STRONG AND WEEK ACIDS / IGCSE/GCSE / GCE- CHEMISTRY

Acids and Alkalis
Strong and Weak Acids - Strength and Concentration.
Acids and alkalis can be described as strong or weak.This does not mean the same as concentrated or dilute.
The strength of an acid or alkali depends on how ionised it is in water.
A strong acid or alkali is completely (100%) ionised. For hydrochloric acid
hydrogen chloride (in water) hydrogen ion + chloride ionHCl(aq) H+(aq) + Cl-(aq)
All of the hydrogen chloride moleculesbecome hydrogen ions and chloride ions in water(see examples for other strong acids).
For sodium hydroxide
sodium hydroxide (in water) sodium ion + hydroxide ionNaOH(aq)
Na+(aq) + OH-(aq)
Sodium hydroxide exists as ions both in water and in the solid.(see examples for other strong alkalis).

A weak acid or alkali is only partly (less than 100%) ionised.
For ethanoic acid
ethanoic acid (in water) hydrogen ion + ethanoic ion

CH3CO2H(aq) H+(aq) + CH3CO2-(aq)
Some of the ethanoic acid molecules become ions in waterbut most of them stay as molecules.The reaction is reversible (shown by the arrow).
For ammonia
ammonia + water ammonium ion + hydroxide ion

NH3(g) + H2O(l) NH4+(aq) + OH-(aq)
Some of the ammonia molecules become ions in waterbut most of them stay as molecules.