Brønsted-Lowry Acids & Bases Theory
- A Brønsted acid is a species that can donate a proton
- For example, hydrogen chloride (HCl) is a Brønsted acid as it can lose a proton to form a hydrogen (H+) and chloride (Cl-) ion
HCl (aq) → H+ (aq) + Cl- (aq)
- A Brønsted base is a species that can accept a proton
- For example, a hydroxide (OH-) ion is a Brønsted 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
Monobasic, dibasic & tribasic acids
- Acids can be classified by the number of bases that they can donate protons to in a reaction, which depends on how many H+ per molecule that they can give up in a reaction
- Acids such as HCl, HNO3, and HCN that contain one ionisable hydrogen atom in each molecule are called monobasic (or monoprotic) acids
- When HCl reacts with NaOH we can see that one hydrogen is replaced by a sodium atom
HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (aq)
- Dibasic (or diprotic) acids contain two ionisable hydrogen atoms per molecule, for example H2SO4
- Ionisation of such acids occurs in two steps
- When H2SO4 reacts with NaOH we can see that two hydrogens are replaced by two sodium atoms
H2SO4 (aq) + 2NaOH (aq) → Na2SO4 (aq) + 2H2O (aq)
- Tribasic (or triprotic) acids contain three ionisable hydrogen atoms per molecule, for example H3PO4
- Ionisation of such acids occurs in three steps
- When H3PO4 reacts with NaOH we can see that three hydrogens are replaced by three sodium atoms
H3PO4 (aq) + 3NaOH (aq) → Na3PO4 (aq) + 3H2O (aq)