What is the primary difference between the products of molten and aqueous electrolysis?

Aqueous electrolysis involves water-derived $H^+$ and $OH^-$ ions, which compete with the electrolyte's ions for discharge. This means the products are determined by reactivity rules rather than just the compound's components.

How do the products of electrolysing aqueous sodium chloride differ from molten sodium chloride?

Molten NaCl produces sodium metal at the cathode and chlorine at the anode. Aqueous NaCl produces hydrogen gas at the cathode because hydrogen is less reactive than sodium, while chlorine is still produced at the anode.

Why is oxygen produced at the anode during the electrolysis of dilute sulfuric acid instead of sulfate ions?

Hydroxide ions from water (or water molecules themselves) are easier to discharge than stable sulfate ions. Consequently, oxygen gas is formed and the sulfate ions remain unchanged in the solution.

What is the common mistake made when identifying the gas that relights a glowing splint?

Students often confuse it with the 'squeaky pop' test. The glowing splint test is specifically for oxygen, which supports combustion, while the 'pop' test is for hydrogen, which is itself flammable.

Why might a student incorrectly predict that sodium metal forms at the cathode during aqueous electrolysis?

This occurs if the student forgets that $H^+$ ions from water are also present. Since sodium is highly reactive, it stays in solution while the less reactive $H^+$ ions are reduced to hydrogen gas.

What error is made if a student uses copper electrodes instead of graphite in a general electrolysis investigation?

The copper anode may dissolve into the solution ($Cu \rightarrow Cu^{2+} + 2e^-$) rather than allowing the discharge of anions. This interferes with the identification of the electrolyte's decomposition products.

How is chlorine gas identified during the electrolysis of aqueous solutions?

Chlorine is identified by placing damp blue litmus paper near the anode. The paper will first turn red due to acidity and then be bleached white by the strong oxidising action of the chlorine.

What observation confirms the presence of hydrogen gas at the cathode?

Hydrogen is confirmed if a lighted splint held at the mouth of the collection tube produces a distinctive 'squeaky pop' sound. This sound is the result of the rapid combustion of hydrogen with oxygen in the air.

What does a pink-brown solid deposit on an electrode typically indicate in electrolysis?

A pink-brown deposit indicates the formation of copper metal. This occurs at the cathode when $Cu^{2+}$ ions gain electrons (reduction) to form neutral copper atoms.

Under what condition is hydrogen gas produced at the cathode in aqueous electrolysis?

Hydrogen gas is produced if the metal ions in the solution are more reactive than hydrogen in the reactivity series. The more reactive metal remains as an ion, allowing the $H^+$ ions to be discharged.

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1. Principles of Chemistry

Practical: Investigate the Electrolysis of Aqueous Solutions

Summary

This practical investigation explores how electrical energy decomposes ionic compounds in water. By observing products at the electrodes and applying specific chemical tests, students can determine the movement of ions and the rules of selective discharge that govern aqueous electrolysis.

1. Definition & Core Concepts

Aqueous Electrolysis Investigation: This practical procedure involves passing a direct current through an ionic solution to identify the resulting chemical products at the electrodes. Unlike molten electrolysis, aqueous solutions contain $H^+$ and $OH^-$ ions from water dissociation, adding complexity to the predicted outcomes.
Inert Electrodes: Graphite or platinum is typically used because these materials do not participate in the chemical reactions. Their role is solely to provide a surface for electron transfer, ensuring that the observations reflect the decomposition of the electrolyte rather than the erosion of the equipment.
Gas Collection Apparatus: Inverted test tubes or graduated cylinders are placed over the electrodes within the electrolyte. This allows for the capture of gaseous products, which can then be identified using specific diagnostic tests to confirm the identity of the discharged ions.

Diagram of an electrolysis cell used to collect gases from aqueous solutions, showing inverted test tubes over graphite electrodes in a beaker.

2. Underlying Principles

3. Methods & Techniques

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions