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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

Part 1 of 6Strong 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.

Part 2 of 6Weak 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.

Part 3 of 6Weak 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.

Part 4 of 6Weak 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

Part 5 of 6Method

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.

Part 6 of 6More 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.

Key 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.

Avoid these mistakes

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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