- Electrolysis Definition: Electrolysis is defined as an electrochemical process where electric current causes ions in an electrolyte to move to opposite electrodes.
- Electrolyte Definition: An electrolyte is a chemical that splits into positive and negative ions when dissolved in water, allowing the solution to conduct electricity.
- Ionic Bonds: Ionic bonds are formed when atoms exchange electrons, creating positive and negative ions that attract each other.
- Electrolysis of Copper Sulfate: During the electrolysis of copper sulfate, copper ions deposit on the cathode, and sulfate ions interact with the anode, demonstrating material transfer.
- Copper vs. Carbon Electrodes: Using copper electrodes deposits copper at the cathode, while carbon electrodes lead to the formation of sulfuric acid and oxygen in the solution.
Electrolysis
The principle of electrolysis is easier to follow after defining an electrolyte and distinguishing ion transport in the liquid from electron transport in the wires. The definition of electrolyte also includes molten ionic substances and solutions formed by ionisation, not only ionic compounds dissolved in water.
Definition of Electrolyte
An electrolyte is a substance whose molten form or solution conducts through mobile ions. Some ionic solids dissociate into their existing ions when they dissolve; some molecular substances ionise by reacting with the solvent. The atoms do not simply split apart. Positive ions are cations and negative ions are anions. Solubility and the degree of dissociation or ionisation depend on the substance, solvent, concentration and temperature.
Principle of Electrolysis
Electrolysis uses an external power supply to drive a non-spontaneous or otherwise controlled electrode reaction. Oxidation always occurs at the anode and reduction at the cathode. In an electrolytic cell connected to a DC source, the anode is normally positive and the cathode negative. Cations migrate toward the cathode, while anions migrate toward the anode, but migration alone does not identify the species that will react. Electrode material, ion concentration and electrode potential also matter.
Two electrodes contact the electrolyte, and an external source applies an electrical potential difference. A suitable DC power supply or battery must provide enough voltage for the required reactions, internal resistance and electrode overpotentials.
The electrodes are electronic conductors connected to the external circuit. At the cathode, a reducible species accepts electrons. At the anode, an oxidisable species releases electrons. The reacting species may be an ion, water, the solvent or the electrode itself. Electrons carry charge in the wires, while ions carry charge through the electrolyte. These coupled paths complete the circuit. Conventional current has the opposite direction to electron motion in the metallic conductors. This charge-transfer process is the basic principle of electrolysis.
Electrolysis of Copper Sulfate
Aqueous copper sulfate, CuSO4, dissociates into hydrated ions. Each CuSO4 formula unit gives one Cu+ + ion and one SO4 − − ion. Modern notation writes these as Cu2+ and SO42−. Water also participates in the possible electrode reactions.
Copper electrodes make both electrode reactions copper-based under normal refining or plating conditions.
Copper ions Cu+ + migrate toward the cathode. At its surface, each Cu+ + ion accepts two electrons and deposits as copper metal. The cathode half-reaction is Cu2+ + 2e− → Cu.
Sulfate SO4 − − moves through the solution to maintain charge balance. At a copper anode, copper metal is oxidised directly to Cu2+ and releases two electrons. Sulfate SO4 − − is not discharged into a neutral SO4 radical under these conditions.
The anode reaction is copper metal forming Cu2+ ions and two electrons. No attack by a neutral SO4 radical is required to explain copper dissolution.
With copper electrodes, copper dissolves at the anode and deposits at the cathode. The SO4 ion remains a spectator, and CuSO4 stays dissociated in solution rather than repeatedly forming intact CuSO4 molecules. The solution contains Cu+ + and SO4 − − ions. Faraday’s law links the ideal deposited mass to total charge; impurities and side reactions can make the measured anode loss and cathode gain differ. During electrolysis of copper sulfate with inert carbon electrodes, copper ions are normally reduced to copper at the cathode while water is oxidised at the anode. This electrolysis produces oxygen gas and hydrogen ions. The sulfate SO4 remains in solution; it does not react as a neutral radical with carbon. Combining the spectator sulfate with increasing hydrogen-ion concentration is a useful bookkeeping description of sulfuric acid formation. Once copper ions become depleted, the cathode reaction can also change. The second SO4 reference in simplified equations must therefore be read as spectator-ion balance, not radical chemistry.
This externally driven set of electrode reactions is electrolysis.
