Ions

• Selective discharge depends on three factors:
  • Selective discharge of cations (influenced by reactivity series).
  • Selective discharge of anions (influenced by ease of discharge, concentration).
• The more reactive a metal, the greater its tendency to form ions; hence, ease of discharge of cations is affected by its position in the reactivity series (e.g., H+ ions).
• OH- ions preferentially discharge forming water and oxygen gas in most cases, unless concentration of other anions (like Cl-) is greater.

Predicting the Products of Electrolysis

• Identify the cations and anions in the electrolyte.
• Determine the anion that is discharged.
• Determine the cation that is discharged.
• Identify the remaining ions in solution after electrolysis.

Electrolysis of Dilute NaCl Solution

• At the cathode: H+ ions are preferentially discharged forming hydrogen gas.
• At the anode: OH- ions are preferentially discharged forming oxygen gas and water.

Electrolysis of Concentrated NaCl Solution

• At the cathode: H+ ions are preferentially discharged forming hydrogen gas.
• At the anode: Cl⁻ ions are preferentially discharged forming chlorine gas.

Electrolysis of CuSO4 Solution (Inert Electrodes)

• At the anode: OH⁻ ions are preferentially discharged forming oxygen gas and water.
• At the cathode: Cu²⁺ ions are preferentially discharged forming copper metal.

Electrolysis of CuSO4 Solution (Copper Electrodes)

• At the anode, copper is oxidized, entering the solution as Cu²⁺.
• At the cathode, Cu²⁺ gains electrons getting reduced to metallic copper.
• The anode decreases in mass, whereas the cathode increases in mass.

Purification of Impure Copper

• The impure copper is used as the anode, and a thin sheet of pure copper is used as the cathode.
• The electrolyte is an aqueous solution of copper(II) sulfate.

Electroplating

• The object to be plated is the cathode.
• The anode is the source of the plating metal.
• The electrolyte is an aqueous solution of a salt of the plating metal.

Questions (Examples)

• Students are asked questions about different electrolysis scenarios (e.g., molten lead(II) bromide, molten copper(II) bromide, dilute/concentrated NaCl, etc...)