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, K_a?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 solutionsE.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

• The conjugate base of HCl is the chloride ion, Cl^-, but since the reverse reaction is virtually non-existent the chloride ion must be a very weak conjugate base

HCl (g) → H^+?(aq)? +? ?Cl^-?(aq)

acid? ? ? ? ? ? ? ? ? ? ? ? ? conjugate base

• In general?strong acids?produce?weak conjugate bases?and?weak acids?produce?strong conjugate bases
• A strong base is also fully ionized and is a good proton acceptor
• For example the hydroxide ion is a strong base and readily accepts protons:

OH^-?(aq)+??H⁺?(aq)? ?? H₂O (l)

• The conjugate acid of the hydroxide ion is water, which is a weak conjugate acid
• In general?strong bases?produce?weak conjugate acids