Overview
Electrolysis of aqueous copper sulfate demonstrates ion discharge at electrodes, with behavior varying based on electrode material. This process is industrially applied to purify copper metal from ore through electrorefining.
Electrolysis with Graphite Electrodes
- Graphite electrodes are inert and do not participate in the reaction.
- Ions present include Cu²⁺, SO₄²⁻, H⁺, and OH⁻ (from water).
- At the cathode, Cu²⁺ is attracted alongside H⁺.
- Copper has lower reactivity than hydrogen, so copper ions are discharged preferentially.
- Reduction occurs: Cu²⁺ + 2e⁻ → Cu (copper metal deposits).
- At the anode, both SO₄²⁻ and OH⁻ are attracted.
- Sulfate ions never discharge at the anode.
- Hydroxide ions are oxidized instead: 4OH⁻ → 2H₂O + O₂ + 4e⁻.
- Oxygen gas and water are produced at the anode.
Electrolysis with Copper Electrodes
- Copper electrodes actively participate in the electrolysis process.
- Same ions are present as with graphite: Cu²⁺, SO₄²⁻, H⁺, OH⁻.
- At the cathode, Cu²⁺ is reduced to copper metal: Cu²⁺ + 2e⁻ → Cu.
- Cathode increases in size as copper layers deposit on it.
- At the anode, copper metal can now be oxidized.
- Copper oxidizes more easily than sulfate or hydroxide ions.
- Oxidation reaction: Cu → Cu²⁺ + 2e⁻ (copper dissolves into solution).
- Copper transfers from anode to cathode via the electrolyte solution.
Copper Refining Process
Setup:
- Anode consists of impure copper ore (rock containing copper).
- Cathode is a thin strip of pure copper metal.
- Electrolyte is aqueous copper sulfate solution.
Process:
- At cathode, Cu²⁺ ions gain electrons and deposit as pure copper.
- Cathode grows progressively larger with pure copper coating.
- At anode, copper atoms from ore oxidize to Cu²⁺ ions.
- Impurities (sand, other minerals) do not dissolve into solution.
- Impurities fall to the bottom as sludge.
- Over time, anode shrinks as copper is extracted.
- Result is high-purity copper metal at the cathode.
| Electrode Setup | Cathode Reaction | Anode Reaction | Outcome |
|---|
| Graphite | Cu²⁺ + 2e⁻ → Cu | 4OH⁻ → 2H₂O + O₂ + 4e⁻ | Copper deposits; oxygen gas produced |
| Copper | Cu²⁺ + 2e⁻ → Cu | Cu → Cu²⁺ + 2e⁻ | Copper transfers from anode to cathode |
| Refining (Copper) | Cu²⁺ + 2e⁻ → Cu | Cu → Cu²⁺ + 2e⁻ | Pure copper extracted from ore |
Key Terms & Definitions
- Electrolysis: Chemical decomposition using direct electric current to drive non-spontaneous reactions.
- Cathode: Negative electrode where reduction occurs; cations attracted here.
- Anode: Positive electrode where oxidation occurs; anions attracted here.
- Inert electrode: Electrode material that does not participate chemically (e.g., graphite).
- Discharge: Ion losing or gaining electrons at electrode surface to form neutral species.
- Preferential discharge: More reactive ion discharged first when multiple species compete.
- Electrorefining: Purification process using electrolysis to separate metals from impurities.