STPM Chemistry Form 6 Notes – Ionic Equilibrium (Part 4)

by BerryBerryTeacher

in Berry Reference (Notes)

In our previous post, Berry Readers were treated to a feast of three chemistry theories namely the Arrhenius Theory, Brønsted-Lowry Acid-Base Theory, Lewis Theory. In this Part 4 of Ionic Equilibrium for STPM Chemistry Form 6 (Kimia Tingkatan 6) from Berry Berry Easy, you’ll also be treated again to three concepts which are the water-based acid-base chemistry, acid dissociation, and base hydrolysis. It is not as hard a theories but definitely trickier to understand. Some students will find this easy while most will struggle with this part. So the key remains, ‘understand’ rather than ‘memorise’ if you want to understand the three concepts presented below.

(Tips: Copy all three concepts on flash cards and flip it around so you cannot see it. The test your understanding and see if you can fully distinguish the difference and state the main characteristics of the concepts. You can also do this with a friend.)

STPM Chemistry Form 6 Notes – Ionic Equilibrium (Part 4)

 

Natural Universal Indicator_Purple cabbage

Natural Universal Indicator_Purple cabbage

 

Water-based Acid-Base Chemistry

2H2O(l) <—-> H3O+(aq) + OH-(aq)

  • water is amphoteric that may act as either an acid or base
  • neutral water naturally dissociates into hydronium and hydroxide
  • only small fraction of water dissociates into solution
  • at 25˚C (in the absence of an acid or base), water dissociates enough to generate a solution with 10-7M H3O+ and 10-7M OH-
  • a neutral aqueous solution, [H3O+ ] = [OH- ]
  • pH = pOH = 7.0

Acid Dissociation

HA(aq) + H2O(l) <—-> H3O+(aq) + A-(aq)

  • Ka = acid dissociation constant
  • Ka = ([H3O+ ] [A- ]) / [HA]
  • Each molecule of an acid when added to water, dissociates to form hydronium ion (H3O+) and its conjugate base upon reaction with one water molecule.
  • pKa = -lg Ka
  • Ka = 10-pKa
  • strength of an acid increases (↑), its Ka (↑) and its pKa (↓)
  • it means that stronger acid have higher Ka values and lower pKa values
  • Ka and pKa of an acid depend on the strength of the acid, but not its concentration

Base Hydrolysis

A-(aq) + H2O(l) <—-> OH-(aq) + HA(aq)

  • Kb = base hydrolysis constant
  • Kb = ([OH- ] [HA]) / [A- ]
  • Each molecule of a base when added to water, hydrolyses to form hydroxide ion (OH- ) and its conjugate acid upon reaction with one water molecule.
  • pKb = -lg Kb
  • Kb = 10-pKb
  • strength of a base increases (↑), its Kb (↑) and its pKb decreases (↓)
  • it means that stronger base have higher Kb values and lower pKb values
  • Kb and pKb of a base depend on the strength of the base, but not its concentration

Overall Relationship

Acid strength Ka pKa Conjugate base strength Kb pKb

The next part, Part 5 will be a relieve to students after two difficult subtopics in a row. In the next post, Berry Readers will learn about the strength of reagents – strong acids, weak acids and very weak acids. So stay tune to Berry Berry Easy’s notes on STPM Chemistry Tingkatan 6/Form 6 – Ionic Equilibrium.

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