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: 14
Grade code: 3.1.2.LI.3
Strand code: 1
Sub-strand code: 2
Content standard code: 3.1.2.CS.1
Indicator code: 3.1.2.LI.3
Theme: PHYSICAL CHEMISTRY
Subtheme: EQUILIBRIA
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This lesson delves into the fascinating world of buffer solutions, which are crucial for maintaining stability in chemical systems. In our bodies, in the soil that grows our food, and in many industries, the ability to control pH is a matter of life, death, and economic success. We will explore what buffers are, how they perform their remarkable job of resisting pH changes, and how to calculate their pH. Understanding buffers is key to appreciating the delicate chemical balances that govern the world around us, from the blood flowing in our veins to the manufacturing of essential medicines in Accra and Tema.
A. Foundation: Weak Acids, Weak Bases and their Ionisation Constants (Ka & Kb)
Before we understand buffers, we must recall the behaviour of weak acids and bases. Unlike strong acids/bases which dissociate completely, weak acids/bases only partially ionise in water, establishing an equilibrium. Weak Acid (HA): `HA(aq) ⇌ H⁺(aq) + A⁻(aq)` The acid dissociation constant, Ka, is the equilibrium constant for this reaction: `Ka = ([H⁺][A⁻]) / [HA]` A smaller `Ka` value means a weaker acid. pKa: For convenience, we use `pKa`. `pKa = -log₁₀(Ka)` A larger `pKa` value indicates a weaker acid. Weak Base (B): `B(aq) + H₂O(l) ⇌ BH⁺(aq) + OH⁻(aq)` The base dissociation constant, Kb, is the equilibrium constant: `Kb = ([BH⁺][OH⁻]) / [B]` A smaller `Kb` value means a weaker base. pKb: Similarly, we use `pKb`. `pKb = -log₁₀(Kb)` A larger `pKb` value indicates a weaker base.
This equilibrium is the foundation of how buffers work. B. What is a Buffer Solution?
A buffer solution is an aqueous solution that resists a change in its pH when a small amount of strong acid or strong base is added to it.