Final answer:
During the electrolysis of an aqueous potassium chloride solution, the gas produced at the negative electrode is hydrogen due to the preferential reduction of water over potassium ions.
Step-by-step explanation:
When a student dissolves potassium in water to form a potassium chloride solution and attempts electrolysis, the expectation might be to see potassium metal at the negative electrode. However, this is not the case. Instead, bubbles of gas are observed due to a specific reduction reaction that takes precedence over the reduction of potassium ions. This phenomenon occurs because the reduction potential of water (-0.83 V) is less negative than that of potassium ions. Thus, it is energetically more favorable for water molecules to undergo reduction at the negative electrode.
During the electrolysis of an aqueous potassium chloride solution, water molecules receive electrons from the cathode and are reduced to produce hydrogen gas (H₂) and hydroxide ions (OH-). On the other hand, at the positive electrode, chlorine gas (Cl₂) is produced from the oxidation of chloride ions (Cl-).
Therefore, the gas produced at the negative electrode is not potassium but rather hydrogen. This outcome would be consistent across various similar electrolysis scenarios involving aqueous solutions of sodium, lithium, potassium, and magnesium. Please note that with molten electrolytes, such as molten sodium chloride, metallic sodium would be produced at the cathode because water is not present to compete in the reduction process.