⚡️ Electrochemistry – Electrolysis
Definition
Electrolysis is the process of breaking down an ionic compound into its constituent elements or ions by passing an electric current through it.
It can happen when the compound is molten or dissolved in water (aqueous solution).
Think of it as a “chemical water‑fall” where ions rush to the electrodes, just like water flows downhill under gravity.
🔍 Examination Tip
When asked to write the reaction for an electrolytic cell, always:
- Identify the electrolyte and its state.
- Write the half‑reactions at the anode and cathode.
- Combine them, cancel spectator ions, and check charge balance.
How Electrolysis Works
- Place the ionic compound in a container with two electrodes (cathode and anode).
- Apply a direct electric current.
- Positive ions (\$\mathrm{Na^+}\$, \$\mathrm{Cl^-}\$, etc.) move toward the cathode (negative electrode) and gain electrons (reduction).
- Negative ions move toward the anode (positive electrode) and lose electrons (oxidation).
- The separated ions recombine as elements or new compounds at the electrodes.
Example: Electrolysis of Molten Sodium Chloride
| Electrode | Half‑Reaction |
|---|
| Cathode (−) | \$\displaystyle \mathrm{Na^+ + e^- \rightarrow Na_{(s)}}\$ |
| Anode (+) | \$\displaystyle \mathrm{2Cl^- \rightarrow Cl2{(g)} + 2e^-}\$ |
Overall: \$\displaystyle \mathrm{NaCl{(l)} \xrightarrow{⚡} Na{(s)} + \tfrac{1}{2}Cl2{(g)}}\$ |
Analogy: The Ionic Waterfall
Imagine a river of ions flowing from the source (the electrolyte) to two waterfalls (the electrodes).
The cathode waterfall pulls in positive ions and gives them a boost (electrons), turning them into solid metal.
The anode waterfall pushes out negative ions, which release their electrons and form gases or new compounds.
Just as water changes form at a waterfall, ions change from dissolved to solid or gas during electrolysis.
Common Electrolytes (Aqueous)
- \$\mathrm{H2O}\$ – produces \$\mathrm{H2}\$ at cathode and \$\mathrm{O_2}\$ at anode.
- \$\mathrm{NaCl{(aq)}}\$ – gives \$\mathrm{Na}\$ metal (cathode) and \$\mathrm{Cl2}\$ gas (anode).
- \$\mathrm{CuSO4{(aq)}}\$ – yields \$\mathrm{Cu}\$ metal (cathode) and \$\mathrm{SO_4^{2-}}\$ remains in solution.
- \$\mathrm{ZnCl2{(aq)}}\$ – produces \$\mathrm{Zn}\$ metal (cathode) and \$\mathrm{Cl_2}\$ gas (anode).
Safety Notes
⚠️ Electrolysis can produce hazardous gases (e.g., \$\mathrm{Cl2}\$, \$\mathrm{H2}\$).
Always perform the experiment in a well‑ventilated area or under a fume hood.
Wear safety goggles and gloves to protect against splashes and hot surfaces.
💡 Quick Review
- Electrolysis = decomposition of an ionic compound by electric current.
- Positive ions → cathode (reduction); negative ions → anode (oxidation).
- Overall reaction = sum of half‑reactions; spectator ions cancel out.
Exam Focus
Key points to remember:
- Identify the electrolyte and its state.
- Write balanced half‑reactions with correct electron transfer.
- Combine and simplify to give the overall reaction.
- Explain the role of each electrode (anode vs cathode).