Electrolysis

Main Ideas

• Electrolysis uses electrical energy to drive a chemical change that would not happen on its own. It is often used to split a compound into simpler substances or its constituent elements.
• Electrolysis involves redox reactions:
  • Reduction is the gain of electrons and occurs at the cathode.
  • Oxidation is the loss of electrons and occurs at the anode.
  • In the electrolytic cells shown, the battery pushes electrons into the cathode and draws them from the anode.
• Some elements occur naturally as diatomic molecules, including chlorine, hydrogen, and oxygen. They are written as Cl₂, H₂, and O₂ rather than as single atoms.

Example 1: Electrolysis of Molten Sodium Chloride

Sodium chloride does not spontaneously break down into sodium and chlorine. A battery supplies the energy needed to drive the reaction. The salt must be molten so its ions can move.

• Sodium ions (Na⁺) move toward the negatively charged cathode and gain electrons, forming sodium metal.
• Chloride ions (Cl⁻) move toward the positively charged anode and lose electrons. The resulting chlorine atoms pair up as Cl₂ gas.

The oxidation-state changes show the electron transfer: sodium changes from +1 to 0 and is reduced, while chlorine changes from −1 to 0 and is oxidized.

The half-reactions are:

• Cathode (reduction): Na⁺ + e⁻ → Na
• Anode (oxidation): 2Cl⁻ → Cl₂ + 2e⁻

To combine the half-reactions, make the electron counts equal, add the equations, and cancel electrons that appear on both sides. The overall reaction is:

2NaCl(l) → 2Na(l) + Cl₂(g)

Example 2: Electrolysis of Water

Water does not spontaneously decompose into hydrogen and oxygen under ordinary conditions. Electrical energy drives the process.

The cell uses electrodes connected to a battery and inverted, water-filled test tubes to collect the gases. An electrolyte, such as sulfuric acid, is added to help the water conduct electricity.

• At the cathode, hydrogen in water is reduced, producing H₂ gas.
• At the anode, oxygen in water is oxidized, producing O₂ gas.

Bubbles form at the electrodes and collect in the test tubes, displacing the water. The balanced overall reaction is:

2H₂O(l) → 2H₂(g) + O₂(g)

The volume of hydrogen produced is twice the volume of oxygen, matching the 2:1 ratio in the balanced equation.

One set of half-reactions presented in the video is:

• Cathode (reduction): 2H₂O + 2e⁻ → H₂ + 2OH⁻
• Anode (oxidation): 2H₂O → O₂ + 4H⁺ + 4e⁻

To combine them, multiply the reduction half-reaction by two so both half-reactions contain four electrons. Add them and cancel the electrons. Then combine H⁺ and OH⁻ into water and simplify. The overall reaction remains 2H₂O → 2H₂ + O₂. Textbooks may write equivalent half-reactions for water electrolysis in different ways.

Speakers and Sources Featured

• Presenter/narrator: Tyler DeWitt, identified from the channel context. The subtitles contain one continuous instructional narration.
• Other speakers or external sources: None identified in the subtitles.