STPM Chemistry Form 6 Notes – Ionic Equilibrium (Part 7)

by BerryBerryTeacher

in Berry Reference (Notes)

Linking heavily to Berry Berry Easy’s Part 5 and Part 6 notes on Ionic Equilibrium for STPM Form 6 Chemistry students, this Part 7 will be the post to link up everything regarding the strength of reagents. While part 5 focuses on acid and part 6 focuses on base, this part is all about the strength of reagents and the pK scales and also a summary of what you have learned in the previous two posts.

(Tips: Try to do first understand and then memorise the tables below. To easily memorise what is in the tables, you will first need to understand the concepts of compound, conjugate, the strength of the acid/base, the value of the pK scale in the examples given. By understanding that, you’ll easily ‘memorise’ everything in these tables.)

STPM Chemistry Form 6 Notes – Ionic Equilibrium (Part 7)

Natural Universal Indicator_Purple cabbage

Natural Universal Indicator_Purple cabbage

Relationship between the strength and the pK scale

  • strength of the reagent is measured by its ability to react in water
  • strong acids are more readily to dissociated in water (do not confuse the concentration of a reactant with its strength)
  • strong acids have very weak conjugate bases
  • strong bases have very weak conjugate acids

These are some shortnotes for the previous ionic equilibrium studynotes

Summary – Strong acids / Very weak bases

Compound Ka / pKa Conjugate Kb / pKb
Strong acid - Very weak base -
HCl Ka >> 1 Cl- Kb < 10-14
(pKa = -7) pKa < 0 (pKb = 21) pKb > 14

Summary – Weak acids / Weak bases

Compound Ka / pKa Conjugate Kb / pKb
Weak acid - Very weak base -
RCOOH 10-14 < Ka < 1 RCOO- 10-14 < Kb < 1
(pKa = 3–5) 0 < pKa < 14 (pKb = 9-11) 0 < pKb < 14

Summary – Very weak acids / Strong bases

Compound Ka / pKa Conjugate Kb / pKb
Very weak acid - Strong base -
CH4 Ka < 10-14 CH3- Kb >> 1
(pKa = 49) pKa > 14 (pKb = -35) pKb < 0

The next part , Part 8 of STPM Chemistry Form 6 notes on Ionic Equilibrium is focused on the Ionic Product of water. Make sure you understand fully on Parts 5-7 before heading to Part 8. The next part is easy but are usually confusing for students who do not like the log scale. So stay tuned, Berry Readers.

Previous post:

Next post: