Definitions

• An acid ionises in solution to form H+ ions.

• A base ionises in solution to form OH- ions.

 

Strength of acid/base

• The strengths of acid and base are dependent on the degree of dissociation.

• Degree of dissociation, α = (Amount Dissociated)/ (Initial Amount)

• Strength of acid depends on

  • α, higher the value, the stronger it is

  • Independent of [H+] and [OH-]

• Strong acids/bases

  • Ionise completely in solution

  • α = 1

  • [acid] = [H+ aq], *

  • [base] = [OH- aq], *

• Weak acids/bases

  • Ionise partially in solution

  • α < 1

  • For weak acid HA,

    • HA(aq) ⇌ H+(aq) + A-(aq)

    • [HA] >> [H+ aq]

  • For weak base B,

    • B(aq) + H2O(l) ⇌ HB+(aq) + OH-(aq)

    • [B] >> [OH- aq]

 

9.1 pH and pOH

• pH

  • The pH of a solution is the negative logarithm to the base 10 of the H+ ion concentration in mol dm^-3.

    • pH= -log10 [H+]

• pOH

  • The pOH of a solution is the negative logarithm to the base 10 of the OH- ion concentration in mol dm^-3.

    • pOH= -log10 [OH-]

• pOH + pH = 14

• [H+] = 10^- pH

• [OH-] = 10^- pOH

 

9.2 Measurement of pH

• Indicators show different colors at a different pH.

• Examples include Universal Indicator, Phenolphthalein, Bromothymol blue and Methyl Orange. (Refer to Mole Concept for pH ranges).

• pH sensor attached to a data logger

• pH meter

 

9.2.1 Acid Dissociation Constant, Ka, and the use of pKa

• Acid dissociation constant

• HA(aq) ⇌ H+(aq) + A-(aq)

• Ka = ( [H+] [A-] / [HA] ) mol dm^-3, constant at a given temperature.

  • Ka is the measure of the strength of the acid.

  • Ka increases with temperature since dissociation is endothermic.

  • Larger Ka implies stronger acid (higher [H+] and [A-])

  • pKa = -log10 Ka

  • Smaller pKa implies stronger acid.

 

9.2.2 Base Dissociation Constant, Kb, and the use of pKb

• Base dissociation constant

• B(aq) + H2O(l) ⇌ HB+(aq) + OH-(aq)

• Kb = ( [HB+][OH-] / [B] ) mol dm^-3, constant at a given temperature

  • Kb is the measure of the strength of a base.

  • Kb increases with temperature since dissociation is constant.

  • Larger Kb implies stronger base (higher [HB+] and [OH-])

  • pKb = -log10 Kb

  • Smaller pKb implies stronger base.

 

9.3 Ionic Product of Water

• H2O(l) ⇌ H+(aq) + OH-(aq) , ΔH >0

• Kw = [H+ (aq)] [OH- (aq)] mol^2 dm^-6 = Ka x Kb

• Kw = 1.00 * 10 ^-14 mol^2 dm^-6 at 298 K

• [H+ (aq)] = [OH- (aq)] in water

• Constant Kw at constant temperature

• [H+] and [OH-] increase with temperature as dissociation is endothermic.

• [H+ aq] = [OH- aq] so pH does not change.

• pKw = -log10 Kw