SHS3 Chemistry · Semester 1 Week 2: Equilibria

Lesson notes

Learning Objectives

Indicator: 3.1.2.LI.2 - Explain the incomplete or partial ionization of weak acids and weak bases and calculate pKa and pKb.

By the end of the lesson, learners can:

Explain what makes an acid or base “weak” in terms of partial or incomplete ionization in aqueous solution, using appropriate examples.
Write the ionization constant expression (Ka) for a weak acid and (Kb) for a weak base from a balanced chemical equation.
Calculate the value of pKa from Ka and pKb from Kb, and convert pKa back to Ka and pKb back to Kb using inverse operations.
Use Ka and Kb values to compare the relative strengths of different weak acids and weak bases.
Apply the relationship between Ka, Kb, and Kw (Kw = Ka × Kb) to find the Ka of a weak acid from its conjugate base’s Kb, and vice versa.

Building on the previous week’s work (3.1.2.LI.1) where learners defined pH and pOH and performed simple calculations, this lesson extends that knowledge to the ionization constants that explain why some acids and bases are weak. The concept of buffers (3.1.2.LI.3) will be introduced next week, so this lesson focuses on the foundation: the constants themselves.

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Curriculum details

Strand: Physical Chemistry (Strand 1)
Sub-strand: Equilibria (1.2)
Content standard: 3.1.2.CS.1 - Demonstrate knowledge and understanding of the pH scale in determining the acidity and alkalinity of strong and weak acids, bases and aqueous solutions. 3.1.2.LO.1 Use the knowledge in pH to distinguish between solutions that are acidic, neutral or basic. 5 Gender Equality and Social Inclusion 6 Socio-Emotional Learning 3.1.2.LO.2 Use your knowledge and understanding of concepts in hydration and hydrolysis to determine the types of salts. 3.1.2.LO.3 Use the understanding of hydrolysis of salts to decide on suitable indicator as well as plot and interpret titration curves. 3.1.2.LO.4 Describe oxidation and reduction reactions and apply their principles to electrochemical cells as well as their importance in everyday life.
Indicator: 3.1.2.LI.2 - Explain the incomplete or partial ionization of weak acids and weak bases and calculate pKa and pKb.
Suggested placement: Semester 1, Week 2 (Week 2 of the year)
Our suggestion, laid out in curriculum order across three terms of twelve weeks. NaCCA does not fix the week, so follow your school's scheme of learning.
Curriculum reference: NaCCA curriculum document, p. 95

Exemplars (from the NaCCA curriculum)

Talk for Learning:
- Compare and contrast between a. Strong and weak acids b. Strong and weak bases
- Explain the ionization constant Ka for weak acids and Kb for weak bases, relate Ka to pKa and Kb to pKb.
Activity-Based Learning: Calculate pKa, pKb, pH and pOH for weak acids and weak bases.
Teaching and Learning Materials:
- * pH scale * pH meter
- * Universal indicator * Calculator
Assessment (3.1.2.AS.2). The document marks these depth-of-knowledge levels for this indicator: Level 3 Strategic reasoning.