4CH1

Acids, Alkalis and Titrations

Inorganic Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 10% of your exam marks.

stable
Medium
Stable10%

Neutralisation ionic equations and titration calculations are among the most reliably tested question types.

What makes a solution acidic or alkaline

  • An acid is a substance that releases hydrogen ions, H+, into aqueous solution
    • Hydrochloric acid: HCl(aq) → H+(aq) + Cl−(aq)
    • Sulfuric acid: H2SO4(aq) → 2 H+(aq) + SO42−(aq)
    • Nitric acid: HNO3(aq) → H+(aq) + NO3−(aq)
  • An alkali is a soluble base that releases hydroxide ions, OH−, into aqueous solution
    • Sodium hydroxide: NaOH(aq) → Na+(aq) + OH−(aq)
    • Potassium hydroxide: KOH(aq) → K+(aq) + OH−(aq)
    • Aqueous ammonia: NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH−(aq)

Neutralisation

  • A reaction is the reaction between an acid and a base (or an alkali) that produces a and water
  • For an alkali, the central chemistry is that H+ ions from the acid combine with OH− ions from the alkali to form water:

H+(aq) + OH−(aq) → H2O(l)

  • This is the net ionic equation for every acid–alkali neutralisation; the spectator ions (e.g. Na+ and Cl−) move from reactants to products unchanged
  • Example: hydrochloric acid neutralised by sodium hydroxide
    • Full equation: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
    • Net ionic equation: as above
  • Example: sulfuric acid neutralised by potassium hydroxide
    • H2SO4(aq) + 2 KOH(aq) → K2SO4(aq) + 2 H2O(l)
Neutralisation of hydrochloric acid by sodium hydroxide: HCl (acid) plus NaOH (base) gives water plus NaCl (salt), split into the two ionic reactions H+ plus OH- forming water and Na+ plus Cl- forming NaCl
Source: Acids, Alkalis & Neutralisation by Save My Exams

Not every acid reaction is a neutralisation

  • A metal reacting with an acid gives a salt and hydrogen gas, not water, so it does not count as a neutralisation (see topic 13 section 1)
  • Neutralisation specifically requires that water be produced alongside the salt — so it is the acid + base / alkali / metal carbonate type reactions that qualify

A practical use: treating acidic soil

  • Many crops grow poorly when the soil pH drops below 7
  • Farmers spread bases such as crushed limestone (calcium carbonate) or quicklime (calcium oxide) on acidic soil
  • These bases neutralise the soil acidity and raise the pH back into the range crops tolerate