GCSE Chemistry - Electrolysis Part 3 - Aqueous Solutions #42

Introduction

This tutorial focuses on the electrolysis of aqueous solutions, explaining how soluble compounds are electrolyzed and the rules for determining which ions are discharged at the electrodes. This information is essential for GCSE Chemistry students preparing for their exams, particularly in understanding key concepts related to electrolysis.

Step 1: Understanding Electrolysis of Soluble Compounds

Electrolysis involves breaking down a compound into its constituent elements using electricity. When dealing with aqueous solutions, the process includes additional ions from water.

Key Concepts:

• Electrolytes: Substances that dissolve in water to produce ions.
• Electrodes: Conductors through which electricity enters or leaves the solution. There are two types:
  • Anode: Positive electrode where oxidation occurs.
  • Cathode: Negative electrode where reduction occurs.

Practical Tip:

Always remember that water can also produce hydroxide ions (OH⁻) and hydrogen ions (H⁺), which can affect the electrolysis process.

Step 2: Identifying Discharged Ions

To determine which ions will be discharged at each electrode, follow these rules:

At the Cathode:

1. If metal ions are present, the metal ions are usually discharged.
2. If no metal ions are present, hydrogen ions (H⁺) will be discharged.

At the Anode:

1. If halide ions (like Cl⁻, Br⁻, I⁻) are present, the halogen will be discharged.
2. If no halide ions are present, hydroxide ions (OH⁻) will be discharged, leading to oxygen gas production.

Common Pitfall:

Do not assume that the ions with the highest concentration will always be discharged. Follow the prioritization rules listed above.

Step 3: Electrolysis of Aqueous Copper Sulfate

When conducting electrolysis of copper sulfate (CuSO₄) solution:

1. Set up your apparatus with copper electrodes.
2. Connect the electrodes to a DC power supply.
3. Observe the following discharges:
   • At the cathode: Copper ions (Cu²⁺) are discharged, depositing copper metal.
   • At the anode: Copper metal oxidizes to form copper ions.

Real-World Application:

This process is used in electroplating, where a metal object is coated with a layer of another metal.

Step 4: Electrolysis of Aqueous Sodium Chloride

For the electrolysis of sodium chloride (NaCl) solution:

1. Use inert electrodes like carbon or platinum.
2. Connect the electrodes to a power supply.
3. Observe the following discharges:
   • At the cathode: Hydrogen ions (H⁺) are discharged, producing hydrogen gas.
   • At the anode: Chloride ions (Cl⁻) are discharged, producing chlorine gas.

Safety Note:

Chlorine gas is toxic; conduct this experiment in a well-ventilated area or under a fume hood.

Conclusion

In this tutorial, we covered the principles of electrolysis in aqueous solutions, including how to determine which ions are discharged at the electrodes. We examined the electrolysis of aqueous copper sulfate and sodium chloride, highlighting key observations and real-world applications. For further study, consider exploring the first two parts of the electrolysis series for foundational knowledge. Understanding these concepts will be crucial for your success in GCSE Chemistry.