Bronsted-Lowry Concept

• Bronsted-Lowry theory defines acid as any substance that donates H+ to its partner in a reaction
• Substance that accepts the H+ is classified as a base
• Example reaction: NH3 (aq) + HCl (aq) → NH4Cl (aq)
• HCl donates H+ and is classified as a Bronsted-Lowry acid
• NH3 accepts H+ and is classified as a Bronsted-Lowry base
• In Bronsted-Lowry theory, even ions can be classified as acid or base
• Water can act as an acid or a base depending on its partner, known as amphoterism
• H2O can act as an acid or a base, exhibiting amphoterism
• Conjugate acid of NH3 is NH4+ while conjugate base of H2O is OH-
• Strong conjugates can undergo hydrolysis, weak conjugates cannot
• NH3 is a weak base, so NH4+ is a strong conjugate acid that can undergo hydrolysis in Eqn. 8
• H2O is a weak acid, so OH- is a strong conjugate base
• Table 1 lists conjugate acid and/or conjugate base of common molecules in aqueous environment
• HCl is a strong acid, so its conjugate base, the chloride ion, will not undergo hydrolysis
• Acetic acid is a weak acid, making acetate ion a strong conjugate base that will undergo hydrolysis
• Water (H2O) and bicarbonate ion (HCO3-) are amphoteric molecules that can form their conjugate acid or base in an aqueous environment
• Both conjugates of water are strong
• As an acid, bicarbonate ion (HCO3-) is a weak acid, and as a base, it is also a weak base
• Both carbonic acid (H2CO3) and carbonate ion (CO32-) can undergo hydrolysis
• Nitrate ion (NO3-) is a weak conjugate base of nitric acid (HNO3) which is a strong acid
• Acetate ion (CH3COO-) is a strong conjugate base of acetic acid (CH3COOH) which is a weak acid
• Acetate ion is a stronger base compared to nitrate ion
• Hydroxide ion (OH-) is produced in the hydrolysis of acetic acid (CH3COOH) with water (H2O)
• Hydrogen phosphate ion (HPO4-) is an amphoteric molecule that can act as an acid or a base
• Hydrolytic reaction for hydrogen phosphate ion:

  • As an acid: HPO4- + H2O -> H2PO4- + OH-
  • As a base: HPO4- + H2O -> HPO42- + H3O+
• Bronsted-Lowry theory focuses on the ability of acids to donate H+ and bases to accept H+
• Conjugate acids are formed when bases accept H+ and conjugate bases are formed when acids donate H+
• Strong acids and bases produce weak conjugates, while weak acids and bases produce strong conjugates
• Strong conjugates can undergo hydrolysis, a Bronsted-Lowry reaction of a molecule with water
• Amphoteric molecules can act as an acid or a base, depending on their partner