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4.8Chemical Reactions

Introduction to Acid-Base Reactions — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 10 parts Lesson 11 of 12 1 quiz

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Lesson summary

4.8 Introduction to Acid-Base Reactions — Notes — AP CHEMISTRY notes: Identify Brønsted-Lowry acids, bases and conjugate pairs and write acid-base reactions.A Brønsted-Lowry acid is a proton (H+) donor.A Brønsted-Lowry base is a proton acceptor.The theory is not limited to water …

  • Strong acids ionise completely
  • Strong acid with strong base
  • Weak acid with strong base
  • Strong acid with weak base
  • Amphoteric
  • Hydronium ion

Section 1 of 10Brønsted-Lowry definitions

1
A Brønsted-Lowry acid is a proton (H+) donor.
2
A Brønsted-Lowry base is a proton acceptor.
3
The theory is not limited to water and applies to gas-phase reactions.
4
Lewis acid-base concepts are not assessed on the AP exam.

Section 2 of 10Water

Water
In water H+ exists as hydronium, H3O+; H+(aq) and H3O+(aq) are used interchangeably.
Water is amphoteric (amphiprotic) because it can donate or accept protons.
  • As a baseHCl + H2O → H3O+ + Cl−.
  • As an acidH2O + NH3 ⇌ NH4+ + OH−.
Water autoionises: 2 H2O(l) ⇌ H3O+(aq) + OH−(aq).

Section 3 of 10Conjugate pairs

Conjugate pairs
An acid donating a proton forms its conjugate base; a base accepting a proton forms its conjugate acid.
A conjugate pair differs by exactly one H+, for example HF/F− and NH4+/NH3.
In CH3COOH + H2O ⇌ CH3COO− + H3O+, the acid and conjugate base are CH3COOH and CH3COO−, and the base and conjugate acid are H2O and H3O+.

Section 4 of 10Relative strength

Relative strength
A strong acid is a good proton donor and its conjugate base is weak.
A weak acid has a strong conjugate base.
A strong base is a good proton acceptor.

Section 5 of 10Strong and weak acids and bases

Strong and weak acids and bases
Strong acids ionise completely
  • HCl, HBr, HI, HNO3, HClO4 and H2SO4 (first proton).
Strong bases are the soluble Group 1 hydroxides and the hydroxides of Ca, Sr and Ba.
Weak acids and bases ionise only partially, so they are written as molecules in net ionic equations, such as HC2H3O2(aq) and NH3(aq).

Section 6 of 10Net ionic equations

1Strong acid with strong base
  • H+(aq) + OH−(aq) → H2O(l).
2Weak acid with strong base
  • HA(aq) + OH−(aq) → A−(aq) + H2O(l).
3Strong acid with weak base
  • H3O+(aq) + NH3(aq) → NH4+(aq) + H2O(l).
4
Reactions involving a strong acid or strong base go essentially to completion.
TypeExampleIonisationpH of 0.1 M
Strong acidHClCompleteAbout 1
Weak acidEthanoic acidPartialAbout 3
Strong baseNaOHCompleteAbout 13
Weak baseNH3PartialAbout 11

Section 7 of 10pH

pH + pOH
14 at 25 °C.
Each pH unit is a tenfold change in H+ concentration.

Section 8 of 10More from the specification

1Acidity order in conjugate pairs
  • Acidity order in conjugate pairs proton transfer equilibria reflect relative base affinities, determining the thermodynamic order of competing acid-base reactions.
2Charge
  • Q = It, measured in coulombs.
    What the symbols mean: Q = charge (C) · I = current (A) · t = time (s)
3Reactants
  • The substances that react together at the start of a reaction.
4Aqueous
  • Dissolved in water.

Section 9 of 10Key terms

TermMeaning
Brønsted-Lowry acidProton donor.
Brønsted-Lowry baseProton acceptor.
Conjugate pairAcid and base differing by one H+.
AmphotericAble to act as an acid or a base.
Hydronium ionH3O+, the form of H+ in water.
Strong acidAcid that ionises completely in water.

Section 10 of 10Exam technique

1
Saying the acid is the species that accepts protons — acids donate
2
Writing a weak acid as ions in a net ionic equation
3
Forgetting that a conjugate pair differs by one proton
4
Saying the conjugate base of a strong acid is strong — it is weak

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