STPM Chemistry Form 6 Notes – Ionic Equilibrium (Part 9)

by BerryBerryTeacher

in Berry Reference (Notes)

Conjugate pairs in chemistry is one of the simplest but most baffling concepts in Form 6 chemistry. Due to the way Brønsted-Lowry defined acids and bases, we have a phenomena between acids and bases where acid can donate a proton, thereafter the remaining chemical is then considered as a base. Confusing? Be confused no further as Berry Berry Easy attempts to shed some light on it for our Berry Readers with Part 9 of Ionic Equilibrium notes for STPM Form 6 Chemistry students. Also in this post, we’ll be learning about physiological compounds and their natural states. This will be a sub-topic filled with new stuffs, so make sure you pay full attention when you are learning this part in class, or at least pay more effort to understand the notes served here.

(Tips: You must try your best to identify or at least memorise common conjugate pairs. This is exceptionally crucial as the ability to name conjugate pairs of acids and bases helps out when you need to compare the strengths of acids and bases. It is also important for buffered solutions and salt hydrolysis.)

STPM Chemistry Form 6 Notes – Ionic Equilibrium (Part 9)

Natural Universal Indicator_Purple cabbage

Natural Universal Indicator_Purple cabbage


Conjugate pairs

  • consists of an acid and a base that exchange one proton
  • acid – deprotonated forms its conjugate base
  • base – protonated forms its conjugate acid

Example:

The conjugate base of HCO3- is which of the following?

A H2CO3
B CO32-
C CO2
D HCO2-

Solution: Conjugate base is formed when an acid loses one proton (eliminate option A = conjugate acid of HCO3-).

Dissociation reaction of bicarbonate into hydronium and carbonate:

HCO3-(aq) + H2O(l) <—-> H3O+(aq) + CO32-(aq)

Answer: B CO32-

Typical conjugate pairs

  • Conjugate pair favours the conjugate acid form in the presence of hydronium (H3O+).
  • Conjugate pair favours the conjugate base form in the presence of hydroxide (OH- ).
  • If pH (solution) > pKa (conjugate acid), the solution is basic relative to the compound, so it is deprotanated.
  • If pH (solution) < pKa (conjugate acid), the solution is acidic relative to the compound, so it is protonated.

Example of common conjugate pairs:

  • carboxylic acids / carboxylates (RCOOH / RCOO- )
  • alkyl ammoniums / alkyl amines (RNH3+ / RNH2)
  • phenols / phenoxides (C6H5OH / C6H5O- )

Example of specific pairs:

  • carbonic acid / bicarbonate (H2CO3 / HCO3- )
  • phosphoric acid / dihydrogen phosphate (H3PO4 / H2PO4- )

Physiological compounds and their natural states

  • pH refers to the surrounding solution (environment) in which the compound exist
  • pKa refers to the conjugate acid that exists in solution
  • compound responds to the pH of the solution
Acid Conjugate Base
RCOOH / pKa = 3 – 5 RCOO- / pKb = 9 – 11
RNH3+ / pKa = 9 -10 RNH2 / pKb = 4 – 5

Relationship of pKa and pKb

  • an acid when deprotonated forms its conjugate base
  • a base when protonated forms its conjugate acid
  • Ka x Kb = 10-14

Example:

What is the pKa for ammonia, given that the pKb for ammonia is 4.7?

A 33
B 9.3
C 7.0
D 4.7

Solution:

  • the answer is not B 9.3! The compound with a pKa of 9.3 is ammonium (NH4+) – the conjugate acid of ammonia
  • ammonia is a weaker acid than ammonium, so it has a pKa greater than 9.3
  • pKa (HA) + pKb (A- ) = 14 is for conjugate pairs, not for the same compound.

Answer: A 33

So there you go, conjugate pairs and physiological compounds. If this part is difficult, wait till you read about the next headache which is the titration curve in Part 10 of the notes on Ionic Equilibrium for STPM Chemistry Form 6 students from Berry Berry Easy.

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