Lesson Notes By Weeks and Term v4 - SHS 3
EQUILIBRIA
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EQUILIBRIA
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Subject: Chemistry
Class: SHS 3
Term: 2nd Term
Week: 17
Grade code: 3.1.2.LI.3
Strand code: 1
Sub-strand code: 2
Content standard code: 3.1.2.CS.4
Indicator code: 3.1.2.LI.3
Theme: PHYSICAL CHEMISTRY
Subtheme: EQUILIBRIA
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This lesson explores the fascinating field of electrochemistry, a key part of chemical equilibria. We will learn how spontaneous chemical reactions (redox reactions) can be harnessed to produce electrical energy. This principle is the foundation of every battery we use, from the small 'Tiger Head' batteries in our radios and remote controls to the large, powerful batteries in cars and solar power systems. Understanding this topic is crucial for appreciating modern technology, energy storage solutions vital for Ghana's development (like storing solar power), and the future of clean transportation.
2.1. Introduction: From Redox Reactions to Electricity An electrochemical cell is a device that converts chemical energy into electrical energy or vice versa. The reactions involved are redox reactions, where electrons are transferred from one substance to another.
There are two main types of electrochemical cells: Voltaic (or Galvanic) Cells: These use a *spontaneous* redox reaction to generate electricity. This is the principle behind all batteries. This is our focus for today. Electrolytic Cells: These use electrical energy to drive a *non-spontaneous* reaction (e.g., electrolysis of water). 2.2. Electrode Potential: The Source of Voltage When a piece of metal (an electrode) is placed in a solution containing its own ions (an electrolyte), a dynamic equilibrium is established at the surface of the metal. Example: Zinc in Zinc Sulphate (ZnSO₄) Some zinc atoms on the rod can lose two electrons and enter the solution as Zn²⁺ ions (Oxidation: Zn(s) → Zn²⁺(aq) + 2e⁻). Simultaneously, some Zn²⁺ ions in the solution can gain two electrons from the rod and become solid zinc atoms (Reduction: Zn²⁺(aq) + 2e⁻ → Zn(s)). This sets up an equilibrium: Zn(s) ⇌ Zn²⁺(aq) + 2e⁻
This separation of charge (negative electrons left on the metal rod and positive ions in the solution) creates an electrical potential difference called the electrode potential.
Factors Affecting Electrode Potential: Nature of the electrode: Different metals have different tendencies to lose or gain electrons. Zinc is more reactive than copper, so it has a greater tendency to lose electrons. Concentration of the electrolyte: Changing the concentration of the ions in solution shifts the equilibrium, changing the potential. Temperature: Affects the rate of reaction and the position of equilibrium. Pressure: Important for gas electrodes (like the hydrogen electrode). 2.3. The Problem of Measurement and the Standard Hydrogen Electrode (SHE) It is impossible to measure the absolute electrode potential of a single half-cell. We can only measure the potential difference between two half-cells. Analogy: You cannot measure the height of a single mountain peak in isolation. You measure it relative to a standard reference point, like sea level.