Topic 4.1: Electrolysis Revision Notes

1. Overview

Electrolysis is a fundamental process in chemistry used to decompose ionic compounds into their constituent elements using electricity. This process is essential for the industrial extraction of reactive metals (like aluminum), the production of useful chemicals like chlorine, and for protecting or decorating metals through electroplating.

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Key Definitions

• Electrolysis: The decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.
• Electrolyte: The molten or aqueous substance that undergoes electrolysis; it contains moving ions which carry the charge.
• Anode: The positive electrode.
• Cathode: The negative electrode.
• Anion: A negatively charged ion (attracted to the anode).
• Cation: A positively charged ion (attracted to the cathode).
• Inert Electrode: Electrodes that do not react with the electrolyte or products (usually graphite/carbon or platinum).

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Core Content

The Electrolytic Cell

General Rules for Electrode Products

1. Metals or Hydrogen are always formed at the cathode (negative electrode).
2. Non-metals (other than hydrogen) are always formed at the anode (positive electrode).

Electrolysis of Molten Lead(II) Bromide ($PbBr_2$)

• Electrolyte: Molten lead(II) bromide, $PbBr_2(l)$
• Observations at Anode (+): Brown fumes of bromine gas are evolved.
• Observations at Cathode (-): A silver bead of molten lead is formed.
• Word Equation: lead(II) bromide → lead + bromine
• Symbol Equation: $PbBr_2(l) \rightarrow Pb(l) + Br_2(g)$

Electrolysis of Concentrated Aqueous Sodium Chloride ($NaCl$)

• Electrolyte: Concentrated $NaCl(aq)$ (Brine).
• Observations at Anode (+): Bubbles of pale green gas (chlorine).
• Observations at Cathode (-): Bubbles of colorless gas (hydrogen).
• Note: Sodium remains in the solution as part of sodium hydroxide ($NaOH$).

Electrolysis of Dilute Sulfuric Acid ($H_2SO_4$)

• Electrolyte: $H_2SO_4(aq)$
• Observations at Anode (+): Bubbles of colorless gas (oxygen).
• Observations at Cathode (-): Bubbles of colorless gas (hydrogen).
• Volume Ratio: The volume of hydrogen produced is twice the volume of oxygen ($2:1$ ratio).

Electroplating

• Purpose: To improve appearance (e.g., silver-plated jewelry) and to increase resistance to corrosion (e.g., chromium-plated car parts).
• Process:
  1. The cathode is the object to be plated (e.g., a spoon).
  2. The anode is the pure metal you want to plate with (e.g., silver).
  3. The electrolyte is an aqueous solution of a soluble salt of the plating metal (e.g., silver nitrate).

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Extended Content (Extended Curriculum Only)

Transfer of Charge

• External Circuit: Electrons flow from the positive terminal of the battery to the anode, and from the cathode to the negative terminal.
• Electrolyte: Charge is carried by the movement of ions. Cations ($+$) move to the cathode; Anions ($-$) move to the anode.
• At Electrodes:
  • Anode: Oxidation occurs (loss of electrons). $2Cl^-(aq) \rightarrow Cl_2(g) + 2e^-$
  • Cathode: Reduction occurs (gain of electrons). $Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)$

Electrolysis of Aqueous Copper(II) Sulfate ($CuSO_4$)

1. Using Inert Electrodes (Carbon/Graphite):
   • Cathode: Pink-brown solid copper coats the electrode.
   • Anode: Bubbles of oxygen gas.
   • Observation: The blue color of the solution fades as $Cu^{2+}$ ions are removed.
2. Using Copper Electrodes:
   • Cathode: Increases in mass as copper is deposited.
   • Anode: Decreases in mass as copper atoms dissolve to become ions.
   • Observation: The blue color of the solution remains constant.

Predicting Products in Aqueous Solutions

When multiple ions are present, the "Ease of Discharge" rules apply:

• At the Cathode: The less reactive element is discharged. (Usually $H^+$ vs. a metal; if the metal is above Hydrogen in the reactivity series, $H_2$ gas forms).
• At the Anode:
  • In concentrated halides (e.g., $NaCl$), the halide ion ($Cl^-$, $Br^-$, $I^-$) is discharged.
  • In dilute solutions or solutions with sulfates/nitrates, oxygen is discharged from $OH^-$ ions.

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Key Equations

Process	Location	Half-Equation
Reduction	Cathode	$M^{n+} + ne^- \rightarrow M$
Oxidation	Anode	$2X^- \rightarrow X_2 + 2e^-$
Hydrogen formation	Cathode	$2H^+(aq) + 2e^- \rightarrow H_2(g)$
Oxygen formation	Anode	$4OH^-(aq) \rightarrow O_2(g) + 2H_2O(l) + 4e^-$
Lead formation	Cathode	$Pb^{2+}(l) + 2e^- \rightarrow Pb(l)$
Chlorine formation	Anode	$2Cl^-(aq) \rightarrow Cl_2(g) + 2e^-$

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Common Mistakes to Avoid

• ❌ Wrong: Saying electrons flow through the electrolyte.
  • ✓ Right: Electrons only flow through the wires; ions carry the charge through the electrolyte.
• ❌ Wrong: Confusing the charges of the electrodes.
  • ✓ Right: Use the mnemonic PANIC: Positive Anode, Negative Is Cathode.
• ❌ Wrong: Forgetting that hydrogen is diatomic ($H_2$) and oxygen is diatomic ($O_2$).
  • ✓ Right: Always write them as $H_2(g)$ and $O_2(g)$ in equations.

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Exam Tips

• Command Words:
  • If asked to "State" an observation, write what you see (e.g., "bubbles," "colorless gas," "pink solid"). Do not just name the gas.
  • If asked to "Describe" the electrolysis, mention the movement of ions, the observations at both electrodes, and the names of the products.
• The 2:1 Ratio: In the electrolysis of water or dilute sulfuric acid, there is always twice as much Hydrogen gas (at the cathode) as Oxygen gas (at the anode) because the formula is $H_2O$.
• Common Contexts: Expect questions about the extraction of Aluminum (using cryolite) or the purification of copper.
• State Symbols: Ensure molten substances are $(l)$ and dissolved substances are $(aq)$. Forgetting these is a common way to lose easy marks.