Acid & Base Dissociation
Strong acids
- A strong acid is an acid that dissociates almost completely in aqueous solutions
- E.g. 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
Diagram showing the dissociation of a strong acid in aqueous solution
In an aqueous solution, a strong acid almost completely dissociates
- 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
- The concentration of H+ / H3O+ ions can be written as [H+ (aq)]
- 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
pH = –log10 [H+ (aq)]
Weak acids
- A weak acid is an acid that partially (or incompletely) dissociates in aqueous solutions
- E.g. most organic acids (ethanoic acid), HCN (hydrocyanic acid), H2S (hydrogen sulfide) and H2CO3 (carbonic acid)
- The position of the equilibrium is more to the left and an equilibrium is established
Diagram showing the dissociation of a weak acid in aqueous solution
In an aqueous solution, a weak acid does not fully dissociate
- The solution is less acidic due to the lower concentration of H+ / H3O+ ions
- Finding the pH of a weak acid is a bit more complicated as now the concentration of H+ ions is not equal to the concentration of acid
- To find the concentration of H+ ions, the acid dissociation constant (Ka) should be used
Acid & equilibrium position table
Strong acid | Weak acid | |
Position of equilibrium | Right | Left |
Dissociation | Completely (→) | Partially () |
H+ concentration | High | Low |
pH | Use [strong acid] for [H+] | Use Ka to find [H+] |
Examples | HCl HNO3 H2SO4 (first ionisation) |
Organic acids, e.g. ethanoic acid HCN H2S H2CO3 |
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
Diagram showing the dissociation of a strong base in aqueous solution
In an aqueous solution, a strong base almost completely dissociates
- 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
- E.g. NH3 (ammonia), amines and some hydroxides of transition metals
- The position of the equilibrium is more to the left and an equilibrium is established
Diagram showing the dissociation of a weak base in aqueous solution
In an aqueous solution, a weak base does not fully dissociate
- The solution is less basic due to the lower concentration of OH- ions
Base & equilibrium position table
Strong base | Weak base | |
Position of equilibrium | Right | Left |
Dissociation | Completely (→) | Partially () |
OH– concentration | High | Low |
Examples | Group 1 metal hydroxides | NH3 Amines Some transition metal hydroxides |
Examiner Tip
- Hydrogen ions in aqueous solutions can be written as either as H3O+ or as H+ however, if H3O+ is used, H2O should be included in the chemical equation:
HCl (g) → H+ (aq) + Cl- (aq)
or
HCl (g) + H2O (l) → H3O+ (aq) + Cl- (aq)
- Remember that some acids are both strong and weak acids – for example, H2SO4 (sulfuric acid) has two hydrogen ions that can ionise.
- H2SO4 acts as a strong acid: H2SO4 → H+ + SO4-
- HSO4- acts as a weak acid: HSO4- ⇌ H+ + SO42-
- Also, don't forget that the terms strong and weak acids and bases are related to the degree of dissociation and not the concentration.
- The appropriate terms to use when describing concentration are dilute and concentrated.