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6.2.7Additional lessons

Electrolytic Cells — IB Chemistry HL Revision Notes & Topical Questions

5 min read 4 parts Lesson 138 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 4Electrolysis Basics

In an electrolytic cell a power supply pushes electrons round the circuit.

Electrolysis
  • Electrolysis The decomposition of an ionic compound by passing an electric current through it as a molten liquid or solution.
Electrolyte
  • Electrolyte The molten or dissolved ionic compound that conducts because its ions are free to move.
Inert electrodes
  • Inert electrodes Graphite or platinum electrodes conduct current but do not take part in the chemical change.
Energy change
  • Energy change Electrical energy is converted into chemical energy, forcing a non-spontaneous reaction to occur.

Part 2 of 4Molten Sodium Chloride

1
Solid sodium chloride does not conduct because the ions are fixed, so it must be melted first.
2
Positive sodium ions move to the negative cathode, where they are reduced: Na⁺ + e⁻ → Na.
3
Negative chloride ions move to the positive anode, where they are oxidised: 2Cl⁻ → Cl₂ + 2e⁻.
4
Electrons travel through the external wire from the anode to the cathode, pushed by the power supply.
5
The overall change is 2NaCl(l) → 2Na(l) + Cl₂(g), which does not happen without the electrical input.

Part 3 of 4Voltaic Versus Electrolytic

Voltaic Versus Electrolytic
Energy conversion
  • Energy conversion Voltaic cells turn chemical energy into electrical energy, whereas electrolytic cells do the opposite.
Spontaneity
  • Spontaneity The voltaic reaction is spontaneous, but the electrolytic reaction is non-spontaneous and needs a power supply.
Polarity
  • Polarity In an electrolytic cell the anode is positive and the cathode is negative, opposite to a voltaic cell.
Constant rules
  • Constant rules In both cells oxidation occurs at the anode, reduction occurs at the cathode, and electrons flow from anode to cathode.
FeatureVoltaic cellElectrolytic cell
AnodeNegativePositive
CathodePositiveNegative
Power sourceNone, it is the sourceExternal supply needed

Part 4 of 4Exam Checklist & Tips

6.2.7 – Key Points for the Exam

  • Electrolysis Decomposition of an electrolyte by an electric current.
  • Electrolyte An ionic compound that conducts electricity when molten or dissolved.
  • Cathode The negative electrode in electrolysis, where reduction occurs.
  • Anode The positive electrode in electrolysis, where oxidation occurs.

Common Mistakes

  • Writing that anions go to the cathode; negative ions move to the positive anode.
  • Saying electrons travel through the molten electrolyte; ions carry the charge there.
  • Forgetting that solid ionic compounds do not conduct.
  • Reversing the polarity of the electrodes when switching between voltaic and electrolytic cells.

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Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A  ⇔  A = I / M  ⇔  I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.

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