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

When a solution of 0,1 M HNO₃dissolves 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^–).

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 k_a 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, k_a, and is written as:

k_a = [H₃O⁺] * [CH₃COO^–] / [CH₃COOH] = 1.76 * 10^-5

The magnitude of k_a provides information about the relative strength of a weak acid, with a smaller k_acorresponding to a weaker acid. The opposite, small pk_a values characterize stronger acids.

Table I.2 below gives the k_a and pk_a 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 k_a and pk_a 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 basesare bases that partially dissociate in water – only partially accept protons from the solvent – and are characterized by base dissociation constant k_b. For example the base dissociation constant k_b for the weak base acetate ion is given by:

CH₃COO^-  +  H₂O  =  OH^-    +     CH₃COOH

k_b =  [OH^-][CH₃COOH] /  [CH₃COO^–]  = 5.71 * 10^-10

The magnitude of k_b provides information about the relative strength of a weak base, with a smaller k_bcorresponding to a weaker base.

Table I.4 below gives the k and pk_b 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 k_band pk_b values for a number of commonly-encountered weak bases

What ranges of k_a 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.

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

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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

 

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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)

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Key Terms

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