反应性 3.2 · 电子转移反应

What would explain this observation?

• An aqueous salt solution can produce different electrode products from the molten salt. Water introduces competing species into the system.
• Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.

Build the model

• Oxidation 氧化 is loss of electrons and reduction 还原 is gain of electrons. In electrolysis, cations move toward the cathode and anions toward the anode. Reduction occurs at the cathode.
• oxidation: Loss of electrons; reduction: Gain of electrons.

Choose evidence that can test it

• Predict products using the specified electrolyte and electrode material. In an aqueous solution, hydrogen or oxygen may form because water-related species compete. Molten salts contain only the ions of the salt.
• Use a low-voltage direct-current supply, approved electrodes, and the school risk assessment. Collect gases only by an approved method. Keep chlorine demonstrations teacher-controlled; do not ask students to generate hazardous gases independently.

Work from known quantities

• State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
• Known: a copper ion gains two electrons. Half-equation: Cu²⁺ + 2e⁻ → Cu. One mole of Cu²⁺ requires two moles of electrons. For 0.050 mol of copper, electron amount = 2 × 0.050 = 0.100 mol.

Example:

How many moles of electrons reduce 0.20 mol of Cu²⁺ to copper? Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.

Check the conclusion and its limits

• Electrode signs depend on the cell type. In an electrolytic cell the cathode is negative; reduction remains the defining process at a cathode in every cell.
• Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.

Warn:

Reduction means loss of electrons. This claim is false: Electrode signs depend on the cell type. In an electrolytic cell the cathode is negative; reduction remains the defining process at a cathode in every cell.

Key:

Redox and electrolysis: Predict products using the specified electrolyte and electrode material. In an aqueous solution, hydrogen or oxygen may form because water-related species compete. Molten salts contain only the ions of the salt.