Strong acid and bases - Weak acid and bases - Dissociation constants and pK's | Chemistry Net

Strong acid and bases - Weak acid and bases - Dissociation constants and pK's

Strong acid and bases - Weak acid and bases - Dissociation constants and pK's

 

Strong acid and bases - Weak acid and bases - Dissociation constants and pK's


Acids are divided into two categories based on the ease with which they can donate protons to the solvent: i) strong acids and  ii) weak acids

Strong acids are acids that completely dissociate in water. The reaction of an acid with its solvent (typically H2O) is called an acid dissociation reaction.
Strong acids, such as HNO3, almost completely transfer their protons to the solvent molecules:
HNO3(aq) + H2O(l) H3O+(aq) + NO3(aq)

In this reaction H2O serves as the base. The hydronium ion, H3O+, is the conjugate acid of H2O, and the nitrate ion is the conjugate base of HNO3. It is the hydronium ion H3O+ that is the acidic species in solution, and its concentration determines the acidity of the resulting solution - pH of a strong acid.

The common strong acids in aqueous solutions are given in Table I.1:

Table I.1: Strong acids in aqueous solutions

Table I.1: Strong acids in aqueous solutions


When a solution of 0,1 M HNO3 dissolves in water dissociates completely to 0,1 M H+ and 0,1 M NO3.
Weak acids, for example acetic acid, cannot completely donate their acidic protons to the solvent. Instead, most of the acid remains undissociated, with only a small fraction present as the conjugate base (CH3COO).


dissociation of acetic acid



Weak acids are acids that dissociate partially in water. The extent of dissociation is given by the equilibrium constant.

Note:

A measure of the relative strength of an acid is: i) the equilibrium constant ka of the dissociation reaction of the acid in water (depends on temperature)  ii) the degree of dissociation α of the acid in water (depends on the concentration of the acid an on temperature).


The equilibrium constant for this reaction is called an acid dissociation constant, ka, and is written as:

ka = [H3O+] * [CH3COO] / [CH3COOH] = 1.76 * 10-5
The magnitude of ka provides information about the relative strength of a weak acid, with a smaller ka corresponding to a weaker acid. The opposite, small pka values characterize stronger acids.

Table I.2 below gives the ka and pka values for a number of commonly-encountered weak acids (25 C).

Compound
ka
pka
Acetic acid (CH3COOH)
1.76 x 10-5
4.75
Adipic acid (CH2)4(COOH)2
3.71 x 10-5   Step 1
3.87 x 10-5   Step 2
4.43
4.41
Benzoic acid (C6H5COOH)
6.46 x 10-5
4.19
Carbonic acid (H2CO3)
4.3 x 10-7
5.61 x 10-11
6.37
10.25
Chloroacetic acid (ClCH2CO2H)
1.4 x 10-3
2.85
Chromic acid (H2CrO4)
1.8 x 10-1
3.2 x 10-7
0.74
6.49
Formic acid (HCOOH)
1.77 x 10-4   (20 C)
3.75
Hydrocyanic acid (HCN)
4.93 x 10-10
9.31
Hydrofluoric acid (HF)
3.53 x 10-4
3.45
Hypobromous acid (HBrO)
2.06 x 10-9
8.69
Hypochlorous acid (HClO)
2.95 x 10-8
7.53
Hypoiodous acid (HIO)
2.3 x 10-11
10.64
Iodic acid (HIO3)
1.69 x 10-1
0.77
Maleic acid
1.42 x 10-2
8.57 x 10-7
1.83
6.07
Nitrous acid (HNO2)
4.6 x 10-4   (12,5 C)
3.37
Oxalic acid (H2C2O4)
5.90 x 10-2
6.40 x 10-5
1.23
4.19
Periodic acid
2.3 x 10-2
1.64
Phenol (C6H5OH)
1.28 x 10-10
9.89
o-Phosphoric acid (H3PO4)
7.52 x 10-3
6.23 x 10-8
2.12
7.21
Table I.2: The ka and pka values for a number of commonly-encountered weak acids


Simirarly, strong bases, such as NaOH, are bases that completely dissociate in water to produce hydroxyl ion OH-:

NaOH    Na+ + OH-

When 0.1M NaOH dissolves in H2O dissociates to 0.1M Na+ and 0.1M  OH-.
The common strong bases in aqueous solutions are given in Table I.3:


Strong bases in aqueous solutions
          Group 1A metal hydroxides        
LiOH, NaOH, KOH, RbOH, CsOH
Group 2A metal hydroxides
Ca(OH)2, Sr(OH)2, Ba(OH)2

Table I.3: Strong bases in water

  
Weak bases are bases that partially dissociate in water – only partially accept protons from the solvent – and are characterized by base dissociation constant kb. For example the base dissociation constant kb for the weak base acetate ion is given by:

CH3COO-  +  H2O  =  OH-    +     CH3COOH


kb =  [OH-][CH3COOH] /  [CH3COO]  = 5.71 * 10-10
The magnitude of kb provides information about the relative strength of a weak base, with a smaller kb corresponding to a weaker base.
Table I.4 below gives the k and pkb values for a number of commonly-encountered weak bases.
 

Compound
kb
pkb
Acetamide (CH3CONH2)
2.5 x 10-13
12.60
Acetanilide (CH3CONHPh)
4.1 x 10-14
13.39
Ammonium hydroxide  (NH4OH)
1.79 x 10-5
4.75
Diethylamine (C2H5NHC2H5)
9.6 x 10-4
3.02
n-Diethylamine
3.65 x 10-8
7.44
Glycine
2.26 x 10-12
11.65
Methylamine
4.38 x 10-4
3.36

Table I.4: The kb and pkb values for a number of commonly-encountered weak bases

What ranges of ka values characterize strong and weak acids? What are the corresponding degree of dissociation α values?

 


Type of acid

ka

pka
Degree of dissociation (α) of acid (%)
Very strong acid
> 0.1
< 1
90-100%
Moderately strong acid
10-3 – 0.1
1-3
30-90%
Weak acid
10-5 – 10-3
3-5
30-5%
Very weak acid
10-15 – 10-5
5-15
5-1%
Extremely weak acid
< 10-15
> 15
< 1%


Why proton transfers occur in acid-base reactions?

 

Acids are proton (H3O+) donors while bases are proton acceptors. An acid, such as HCl, donates a proton to a suitable base like H2O because the products formed (H3O+ and Cl-) are of lower free energy (ΔG) than the reactants and this means energetically more favorable.   
HCl(aq) + H2O(l) H3O+(aq) + Cl(aq)

The free energy (ΔG) for the ionization reaction of HCl in Η2Ο is -40 kJ/mol. Such a large negative ΔG shows that the reaction lies well over to the right – formation of products is highly favorable due to the lower free energy level (Fig. I.1). Therefore, proton transfers occur in acid-base reactions in solution because are energetically favorable since the products are of lower energy than the reactants.

Fig. I.1: Formation of products -  H3O+ transfer – is highly favored in the ionization of HCl in water due to the lower energy attained. The ΔG for the reaction is -40 kJ/mol.


Fig. I.1: Formation of products -  H3O+ transfer – is highly favored in the ionization of HCl in water due to the lower energy attained. The ΔG for the reaction is -40 kJ/mol.
In the gas phase, however, ionization of  HCl(g) happens with much difficulty and ΔG = +1347 kJ/mol.  This means that HCl(g) does not spontaneously ionize in the gas phase—it does not lose protons at all. Why then is HCl such a strong acid in water? The key to this problem is, of course, the water. In the gas phase we would have to form an isolated proton (H3O+) and chloride ion and this is energetically very unfavourable. In contrast, in aqueous solution the proton is strongly attached to a water molecule to give the very stable hydronium ion, H3O+, and the ions are no longer isolated but solvated (Fig. I.2).
 
Fig. I.2: Proton solvated – attached to water molecules.

Fig. I.2: Proton solvated – attached to water molecules


Relevant Posts - Relevant Videos

Ionic equilibrium - A general expression of the pH of  a strong acid

Chemical Equilibrium Calculations in Analytical Chemistry

pH of a strong acid – Examples

Strong Acids & Bases: pH Calculations involving mixtures of strong acids and bases

 


References

  1. J-L. Burgot “Ionic Equilibria in Analytical Chemistry”,Springer Science & Business Media, 2012
  2. J.N. Butler “Ionic Equilibrium – Solubility and pH calculations”, Wiley – Interscience, 1998
  3. Clayden,Greeves, Waren and Wothers “Organic Chemistry”, Oxford, 2012
  4. D. Harvey,“Modern Analytical Chemistry”, McGraw-Hill Companies Inc., 2000
  5. Toratane Munegumi, World J. of Chem. Education, 1.1, 12 (2013)


Key Terms

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