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8.4Acids and Bases

Acid-Base Reactions and Buffers — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 8 parts Lesson 9 of 16 2 quizzes

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Lesson summary

8.4 Acid-Base Reactions and Buffers — AP CHEMISTRY notes: Predict the species and pH after mixing acids and bases, and recognise when a buffer forms.Reaction — H+ + OH− → H2O, completepH — found from the moles of excess reagent divided by total volume20 mL of 0.200 M HCl and 15 m…

  • Base in excess
  • Equal moles
  • Acid in excess
  • Equilibrium
  • Neutralisation
  • Buffer region

Section 1 of 8Strong acid + strong base

Reaction
  • H+ + OH− → H2O, complete
pH
  • pH found from the moles of excess reagent divided by total volume
  • 20 mL of 0.200 M HCl and 15 mL of 0.200 M KOH leave 0.001 mol H+ in 35 mL, so pH = 1.54.

Section 2 of 8Weak base + strong acid

Reaction
  • B + H3O+ → BH+ + H2O, complete
Base in excess
  • Base in excess a buffer forms, with pOH = pKb + log([BH+]/[B])
Equal moles
  • Equal moles the solution is slightly acidic because BH+ is a weak acid
Acid in excess
  • Acid in excess pH from excess H3O+
  • 25 mL of 0.150 M NH3 with 15 mL of 0.150 M HCl: pOH = 4.75 + log(2.25/1.50) = 4.93, pH = 9.07.

Section 3 of 8Weak acid + strong base

Reaction
  • HA + OH− → A− + H2O, complete
Acid in excess
  • Acid in excess a buffer forms, with pH = pKa + log([A−]/[HA])
Equal moles
  • Equal moles the solution is slightly basic because A− is a weak base
Base in excess
  • Base in excess pH from excess OH−
  • 50 mL of 0.100 M NaOH with 75 mL of 0.200 M ethanoic acid: pH = 4.74 + log(5.00/10.0) = 4.45.

Section 4 of 8Weak acid + weak base

Equilibrium
  • HA + B ⇌ A− + HB+, favouring the side with the weaker acid and weaker base
K > 1
  • K > 1 products favoured; K < 1 — reactants favoured
Equal moles
  • Equal moles compare Ka of the acid with Kb of the base to decide whether the solution is acidic, neutral or basic

Section 5 of 8Method

1
Write the reaction and calculate moles of each reactant.
2
Complete the strong-reaction stoichiometry to find the moles left.
3
Decide: excess strong reagent, buffer or equivalence.
4
Calculate the pH from the matching expression.

Section 6 of 8More from the specification

More from the specification
Carboxylic acid functional group
  • Carboxylic acid functional group the carboxyl group comprises a carbonyl and hydroxyl unit, conferring acidic proton donating properties.
Proton
  • Positive particle in the nucleus.
Conjugate acid
  • The species formed when a base accepts a proton.
Ionic equation
  • An equation showing only the ions that react.
Concentration
  • Moles ÷ volume in dm³, or g/dm³.
Density
  • Mass divided by volume.
Aqueous
  • Dissolved in water.

Section 7 of 8Key terms

TermMeaning
NeutralisationReaction of an acid with a base.
Excess reagentReactant left after the reaction.
Buffer regionMixture of weak species and conjugate that resists pH change.
EquivalenceEqual moles of acid and base reacted.

Section 8 of 8Exam technique

Avoid these mistakes
Using initial concentrations in the Henderson-Hasselbalch equation after reaction — use moles remaining.
Forgetting to add volumes.
Calling equivalence of a weak acid with a strong base neutral — it is basic.

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