Question 15
Give reasons for the following changes —
(i) pure water a non-electrolyte — becomes an electrolyte on addition of dil. H2SO4
(ii) Blue colour of aq. CuSO4 — turns almost colourless on it's electrolysis using Pt electrodes.
(i) Pure water is almost a non-electrolyte and will not normally conduct electricity. It consists almost entirely of molecules. It can be electrolytically decomposed by addition of traces of dil. H2SO4 which dissociates as: H+1 and SO42- and help in dissociating water into H+1 and OH- ions, water being a polar solvent. Hence, pure water a non-electrolyte, becomes an electrolyte on addition of dil. H2SO4
(ii) The blue colour of CuSO4 solution is due to the Cu2+ ions. During it's electrolysis using Pt electrodes, at the cathode Cu2+ ions are discharged as neutral copper atoms by accepting electrons. These are not replaced by Cu2+ ions from the Anode because at the Anode, OH- ions are discharged. Hence, the blue colour of CuSO4 solution fades and it becomes almost colourless on it's electrolysis using Pt electrodes.
Chapter Overview: Electrolysis
Electrolysis is the process of decomposing an ionic compound (electrolyte) by passing an electric current through it in its molten or aqueous state. The apparatus consists of an electrolyte, two electrodes (anode and cathode), and a battery. Cations migrate to the cathode (reduction) and anions migrate to the anode (oxidation). The chapter covers electrolysis of molten lead bromide, acidified water, aqueous copper sulphate (with different electrodes), and concentrated HCl. The selective discharge theory explains which ions are preferentially discharged at electrodes when multiple ions are present. The electrochemical series helps predict the order of discharge. Applications include electroplating, electrorefining of metals, and extraction of reactive metals (like aluminium from alumina). Students must write electrode reactions (half-equations) showing electron gain at cathode and electron loss at anode, and understand the difference between electrolytes and non-electrolytes. This chapter requires understanding both the theory and practical applications of electrolysis.
Key Definitions & Electrode Reactions
| Term | Definition |
|---|---|
| Electrolysis | Chemical decomposition of an electrolyte by passing electric current through it |
| Electrolyte | Substance that conducts electricity in molten or aqueous state and is decomposed |
| Cathode | Negative electrode; cations are reduced here (gain electrons) |
| Anode | Positive electrode; anions are oxidised here (lose electrons) |
| Selective Discharge | When multiple ions are present, the ion lower in the electrochemical series is discharged first |
| Electroplating | Coating a metal object with a thin layer of another metal using electrolysis |
| Strong Electrolyte | Completely ionised in solution (e.g., NaCl, HCl, NaOH) |
| Weak Electrolyte | Partially ionised in solution (e.g., CH3COOH, NH4OH) |
Must-Know Concepts
- Electrolysis of acidified water: Cathode: 4H+ + 4e− → 2H2↑; Anode: 4OH− → 2H2O + O2↑ + 4e−
- H2 and O2 are produced in 2:1 volume ratio at cathode and anode respectively
- CuSO4 with Cu electrodes: Cu deposited at cathode, Cu dissolved from anode (copper refining)
- CuSO4 with Pt/C electrodes: Cu deposited at cathode, O2 at anode, solution turns acidic
- Molten PbBr2: Cathode: Pb2+ + 2e− → Pb; Anode: 2Br− → Br2 + 2e−
- Non-electrolytes: sugar solution, alcohol, kerosene (no free ions)
- For electroplating: object to be plated = cathode, plating metal = anode, salt of plating metal = electrolyte
Electrolyte vs Non-Electrolyte
| Property | Electrolyte | Non-Electrolyte |
|---|---|---|
| Bonding | Ionic or polar covalent | Non-polar covalent |
| Ions | Contains free ions | No free ions |
| Conductivity | Conducts when molten/dissolved | Does not conduct |
| Examples | NaCl, HCl, CuSO4 | Sugar, alcohol, urea |
Important Diagrams to Practice
- Electrolysis cell setup with labelled anode, cathode, electrolyte, and battery
- Electrolysis of acidified water showing gas collection in inverted test tubes
- Electroplating setup (e.g., silver plating on a spoon)
- Electrolytic refining of copper
Common Mistakes
- Confusing anode (+) and cathode (−) in electrolysis (opposite of electrochemical cells)
- Writing "oxidation at cathode" (cathode is always reduction in electrolysis)
- Forgetting that solid ionic compounds do not conduct electricity (ions are not free to move)
- Not balancing electrons in half-equations
- Confusing electrolysis of CuSO4 with Cu electrodes vs inert electrodes (products differ)
Scoring Tips
- Always write separate half-equations for cathode and anode reactions with electron transfer shown
- Specify whether electrodes are active (Cu, Ag) or inert (Pt, graphite) as this affects products
- For electroplating questions, clearly state: cathode = object, anode = plating metal, electrolyte = salt
- Remember: OILRIG (Oxidation Is Loss, Reduction Is Gain of electrons)
Frequently Asked Questions
Why is dilute H2SO4 added to water during electrolysis?
Pure water is a very poor conductor of electricity. Adding dilute H2SO4 increases the number of H+ and OH− ions, making the solution a better conductor. The acid itself is not consumed in the process.
Why does the colour of CuSO4 solution fade during electrolysis with inert electrodes?
Cu2+ ions (which give the blue colour) are deposited as copper metal at the cathode. Since the anode is inert, no copper dissolves to replace the Cu2+ ions, so the solution gradually becomes colourless and acidic (H2SO4 remains).
How is aluminium extracted by electrolysis?
Aluminium is extracted from purified alumina (Al2O3) dissolved in molten cryolite (Na3AlF6) by electrolysis. Cryolite lowers the melting point from 2050°C to about 950°C. At cathode: Al3+ + 3e− → Al. At anode: 2O2− → O2 + 4e−.