• A?Br?nsted acid?is a species that can donate a proton
  • For example, hydrogen chloride (HCl) is a Br?nsted acid as it can lose a proton to form a hydrogen (H+) and chloride (Cl-) ion

   

HCl (aq) → H+?(aq) + Cl-?(aq)

• A?Br?nsted base?is a species that can accept a proton
  • For example, a hydroxide (OH-) ion is a Br?nsted base as it can accept a proton to form water

   

OH-?(aq) + H+?(aq) → H2O (l)

Weak acids dissociating

• In an equilibrium reaction, the products are formed at the same rate as the reactants are used
• This means that at equilibrium, both reactants and products are present in the solution
• For example, ethanoic acid (CH3COOH) is a weak acid that partially dissociates in solution
• When equilibrium is established there are CH3COOH, H2O, CH3COO-?and H3O+?ions present in the solution
• The species that can?donate?a proton are?acids?and the species that can?accept?a proton are?bases

CH3COOH (aq)? ?+? H2O (l)? ? ?? CH3COO-?(aq)? ? ?+? ?H3O+?(aq)

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

• The reactant CH3COOH is linked to the product CH3COO-?by the transfer of a?proton?from the acid (CH3COOH) to the base (CH3COO-)
• Similarly, the H2O molecule is linked to H3O+?ion by the transfer of a proton
• These pairs are therefore called?conjugate acid-base pairs
• A?conjugate acid-base pair?is two species that are different from each other by an H+?ion
  • Conjugate?here means related
  • In other words, the acid and base are related to each other by one proton difference

   

Answers

The pairs in the order acid/base are:

1. 1. HCO3-?and CO32-?; H3O+?and H2O
   2. H3O+?and H2O ;??(CO2?+ H2O) and HCO3-
   3. H2SO4?and HSO4-??;?(NO2+?+ H2O) and HNO3
   4. HSO4-?and SO42-?; H2O and OH-