Electrochemistry is the study of how chemical energy can be converted into electrical energy and vice versa. This guide covers the two main types of cells: electrochemical cells (like batteries) and electrolytic cells (used for processes like electroplating), as well as the important topic of corrosion.

1. Key Definitions: Oxidation and Reduction

This is the foundation for the entire chapter. Remember the mnemonic OIL RIG:

• Oxidation Is Loss of electrons. (e.g., Zn → Zn²⁺ + 2e⁻)

• Reduction Is Gain of electrons. (e.g., Cu²⁺ + 2e⁻ → Cu)

2. Electrochemical Cells (Galvanic Cells)

These cells generate electricity from a spontaneous chemical reaction. Think of them as simple batteries.

• Function: Convert chemical energy into electrical energy.

• Components: Two different metals (electrodes) placed in an electrolyte (e.g., dilute acid).

• How it Works (e.g., Zinc and Copper in acid):

  • Anode (Negative Terminal): The more reactive metal (Zinc). It undergoes oxidation (loses electrons) and dissolves. Anode reaction: Zn(s) → Zn²⁺(aq) + 2e⁻

  • Cathode (Positive Terminal): The less reactive metal (Copper). Reduction occurs here as hydrogen ions from the acid gain electrons to form hydrogen gas. Cathode reaction: 2H⁺(aq) + 2e⁻ → H₂(g)

  • Electron Flow: Electrons flow from the negative anode (Zn) to the positive cathode (Cu) through the external wire, creating an electric current.

• Mnemonic:

  • An Ox: Anode is Oxidation.

  • Red Cat: Reduction at the Cathode.

3. Electrolytic Cells & Electrolysis

These cells use electricity to force a non-spontaneous chemical reaction to happen.

• Function: Convert electrical energy into chemical energy.

• Electrolysis: The process of using electricity to break down a compound (electrolyte).

• Components: A power source, two electrodes (often inert, like carbon or platinum), and an electrolyte (a molten salt or an aqueous solution).

• How it Works:

  • Anode (Positive Electrode): Connected to the positive (+) terminal of the power source. Anions (negative ions) move here and undergo oxidation (lose electrons).

  • Cathode (Negative Electrode): Connected to the negative (-) terminal of the power source. Cations (positive ions) move here and undergo reduction (gain electrons).

Key Difference: In an electrochemical cell, the anode is negative. In an electrolytic cell, the anode is positive. However, oxidation always happens at the anode, and reduction always happens at the cathode in both types of cells.

4. Applications of Electrolysis

• Electroplating: Coating an object with a thin layer of another metal.

  • Object to be plated: Made the cathode (-).

  • Plating metal: Made the anode (+).

  • Electrolyte: Must be a solution containing ions of the plating metal (e.g., use copper sulfate solution to plate with copper).

• Extraction of Metals: Highly reactive metals like sodium are extracted from their molten salts using electrolysis (e.g., in a Down's cell).

5. Corrosion: The Rusting of Iron

• Corrosion: The gradual destruction of a metal due to its reaction with the environment.

• Rusting: The specific name for the corrosion of iron.

• Two Essential Conditions for Rusting:

  • Oxygen (from the air)

  • Water (or moisture)

• The Process: Rusting is an electrochemical process. Tiny areas on the iron surface act as anodes and cathodes.

• Factors that Speed Up Rusting:

  • Presence of salts (why cars rust faster near the coast).

  • Presence of acids (acid rain).

Preventing Rusting

1. Barrier Methods: Physically block oxygen and water from reaching the iron.

   • Painting, Oiling, Greasing.

   • Coating with a less reactive metal like tin (but if the coating is scratched, the iron will rust quickly).

2. Sacrificial Protection (Cathodic Protection): Connecting iron to a more reactive metal. The more reactive metal corrodes ("is sacrificed") instead of the iron.

   • Galvanizing: Coating iron with a layer of zinc. Zinc is more reactive than iron, so even if the surface is scratched, the zinc will corrode first, protecting the iron. This is a very common exam example.

Exam Tips & Tricks

1. Cell Types: Be crystal clear on the difference between an electrochemical cell (makes electricity, anode is negative) and an electrolytic cell (uses electricity, anode is positive).

2. OIL RIG is Your Best Friend: Use it to determine which reaction is oxidation and which is reduction. This helps you identify the anode and cathode.

3. Electroplating Setup: This is a classic diagram question. Remember: The item you want to plate is ALWAYS the cathode.

4. Rusting Conditions: If a question asks for the conditions for rusting, you MUST mention both oxygen and water.

Galvanizing vs. Tin Plating: Understand the difference. Galvanizing (zinc) offers sacrificial protection even when scratched. Tin plating only works as a barrier; if scratched, the iron underneath rusts faster.