Hydrolysis of Salts December 2015

What we know so far…. Bronsted-Lowry Acids Bronsted-Lowry Bases Donate H+ (protons) Criteria to be an acid: Must have an H+ to give away Ka > 10-14 Can be neutral (HNO2), positive (NH4+ - the conj acid of NH3 ) or negative (HSO3- - the second ionization of H2SO3) Accept H+ (protons) Criteria to be a base: Must have a lone pair of electrons in order to attract an H+ Kb > 10-14 *see next slide Can be neutral (NH3) or negative (CN-) but NEVER positive

Why > 10-14? Consider Cl-: Problem: can’t act as an base because HCl is a strong acid Kb is too small (Kb = ___); won’t make enough OH- to influence the pH Consider the base B with Kb = 1x10-15 B(aq) + H2O  BH+(aq) + OH-(aq) E 0.10-x x x Calculate x, pOH and pH. What is unusual about your answer?

Salts Salts: are ionic compounds that completely dissociate in water What are salts? Recall the 2 products that are formed when an acid and a base react: Acid + Base  ______ + ______ i.e. KOH(aq) + HBr(aq)  KBr(aq) + H2O(l) Salts: are ionic compounds that completely dissociate in water water surrounds all anions and cations Can these ions interact with the surrounding water like acids and bases do?

Hydrolysis of Salts Conclusion: Consider the following salt solutions: NaCl(aq) pH = 7 NaNO2(aq) pH > 7 NH4Cl(aq) pH < 7 Conclusion: something in the NaNO2 solution is accepting H+ from H2O to produce OH- something in the NH4Cl solution is donating H+ to H2O to produce H3O+ nothing is donating/accepting H+ in the NaCl solution.

Consider the basic nature of NaNO2 and the criteria to be a base. What part of NaNO2 is acting as a base? Write the ionization reaction that represents this. What is the Kb for NO2-? What is the pH of a 0.10M solution of NaNO2?

Consider the acidic nature of NH4Cl and the criteria to be an acid. What part of NH4Cl is acting as an acid? Write the ionization reaction that represents this. What is Ka for NH4+? What is the pH of a 0.25M NH4Cl solution?

Consider the neutral nature of NaCl Consider the neutral nature of NaCl. Why are Na+ or Cl- unable to interact with water that surrounds them?

Hydrolysis of Salts When anions or cations or BOTH in a salt solution interact with the water that surrounds them to produce acidic or basic solutions.