1.4Electrolytic Cell
Electrolysis is a chemical process where electrical energy is transformed into chemical energy to decompose compounds in the molten state or aqueous solution into their constituent elements.
Learning Objectives
- Describe electrolysis.
- Describe electrolysis of molten compound through activities.
- Explain factors that affect electrolysis of aqueous solution through experiment.
- Compare voltaic cell and electrolytic cell.
- Describe electroplating and purification of metal by electrolysis through activities.
Summary of Factors Affecting Selective Discharge of Ions at Electrodes
A rapid decision-tree style reference when predicting products at the anode and cathode during aqueous electrolysis.
Cathode
Always select the cation with the higher (more positive / less negative) \(E^0\) value.
Anode
Check electrode type first. If active metal, metal dissolves → oxidation. If inert (C or Pt), check halide concentration. High concentration → halide discharged. Otherwise, select ion with less positive \(E^0\) value (usually \(\text{OH}^- \rightarrow \text{O}_2\)).
Brief Explanation
Understand the key ideas behind electrolysis and selective discharge.
Electrolysis of Molten Compounds
Anions move to the anode and undergo oxidation. Cations move to the cathode and undergo reduction.
Electrolysis of Aqueous Solutions
Besides solute ions, \(\text{H}^+\) and \(\text{OH}^-\) from the partial dissociation of water are also present. Selective discharge depends on \(E^0\) value, concentration and electrode type.
Electrolysis of Molten \(\text{PbBr}_2\)
Ions present: \(\text{Pb}^{2+}\) and \(\text{Br}^-\). \(\text{Pb}^{2+}\) ions move to the cathode; \(\text{Br}^-\) ions move to the anode.
Cathode: \(\text{Pb}^{2+} + 2e^- \rightarrow \text{Pb}\); a grey solid forms.
Anode: \(2\text{Br}^- \rightarrow \text{Br}_2 + 2e^-\); brown gas is released.
Overall ionic equation: \(\text{Pb}^{2+}(\text{l}) + 2\text{Br}^-(\text{l}) \rightarrow \text{Pb}(\text{s}) + \text{Br}_2(\text{g})\).
Electrolysis of \(\text{CuSO}_4(\text{aq})\) with Carbon Electrodes
All ions present: \(\text{Cu}^{2+}\), \(\text{H}^+\), \(\text{SO}_4^{2-}\) and \(\text{OH}^-\).
Cathode: \(\text{Cu}^{2+}\) is discharged because \(E^0=+0.34\text{ V}\) is more positive than \(0.00\text{ V}\) for \(\text{H}^+\). \(\text{Cu}^{2+}+2e^-\rightarrow\text{Cu}\); a brown solid is deposited.
Anode: \(\text{OH}^-\) is discharged because \(E^0=+0.40\text{ V}\) is less positive than \(+2.01\text{ V}\) for \(\text{SO}_4^{2-}\). \(4\text{OH}^-\rightarrow\text{O}_2+2\text{H}_2\text{O}+4e^-\); colourless gas bubbles form.
Effect of Concentration: HCl
With carbon electrodes, \(\text{H}^+\) is discharged at the cathode in both solutions: \(2\text{H}^+ + 2e^- \rightarrow \text{H}_2\).
- Concentrated HCl (\(1.0\text{ mol dm}^{-3}\)): High \(\text{Cl}^-\) concentration; \(\text{Cl}^-\) is discharged at the anode. \(2\text{Cl}^-\rightarrow\text{Cl}_2+2e^-\).
- Very dilute HCl (\(0.0001\text{ mol dm}^{-3}\)): \(\text{OH}^-\) is discharged at the anode based on \(E^0\). \(4\text{OH}^-\rightarrow\text{O}_2+2\text{H}_2\text{O}+4e^-\).
Effect of Electrode Type: Active Copper
In \(\text{CuSO}_4(\text{aq})\) with copper electrodes, the copper anode dissolves and no anion is discharged: \(\text{Cu}(\text{s})\rightarrow\text{Cu}^{2+}(\text{aq})+2e^-\).
At the cathode, \(\text{Cu}^{2+}+2e^-\rightarrow\text{Cu}(\text{s})\). The rates of \(\text{Cu}^{2+}\) formation and discharge are equal, so the ion concentration and blue colour remain unchanged.
| Aspect | Voltaic Cell | Electrolytic Cell |
|---|---|---|
| Energy conversion | Chemical → electrical | Electrical → chemical |
| Anode / cathode | Negative anode; positive cathode | Positive anode; negative cathode |
| Reactions | Oxidation at anode; reduction at cathode | Oxidation at anode; reduction at cathode |
| Electrons in external wire | Anode → cathode | Anode → cathode |
| Electrodes | Two different metals | May use the same type of metal |
Electroplating and Metal Purification Activities
Copper-plating an iron spoon: Connect the iron spoon as the cathode to the battery's negative terminal and a copper plate as the anode to the positive terminal. Dip both in \(\text{CuSO}_4(\text{aq})\). Anode: \(\text{Cu}\rightarrow\text{Cu}^{2+}+2e^-\); cathode: \(\text{Cu}^{2+}+2e^-\rightarrow\text{Cu}\), forming a copper layer on the spoon.
Copper purification: Use impure copper as the anode, pure copper as the cathode and \(\text{CuSO}_4(\text{aq})\) as the electrolyte. Copper dissolves from the anode and is deposited as pure copper on the cathode; impurities remain behind.
| Comparison | Conductor | Electrolyte |
|---|---|---|
| State | Solid or molten state | Molten state or aqueous solution |
| Chemical change | No chemical change | Undergoes chemical decomposition |
| Charge carrier | Free-moving electrons | Free-moving ions |
| Conductivity with temperature | Decreases as temperature increases | Increases as temperature increases |
| Examples | Metals and graphite | Ionic compounds, acids and alkalis |
Try These Questions First
Answer mentally, then check the explanation.
Molten lead(II) bromide is electrolysed using carbon electrodes. State the observation and half-equation at the cathode.
Why are \(\text{OH}^-\) ions discharged at the anode instead of \(\text{SO}_4^{2-}\) ions during the electrolysis of dilute copper(II) sulphate solution using carbon electrodes?
In an experiment to electroplate an iron key with silver, state the metal used as the anode and suggest a suitable electrolyte.
Common Mistakes
Avoid these errors when predicting electrolysis products.
Common Misconceptions
Compare each misconception with the correct fact.
Electrons travel through the electrolyte solution from cathode to anode.
Electrons only flow through the external metallic connecting wires and electrodes. Electrical conduction inside the electrolyte is carried entirely by free-moving ions.
Concentrated solutions will always discharge halide ions at the anode indefinitely.
As electrolysis proceeds, the concentration of halide ions drops. Once the concentration becomes low, hydroxide ions (\(\text{OH}^-\)) will be discharged instead, producing oxygen gas.
The blue colour of copper(II) sulphate solution always fades during electrolysis.
The colour only fades when inert electrodes (carbon/platinum) are used. When copper electrodes are used, the blue colour intensity remains unchanged because the rate of \(\text{Cu}^{2+}\) formation at the anode equals the rate of \(\text{Cu}^{2+}\) discharge at the cathode.
Summary
Key facts to remember.
- Electrolytes conduct electricity in molten or aqueous states due to free-moving ions and decompose into elements.
- Conductors conduct electricity in solid or molten states via delocalised electrons without decomposing.
- Electrolytic cell: Converts electrical energy to chemical energy; Anode is positive (oxidation); Cathode is negative (reduction).
- Factors affecting aqueous electrolysis: \(E^0\) value, concentration of halide ions, and nature of electrode.
- Industrial applications: Electroplating (object at cathode, plating metal at anode) and purification of metals (impure metal at anode, pure metal at cathode).
Quick Activity
Complete all parts before checking your score.
A. Objective Quiz
Which of the following substances can conduct electricity in the molten state and undergo chemical decomposition?
What product is formed at the anode when \(1.0\text{ mol dm}^{-3}\) of hydrochloric acid is electrolysed using carbon electrodes?
B. Fill in the Blanks
In an electrolytic cell, oxidation takes place at the ______, which is connected to the positive terminal of the battery.
During the purification of impure copper via electrolysis, the impure copper is made the ______ of the cell.
C. Match the Process with Its Characteristic
Drag each process on desktop. On mobile, tap a process and then tap the matching box.
Answer Choices
Molten \(\text{PbBr}_2\) cathode Electroplating an iron spoon Dilute \(\text{H}_2\text{SO}_4\) anode Copper anode in \(\text{CuSO}_4\)Characteristic / Setup
Keywords
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