Strong & Weak Acids & Bases
Strong acids
- A strong acid is an acid that dissociates almost completely in aqueous solutions
- HCl (hydrochloric acid), HNO3 (nitric acid) and H2SO4 (sulfuric acid)
- The position of the equilibrium is so far over to the right that you can represent the reaction as an irreversible reaction
The diagram shows the complete dissociation of a strong acid in aqueous solution
- The solution formed is highly acidic due to the high concentration of the H+/H3O+ ions
- Since the pH depends on the concentration of H+/H3O+ ions, the pH can be calculated if the concentration of the strong acid is known
pH is the negative log of the concentration of H+/H3O+ ions and can be calculated if the concentration of the strong acid is known using the stoichiometry of the reaction
Weak acids
- A weak acid is an acid that partially (or incompletely) dissociates in aqueous solutions
- Eg. most organic acids (ethanoic acid), HCN (hydrocyanic acid), H2S (hydrogen sulfide) and H2CO3 (carbonic acid)
- The position of the equilibrium is more over to the left and an equilibrium is established
The diagram shows the partial dissociation of a weak acid in aqueous solution
- The solution is less acidic due to the lower concentration of H+/H3O+ ions
- Finding the pH of a weak acid requires using the acid dissociation constant, Ka but this not required at Standard Level, but only at Higher Level and is covered in Topic 18
Acid & Equilibrium Position Table
Strong bases
- A strong base is a base that dissociates almost completely in aqueous solutions
E.g. group 1 metal hydroxides such as NaOH (sodium hydroxide)
- The position of the equilibrium is so far over to the right that you can represent the reaction as an irreversible reaction
The diagram shows the complete dissociation of a strong base in aqueous solution
- The solution formed is highly basic due to the high concentration of the OH- ions
Weak bases
- A weak base is a base that partially (or incompletely) dissociates in aqueous solutions
- NH3 (ammonia), amines and some hydroxides of transition metals
- The position of the equilibrium is more to the left and an equilibrium is established
The diagram shows the partial dissociation of a weak base in aqueous solution
- The solution is less basic due to the lower concentration of OH- ions
Base & Equilibrium Position Table