STPM Chemistry Form 6 Notes – Ionic Equilibrium (Part 6)

by BerryBerryTeacher

in Berry Reference (Notes)

Continuing on the determination of reagent strength, in this Part 6 of Ionic Equilibrium for STPM Chemistry Form 6 (Kimia Tingkatan 6) from Berry Berry Easy, Berry Readers will learn about reagent strengths in terms of bases such as strong bases, weak bases and very weak bases. This post should not be read in isolation but should instead be read with Part 5. So do not proceed to read this post unless you have fully understand Part 5. Similarly, try to understand the Kb/pKb values and why are they so. Understanding the values (general range) of the pKb values is crucial to understand this part.

(Tips: Please do not memorise the values in the tables below, but instead try to memorise the range of the values of the reagent strength. Take note also that for acid, it is written as Ka, while for base, it is written as Kb. Learning the correct notation is crucial to avoid dropping silly marks in exams)

STPM Chemistry Form 6 Notes – Ionic Equilibrium (Part 6)

Natural Universal Indicator_Purple cabbage

Natural Universal Indicator_Purple cabbage

Strong bases

MOH(s) + H2O(l) <—-> M+(aq) + OH-(aq)

  • strong bases react completely when mixed with water
  • strong bases fully hydrolyse water and completely ionize when added water
  • Kb = ([M+ ] [OH- ]) / [MOH] >> 1
  • pKb < 0
  • all the strong bases can be classified as either hydrides, hydroxides, alkoxides, amines or carbides

Example:

Base Name pKb
LiOH Lithium hydroxide -0.36
Ba(OH)2 Barium hydroxide -2.02
Sr(OH)2 Strontium hydroxide -2.19

Weak bases

A-(aq) + H2O(l) <—-> HA(aq) + OH-(aq)

  • weak bases are bases that only react partially when dissolved into water
  • weak bases partially hydrolyse water
  • weak bases do not fully ionise into when added to water
  • Kb = ([HA] [OH- ]) / [A- ] where  10-14 < Kb < 1
  • 0 < pKb < 14
  • weak bases are the conjugate bases of the acids
  • common weak bases: carboxylates (RCOO- ), alkyl amines (RNH2), bicarbonate (HCO3- ), carbonate (CO32- ), phosphate (PO43- ) and phonoxides (C6H5O- )

Example:

Base Name pKb
BeOH Beryllium hydroxide 10.30
C6H5NH2 Aniline 9.4
NH3 Ammonia 4.75
CH3NH2 Methylamine 3.36
Zn(OH)2 Zinc Hydroxide 3.02
Ca(OH)2 Calcium hydroxide 2.37
CsOH Caesium hydroxide 1.76
KOH Potassium hydroxide 0.5
NaOH Sodium hydroxide 0.2

Very weak bases

A-(aq) + H2O(l) <—-> HA(aq) + OH-(aq)

  • very weak bases ionise less than water (Kw = 1.0 x 10-14)
  • Kb = ([HA] [OH- ]) / [A- ] where Kb < 10-14
  • pKb > 14

The next part, Part 7 in this series Berry Berry Easy notes for STPM Chemistry : Ionic Equilibrium attempts to summarise the strength of reagents in tabulated format for easy viewing. So make sure you do not miss it, a very important post!!!

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