18.2 Calculations involving acids and bases

The strengths of acids and the acid dissociation constant

• Strong acids and bases:
  • Since strong acids are assumed to completely dissociate in water, the concentration of each of the two ions produced is the same as the initial concentration of a strong monoprotic acid (one hydrogen)

• Weak acids and bases:
  • Since these do not complete dissociate, the concentration of products is not the same
  • The way to solve for concentrations is using the equilibrium expression
  • Acid dissociation constant:
    • 
  • Base dissociation constant:
    • 

Calculating K_a and K_b

1. Write out the complete reaction
2. Complete an ICE table
3. Determine K_a or K_b

K_a and K_b for a conjugate acid-base pair

• 
• Strong acid
  • Large K_a
  • Weaker conjugate base
  • Smaller K_b of conjugate base
• Strong base
  • Large K_b
  • Weaker conjugate acid
  • Smaller K_a of conjugate acid

The temperature dependence of K_w

• K_w is defined only at 25°C
• The ionization of water is endothermic, so as temperature increases, equilibrium shifts right
• Increase in concentration of H⁺ results in decrease in pH

pK_a and pK_b

• The pH scale can be applied to K_a and K_b to get pK_a and pK_b