Topic 4: Electrochemistry

📝 Study Notes

<div class="main-heading">📌 Topic 4: Electrochemistry</div>

<div class="sub-heading">4.1 Electrolysis</div>

<div class="point-item">
<div class="point-question">
<span><span class="point-number">1</span> Define electrolysis as the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current</span>
<span class="icon">▼</span>
</div>
<div class="point-answer">
<div class="answer-content">
<p><strong>Definition:</strong> Electrolysis is the <strong>decomposition</strong> of an ionic compound when <strong>molten</strong> or in <strong>aqueous solution</strong> by the passage of an <strong>electric current</strong>.</p>
<p><strong>Why Only Ionic Compounds?</strong></p>
<ul><li><strong>Solid state:</strong> Ions are fixed in place in the lattice, cannot move → no conductivity</li><li><strong>Molten or dissolved:</strong> Ions are <strong>free to move</strong> and carry charge → conducts electricity</li></ul>
<div class="key-definition">💡 Covalent compounds cannot undergo electrolysis because they have no free ions or electrons to carry charge.</div>
</div>
</div>
</div>

<div class="point-item">
<div class="point-question">
<span><span class="point-number">2</span> Identify in simple electrolytic cells: (a) the anode as the positive electrode, (b) the cathode as the negative electrode, (c) the electrolyte as the molten or aqueous substance that undergoes electrolysis</span>
<span class="icon">▼</span>
</div>
<div class="point-answer">
<div class="answer-content">
<p><strong>Key Terms:</strong></p>
表
<tr style="background:#1E3A8A; color:white;"><th>Term</th><th>Definition</th><th>What it attracts</th> 表
<tr><td><strong>Electrode</strong></td><td>Rod (metal or graphite) that conducts electricity into/out of the electrolyte</td><td>-</td> </tr>
<tr style="background:#F9FAFB;"><td><strong>Anode</strong></td><td><strong>Positive</strong> electrode</td><td>Anions (negative ions)</td> </tr>
<tr><td><strong>Cathode</strong></td><td><strong>Negative</strong> electrode</td><td>Cations (positive ions)</td> </tr>
<tr style="background:#F9FAFB;"><td><strong>Electrolyte</strong></td><td>Ionic compound in molten or dissolved solution that conducts electricity</td><td>-</td> </tr>
<tr><td><strong>Anion</strong></td><td>Negatively charged ion</td><td>Attracted to anode</td> </tr>
<tr style="background:#F9FAFB;"><td><strong>Cation</strong></td><td>Positively charged ion</td><td>Attracted to cathode</td> </tr>
表
<div class="key-definition">💡 <strong>Memory Aid:</strong> RED CAT → REDuction at CAThode, AN OX → ANode for OXidation</div>
</div>
</div>
</div>

<div class="point-item">
<div class="point-question">
<span><span class="point-number">3</span> Identify the products formed at the electrodes during the electrolysis of: (a) molten lead(II) bromide, (b) concentrated aqueous sodium chloride, (c) dilute sulfuric acid</span>
<span class="icon">▼</span>
</div>
<div class="point-answer">
<div class="answer-content">
<p><strong>(a) Molten Lead(II) Bromide (PbBr₂)</strong></p>
表
<tr style="background:#1E3A8A; color:white;"><th>Electrode</th><th>Ion</th><th>Product</th><th>Observation</th> 表
英语<td>Cathode (−)</td><td>Pb²⁺</td><td>Grey lead metal</td><td>Lead deposits on electrode</td> </tr>
<tr style="background:#F9FAFB;"><td>Anode (+)</td><td>Br⁻</td><td>Brown bromine gas</td><td>Bubbling at anode, pungent smell</td> </tr>
表
<p><strong>(b) Concentrated Aqueous Sodium Chloride (Brine)</strong></p>
表
<tr style="background:#1E3A8A; color:white;"><th>Electrode</th><th>Ion Discharged</th><th>Product</th><th>Observation</th> 表
英语<td>Cathode (−)</td><td>H⁺ (from water)</td><td>Hydrogen gas</td><td>Bubbling</td> </tr>
<tr style="background:#F9FAFB;"><td>Anode (+)</td><td>Cl⁻</td><td>Chlorine gas</td><td>Bubbling, pungent smell, bleaches damp litmus</td> </tr>
表
<p><strong>(c) Dilute Sulfuric Acid (H₂SO₄)</strong></p>
表
<tr style="background:#1E3A8A; color:white;"><th>Electrode</th><th>Ion Discharged</th><th>Product</th><th>Observation</th> 表
英语<td>Cathode (−)</td><td>H⁺</td><td>Hydrogen gas</td><td>Bubbling, squeaky pop test</td> </tr>
<tr style="background:#F9FAFB;"><td>Anode (+)</td><td>OH⁻ (from water)</td><td>Oxygen gas</td><td>Bubbling, relights glowing splint</td> </tr>
表
<div class="key-definition">💡 Volume ratio in dilute sulfuric acid: Hydrogen : Oxygen = 2 : 1</div>
</div>
</div>
</div>

<div class="point-item">
<div class="point-question">
<span><span class="point-number">4</span> State that metals or hydrogen are formed at the cathode and that non-metals (other than hydrogen) are formed at the anode</span>
<span class="icon">▼</span>
</div>
<div class="point-answer">
<div class="answer-content">
<p><strong>At the Cathode (Negative Electrode):</strong></p>
<ul><li><strong>Metals</strong> are produced (if metal ions are discharged)</li><li><strong>Hydrogen</strong> is produced (if H⁺ ions are discharged)</li></ul>
<p><strong>At the Anode (Positive Electrode):</strong></p>
<ul><li><strong>Non-metals</strong> (other than hydrogen) are produced</li><li>Examples: Chlorine (Cl₂), Bromine (Br₂), Iodine (I₂), Oxygen (O₂)</li></ul>
<div class="key-definition">💡 The product at each electrode depends on which ions are discharged first (based on reactivity).</div>
</div>
</div>
</div>

<div class="point-item">
<div class="point-question">
<span><span class="point-number">5</span> Predict the identity of the products at each electrode for the electrolysis of a binary compound in the molten state</span>
<span class="icon">▼</span>
</div>
<div class="point-answer">
<div class="answer-content">
<p><strong>Rule:</strong> For molten binary compounds (two elements), the products are always the elements themselves.</p>
<p><strong>Worked Example 1: Molten Potassium Chloride (KCl)</strong></p>
<ul><li>Cathode: K⁺ + e⁻ → K (potassium metal)</li><li>Anode: 2Cl⁻ → Cl₂ + 2e⁻ (chlorine gas)</li></ul>
<p><strong>Worked Example 2: Molten Magnesium Oxide (MgO)</strong></p>
<ul><li>Cathode: Mg²⁺ + 2e⁻ → Mg (magnesium metal)</li><li>Anode: 2O²⁻ → O₂ + 4e⁻ (oxygen gas)</li></ul>
<div class="key-definition">💡 The metal is always produced at the cathode; the non-metal is always produced at the anode.</div>
</div>
</div>
</div>

<div class="point-item">
<div class="point-question">
<span><span class="point-number">6</span> State that metal objects are electroplated to improve their appearance and resistance to corrosion</span>
<span class="icon">▼</span>
</div>
<div class="point-answer">
<div class="answer-content">
<p><strong>What is Electroplating?</strong> Coating one metal with a layer of another metal using electrolysis.</p>
<p><strong>Reasons for Electroplating:</strong></p>
<ul><li><strong>Corrosion protection:</strong> Prevents rusting/oxidation</li><li><strong>Improved appearance:</strong> Makes objects look more attractive</li><li><strong>Increased durability:</strong> Adds a hard, wear-resistant layer</li></ul>
<p><strong>Examples:</strong> Galvanising (zinc on steel), chromium plating, silver plating, gold plating</p>
</div>
</div>
</div>

<div class="point-item">
<div class="point-question">
<span><span class="point-number">7</span> Describe how metals are electroplated</span>
<span class="icon">▼</span>
</div>
<div class="point-answer">
<div class="answer-content">
<p><strong>Setup for Electroplating:</strong></p>
<ul><li><strong>Cathode (−):</strong> Object to be plated</li><li><strong>Anode (+):</strong> Pure metal to be plated</li><li><strong>Electrolyte:</strong> Aqueous salt of the plating metal</li></ul>
<p><strong>Example: Plating Iron with Tin</strong></p>
<ul><li>Cathode: Iron strip (object to be plated)</li><li>Anode: Tin metal</li><li>Electrolyte: Tin(II) chloride solution (SnCl₂)</li></ul>
<p><strong>Half-Equations:</strong></p>
<ul><li>Cathode: Sn²⁺(aq) + 2e⁻ → Sn(s) (tin deposited on iron)</li><li>Anode: Sn(s) → Sn²⁺(aq) + 2e⁻ (tin dissolves)</li></ul>
<div class="key-definition">💡 The mass of the cathode increases (metal deposited), while the mass of the anode decreases (metal dissolves).</div>
</div>
</div>
</div>

<div class="point-item">
<div class="point-question">
<span><span class="point-number">8</span> Describe the transfer of charge during electrolysis <span class="supplement-badge">Supplement</span></span>
<span class="icon">▼</span>
</div>
<div class="point-answer">
<div class="answer-content">
<p><strong>How Charge Flows in Electrolysis:</strong></p>
<p><strong>(a) External Circuit:</strong> Electrons flow from the negative terminal of the power supply to the cathode, then from the anode back to the positive terminal.</p>
<p><strong>(b) At the Electrodes:</strong></p>
<ul><li><strong>Cathode (−):</strong> Cations gain electrons → <strong>reduction</strong></li><li><strong>Anode (+):</strong> Anions lose electrons → <strong>oxidation</strong></li></ul>
<p><strong>(c) In the Electrolyte:</strong> Ions move and carry charge</p>
<ul><li><strong>Cations (+)</strong> move towards the cathode</li><li><strong>Anions (−)</strong> move towards the anode</li></ul>
<div class="key-definition">💡 <strong>Memory Aids:</strong> OIL RIG (Oxidation Is Loss, Reduction Is Gain), RED CAT (REDuction at CAThode), AN OX (ANode for OXidation)</div>
</div>
</div>
</div>

<div class="point-item">
<div class="point-question">
<span><span class="point-number">9</span> Identify the products during electrolysis of aqueous copper(II) sulfate using inert electrodes and using copper electrodes <span class="supplement-badge">Supplement</span></span>
<span class="icon">▼</span>
</div>
<div class="point-answer">
<div class="answer-content">
<p><strong>Using Inert Electrodes (Graphite/Platinum):</strong></p>
表
<tr style="background:#1E3A8A; color:white;"><th>Electrode</th><th>Ion Discharged</th><th>Product</th><th>Observation</th> 表
英语<td>Cathode (−)</td><td>Cu²⁺</td><td>Copper metal</td><td>Pink/brown deposit on electrode</td> </tr>
<tr style="background:#F9FAFB;"><td>Anode (+)</td><td>OH⁻ (from water)</td><td>Oxygen gas</td><td>Bubbling, relights glowing splint</td> </tr>
表
<p><strong>Half-Equations:</strong> Cathode: Cu²⁺(aq) + 2e⁻ → Cu(s) | Anode: 4OH⁻(aq) → O₂(g) + 2H₂O(l) + 4e⁻</p>
<p><strong>Using Copper Electrodes (Active Electrodes):</strong></p>
表
<tr style="background:#1E3A8A; color:white;"><th>Electrode</th><th>Process</th><th>Observation</th> 表
英语<td>Cathode (−)</td><td>Cu²⁺ reduced to Cu</td><td><strong>Gains mass</strong> (copper deposited)</td> </tr>
<tr style="background:#F9FAFB;"><td>Anode (+)</td><td>Cu oxidised to Cu²⁺</td><td><strong>Loses mass</strong> (copper dissolves)</td> </tr>
表
<div class="key-definition">💡 The concentration of Cu²⁺ ions remains constant because copper dissolving at the anode replaces copper deposited at the cathode.</div>
</div>
</div>
</div>

<div class="point-item">
<div class="point-question">
<span><span class="point-number">10</span> Predict the products for electrolysis of a halide compound in dilute or concentrated aqueous solution <span class="supplement-badge">Supplement</span></span>
<span class="icon">▼</span>
</div>
<div class="point-answer">
<div class="answer-content">
<p><strong>Rule for Anode Products:</strong> The concentration of the solution affects which ion is discharged at the anode.</p>
表
<tr style="background:#1E3A8A; color:white;"><th>Solution</th><th>Ions Present</th><th>Product at Anode</th><th>Reason</th> 表
英语<td><strong>Concentrated halide</strong></td><td>Cl⁻, Br⁻, I⁻</td><td>Halogen gas (Cl₂, Br₂, I₂)</td><td>Halide ions are discharged</td> </tr>
<tr style="background:#F9FAFB;"><td><strong>Dilute halide</strong></td><td>Cl⁻, Br⁻, I⁻ + OH⁻</td><td>Oxygen gas</td><td>OH⁻ is discharged instead</td> </tr>
表
<p><strong>Examples:</strong></p>
<ul><li><strong>Concentrated NaCl:</strong> Cl⁻ discharged → chlorine gas at anode</li><li><strong>Dilute NaCl:</strong> OH⁻ discharged → oxygen gas at anode</li></ul>
<div class="key-definition">💡 Halide ions are discharged at the anode in <strong>concentrated</strong> solutions. In dilute solutions, OH⁻ ions are discharged instead.</div>
</div>
</div>
</div>

<div class="point-item">
<div class="point-question">
<span><span class="point-number">11</span> Construct ionic half-equations for reactions at the anode and cathode <span class="supplement-badge">Supplement</span></span>
<span class="icon">▼</span>
</div>
<div class="point-answer">
<div class="answer-content">
<p><strong>Common Half-Equations:</strong></p>
表
<tr style="background:#1E3A8A; color:white;"><th>Product</th><th>Half-Equation</th><th>Type</th> 表
英语<td>Hydrogen</td><td>2H⁺ + 2e⁻ → H₂</td><td>Reduction (cathode)</td> </tr>
<tr style="background:#F9FAFB;"><td>Metal (e.g., Cu)</td><td>Cu²⁺ + 2e⁻ → Cu</td><td>Reduction (cathode)</td> </tr>
<tr><td>Chlorine</td><td>2Cl⁻ → Cl₂ + 2e⁻</td><td>Oxidation (anode)</td> </tr>
<tr style="background:#F9FAFB;"><td>Bromine</td><td>2Br⁻ → Br₂ + 2e⁻</td><td>Oxidation (anode)</td> </tr>
<tr><td>Iodine</td><td>2I⁻ → I₂ + 2e⁻</td><td>Oxidation (anode)</td> </tr>
<tr style="background:#F9FAFB;"><td>Oxygen (from OH⁻)</td><td>4OH⁻ → O₂ + 2H₂O + 4e⁻</td><td>Oxidation (anode)</td> </tr>
表
<p><strong>Worked Example: Molten Lead(II) Bromide</strong></p>
<ul><li>Cathode: Pb²⁺ + 2e⁻ → Pb(l)</li><li>Anode: 2Br⁻ → Br₂(g) + 2e⁻</li></ul>
<p><strong>Worked Example: Concentrated Sodium Chloride</strong></p>
<ul><li>Cathode: 2H⁺ + 2e⁻ → H₂(g)</li><li>Anode: 2Cl⁻ → Cl₂(g) + 2e⁻</li></ul>
</div>
</div>
</div>

<div class="sub-heading">4.2 Hydrogen-oxygen fuel cells</div>

<div class="point-item">
<div class="point-question">
<span><span class="point-number">1</span> State that a hydrogen-oxygen fuel cell uses hydrogen and oxygen to produce electricity with water as the only chemical product</span>
<span class="icon">▼</span>
</div>
<div class="point-answer">
<div class="answer-content">
<p><strong>What is a Fuel Cell?</strong> An electrochemical cell that converts chemical energy from a fuel into electrical energy.</p>
<p><strong>Hydrogen-Oxygen Fuel Cell:</strong></p>
<ul><li><strong>Fuel:</strong> Hydrogen (H₂)</li><li><strong>Oxidant:</strong> Oxygen (O₂)</li><li><strong>Only product:</strong> Water (H₂O)</li></ul>
<p><strong>Overall Reaction:</strong> 2H₂(g) + O₂(g) → 2H₂O(l)</p>
<p><strong>Half-Equations:</strong></p>
<ul><li>Anode (−): H₂(g) → 2H⁺(aq) + 2e⁻</li><li>Cathode (+): O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l)</li></ul>
</div>
</div>
</div>

<div class="point-item">
<div class="point-question">
<span><span class="point-number">2</span> Describe the advantages and disadvantages of using hydrogen-oxygen fuel cells in comparison with gasoline/petrol engines <span class="supplement-badge">Supplement</span></span>
<span class="icon">▼</span>
</div>
<div class="point-answer">
<div class="answer-content">
<p><strong>Advantages of Hydrogen Fuel Cells:</strong></p>
<ul><li>✅ Only product is <strong>water</strong> (no pollution, no CO₂)</li><li>✅ Renewable (hydrogen can be produced from water)</li><li>✅ Higher energy per kilogram than petrol/diesel</li><li>✅ No moving parts → no power loss in transmission</li><li>✅ Quieter than petrol engines (less noise pollution)</li></ul>
<p><strong>Disadvantages of Hydrogen Fuel Cells:</strong></p>
<ul><li>❌ Hydrogen production may use fossil fuels (releases CO₂)</li><li>❌ Electrolysis of water requires large amounts of electricity</li><li>❌ Expensive materials (catalysts like platinum)</li><li>❌ Hydrogen is difficult and expensive to store</li><li>❌ Highly flammable and explosive under pressure</li><li>❌ Few hydrogen filling stations available</li><li>❌ Less efficient at low temperatures</li></ul>
<div class="key-definition">💡 Petrol engines produce CO₂, CO, and oxides of nitrogen. Fuel cells produce only water.</div>
</div>
</div>
</div>

<div class="warning-box">
<strong>⚠️ Common Mistakes to Avoid:</strong><br><br>
• ❌ "Electrons flow through the electrolyte" → ✅ Ions move in electrolyte; electrons flow in external circuit<br>
• ❌ "Positive ions go to anode" → ✅ Cations (+) go to cathode (−)<br>
• ❌ "Anode is negative" → ✅ Anode is positive (+)<br>
• ❌ "Cathode is positive" → ✅ Cathode is negative (−)<br>
• ❌ "Hydrogen fuel cells produce CO₂" → ✅ Only product is water<br>
• ❌ "All metals are produced at cathode" → ✅ Only metals below hydrogen in reactivity series; hydrogen is produced for reactive metals
</div>

🎥 Video Lessons

No videos added yet.

📝 Practice Questions

Practice All Questions →

4.1 Electrolysis

📝 0 questions 🎥 0 videos

Practice →

🎯 Live Classes

No upcoming classes for this topic.