STPM Chemistry Form 6 Notes – Ionic Equilibrium (Part 5)

by BerryBerryTeacher

in Berry Reference (Notes)

As promised, a relatively easy post this time on Part 5 of Ionic Equilibrium for STPM Chemistry Form 6 (Kimia Tingkatan 6) from Berry Berry Easy. In this part, you’ll learn all about the determination of reagent strength in terms of acids or bases such as strong acids, weak acids and very weak acids. While this might seemed easy, some students fail to produce answers when asked in exams. With the lack of calculation and numbers in this post, try to understand the pKa values and why are they so. Understanding the values (general range) of the pKa values is crucial to understand this part.

(Tips: While you definitely do not need to memorise the tables shown below but it is useful to have in your head the range of the values of the strength of a reagent (Ka). For those who want to challenge yourself, you may also memorise maybe 2-3 common reagents and their strength values. )

STPM Chemistry Form 6 Notes – Ionic Equilibrium (Part 5)

Natural Universal Indicator_Purple cabbage

Natural Universal Indicator_Purple cabbage

Determination of Reagent Strength

pKa(HA) + pKb(A-) = 14

  • strength of a reagent is determined strictly from Ka (or pKa) and Kb (or pKb) is a measure of the completeness of a reaction in water
  • dissociation = ionisation = electrolytic nature
  • the stronger an acid = the more readily the acid dissociates into water
  • the stronger a base = the more readily the base hydrolysis into water

(*Notes: Not to confuse the concentration of a reactant with its strength (a highly concentrated weak acid may have a lower pH than a strong acid in low concentration.)

Strong acids

HX(aq) + H2O(l) <—-> H3O+(aq) + X-(aq)

  • strong acids are acids that dissociate fully when mixed with water
  • strong acids ionise completely into hydronium ion and a conjugate base when added to water
  • reactants convert completely into products, so the Ka value of a strong acid is very large
  • the equilibrium constant for the acid dissociation reaction has a large numerator and a minuscule denominator
  • Ka = ([H3O+] [X-]) / [HX] >> 1
  • pKa < 0
  • all the strong acids can be classified as either haloacids or oxyacids

Example:

Acid Name pKa
HCl Hydrochloric acid -7
HBr Hydrobromo acid -7
HI Hydroiodic acid -9
H2SO4 Sulfuric acid -9
HNO3 Nitric acid -2
HClO4 Perchloric acid -10

Weak acids

HX(aq) + H2O(l) <—-> H3O+(aq) + X-(aq)

  • weak acids are acids that only dissociate partially when dissolved into water
  • weak acids do not fully ionize into hydronium ion and conjugate base when added to water
  • Ka = ([H3O+] [X-]) / [HX] where 1 > Ka > 10-14
  • 0 < pKa < 14
  • Weak acids can be classified in a number of different classification as oxyacids of low oxidation state, one haloacid, carboxylic acids, alkyl ammoniums, and phenol

Example:

Acid Name pKa
H2SO3 Sulfurous acid 1.82
HClO2 Chlorous acid 1.90
HF Hydrofluoric acid 3.15
HNO2 Nitrous acid 3.41
HCO2H Formic acid 3.74
H3CCO2H Acetic acid 4.74
H2CO3 Carbonic acid 6.36
HClO Hypochlorous acid 7.46
HBrO Hypobromous acid 8.72
NH4+ Ammonium 9.26
HCN Hydrogen cyanide 9.32
HIO Hypoiodous acid 10.66

Very weak acids

HX(aq) + H2O(l) <—-> H3O+(aq) + X-(aq)

  • very weak acid dissociates less than water (Kw = 1.0 x 10-14)
  • Ka = ([H3O+] [X-]) / [HX] where Ka < 10-14
  • pKa > 14

The next part simply continues on this topic but for bases, so stay tune to Berry Berry Easy for Part 6 of STPM Chemistry notes on Ionic Equilibrium.

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