Chemistry Year 12 - Module 6 - Lesson 12
Ka, pKa & Comparing Acid Strengths
1. Key Ideas
Every soft drink you consume contains carbonic acid — and the Ka values of its two successive ionisation steps explain exactly why carbonated drinks erode tooth enamel at pH 3.5 but not at pH 5.5, and why the second ionisation barely contributes to acidity at all.
- Larger Ka = stronger acid; smaller pKa = stronger acid
- Why successive Ka values decrease (electstatic explanation: harder to remove H⁺ from increasingly negative ion)
2. Success Criteria
By the end, you should be able to:
- Larger Ka = stronger acid; smaller pKa = stronger acid
- Ka × Kb = Kw applies to any conjugate acid-base pair
- For polyprotic acids, Ka₁ ≫ Ka₂ ≫ Ka₃
3. Key Terms
4. Activity: Build the Lesson Map
Use the lesson to complete the table. Keep answers brief but specific.
| Prompt | Your answer |
|---|---|
| Main concept | |
| Important example | |
| Common mistake to avoid | |
| How this links to the next lesson |
5. Short Answer Questions
1. Explain this lesson goal in your own words: "Larger Ka = stronger acid; smaller pKa = stronger acid". Use one specific example from the lesson.
2. Apply this idea to a new example: "Ka × Kb = Kw applies to any conjugate acid-base pair". Show your reasoning clearly.
3. Analyse why this idea matters for understanding Ka, pKa & Comparing Acid Strengths: "For polyprotic acids, Ka₁ ≫ Ka₂ ≫ Ka₃".
6. Extend: Apply the Idea
A student gives a memorised answer about Ka, pKa & Comparing Acid Strengths but does not use evidence or reasoning.
Improve the answer by writing a stronger response that uses accurate terminology, a relevant example and a clear explanation.
7. Multiple Choice
1. What is the best first step when answering a question about Ka, pKa & Comparing Acid Strengths?
A. Identify the key concept being tested
B. Write every fact from memory
C. Ignore the command word
D. Skip examples and evidence
2. Which answer would show stronger understanding of Ka, pKa & Comparing Acid Strengths?
A. An answer with accurate terms and reasoning
B. A copied definition only
C. A single-word response
D. An answer with no example
3. What should you do if a question asks you to explain?
A. Link the idea to a reason or cause
B. List unrelated facts
C. Only draw a diagram
D. Write the shortest possible answer
8. Success Criteria Proof
Finish with evidence that you can do each success criterion.