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5.3Equilibrium and Reaction Rates

Ionic equilibria — EST 2 Chemistry Revision Notes & Topical Questions

5 min read 8 parts Lesson 3 of 3 1 quiz

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Lesson summary

Ionic equilibria EST Chemistry notes: Calculate pH of weak acids.Larger Ka (smaller pKa) means a stronger acid.Buffers work best when pH ≈ pKa.Blood is buffered by H2CO3/HCO3− near pH 7.4.For AgCl, Ksp = [Ag+][Cl−] = s².Percent ionisation of a weak acid increases on dilution.Calc…

  • Adding acid to a buffer
  • Adding base
  • 0.10 M acetic acid, Ka = 1.8 × 10−5. Find pH
  • Ksp of AgCl is 1.8 × 10−10. Find molar solubility
  • Ionic equilibria EST Chemistry: what to do next
  • Common-ion effect

Part 1 of 8Key ideas

1
Larger Ka (smaller pKa) means a stronger acid.
2
Buffers work best when pH ≈ pKa.
3
Blood is buffered by H2CO3/HCO3− near pH 7.4.
4
For AgCl, Ksp = [Ag+][Cl−] = s².
5
Percent ionisation of a weak acid increases on dilution.

Part 2 of 8Learning objectives

Learning objectives
Calculate pH of weak acids.
Explain how buffers work.
Use Ksp and the common-ion effect.

Part 3 of 8Essential knowledge

Adding acid to a buffer
A− absorbs H+.
Adding base
HA neutralises OH−.
Titration curve of weak acid with strong base has equivalence pH > 7.
At half-equivalence, pH = pKa.
Indicators are themselves weak acids.

Part 4 of 8Process & sequence

1
Write the ionisation equation.
2
Set up an ICE table with x.
3
Assume x is small compared with initial concentration.
4
Solve Ka = x²/C for x = [H+].
5
pH = −log x; check the 5% rule.

Weak acid pHwrite these steps in this order; each step is a marking point.

  1. Ionisation equation
  2. ICE table
  3. Ka = x²/C
  4. Solve for x
  5. pH = −log x

Part 5 of 8Worked examples

10.10 M acetic acid, Ka
1.8 × 10−5. Find pH.
  • x = √(1.8 × 10−6) = 1.34 × 10−3; pH = 2.87.
2Ksp of AgCl is 1.8 × 10−10. Find molar solubility.
Ksp of AgCl is 1.8 × 10−10. Find molar solubility.
  • s = √(1.8 × 10−10) = 1.3 × 10−5 M.

Part 6 of 8Summary

Summary
Weak acids: use Ka and ICE.
Buffers resist pH change.
Precipitate forms if Qsp > Ksp.

Part 7 of 8Keep going

Ionic equilibria EST Chemistry: what to do next

These Ionic equilibria EST Chemistry notes covered the key ideas, worked examples and exam technique for this topic. Check your Ionic equilibria EST Chemistry understanding with the practice questions below, then reread any Ionic equilibria EST Chemistry point you were unsure about before moving on.

1
Practice: Ionic equilibria EST Chemistry practice questions
2
Previous lesson: 5.2 Reaction rates
3
Next lesson: 6.1 Energy changes
4
Course: EST 2 Chemistry — all units
5
Official syllabus: OpenStax free science textbooks

Ionic equilibria EST Chemistry revision notes

Part 8 of 8Compare: Compare

Salt solutions
Salt fromExamplepH
Strong acid + strong baseNaCl7
Strong acid + weak baseNH4Cl< 7
Weak acid + strong baseCH3COONa> 7
Key relationships
RelationshipUse
Ka × Kb = KwConjugate pair constants
[H+] ≈ √(KaC)Weak acid pH
pH = pKa + log([A−]/[HA])Buffer pH
Qsp > KspPrecipitate forms

Key terms

Command terms

Command terms
Identify / Describe (FRQ)
A short, specific statement; describing requires relevant characteristics, not a single word.
Explain / Justify (FRQ)
Give the cause-and-effect reasoning or evidence; a restated fact does not score.
Calculate
Show setup, substitution and the answer with units.
Predict
State the expected outcome; add a reason only if asked to justify.
MCQ technique
Read every option; eliminate distractors that are true statements but do not answer the question; watch for NOT/EXCEPT.

Exam technique

Keywords the mark scheme looks for in this lesson

Keywords the mark scheme looks for in this lesson
Ka · Kb · Weak acid · Buffer · Ksp · Common-ion effect · Hydrolysis
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

1
Answer exactly what is asked; extra correct material does not earn credit, and contradictory statements can cancel credit (AP FRQ).
2
Use precise vocabulary (e.g. “active site”, “concentration gradient”, “allele”), never “it/stuff/things”.
3
Show units and working in calculations.
4
In MCQs, predict the answer before reading the options, then eliminate.

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