3.1.12 pH of salt solutions (HL)

chem whatarethemechanismsofchemicalchange

Reactivity 3.1.12 - the pH of a salt solution depends on the relative strengths of the parent acid and base

see 3.1.10 and 3.1.11 acid and base dissociation constants (HL)

the salt formed in a neutralisation reaction is an ionic compound:

• the cation is the conjugate acid of the parent base

• the anion is the conjugate base from the parent acid

• neutralisation reactions do not all form neutral aqueous solutions

• the of a salt solution depends on whether and to what extent their ions react with water and hydrolyse it - releasing or ions

the relative strengths of the conjugate acids and bases in the salt determine the extent of hydrolysis reactions

if HA is weak, then the equilibrium tends towards the

salt of strong acid and strong base - no hydrolysis

• weak conjugate acid
• weak conjugate base
• no hydrolysis

salt of weak acid and strong base - anion hydrolysis

• weak acid → strong conjugate base
• strong conjugate base → hydrolysis of
• overall end basic solution
• strong base → weak conjugate acid (no hydrolysis)

salt of strong acid and weak base - cation hydrolysis

• strong acid → weak conjugate base (no hydrolysis)
• weak acid → strong conjugate acid
  • hydrolysis of
• overall acidic solution

salt of weak acid and weak base

• two strong conjugates
• both carry out hydrolysis
• dependent upon and

3.1.10 and 3.1.11 acid and base dissociation constants (HL)

challenge questions

7. formulate equations for reactions that produce the salt ammonium nitrate from its parent acid and base and deduce the of its solution.

   at high temperatures, solid decomposes to produce nitrogen, oxygen and water. formulate equations for this reaction and suggest why it can be explosive.

2 moles of solid releases 7 moles of gas, causes rapid expansion and increase in pressure. ignites easily