CHAPTER 3 THEORETICAL FRAMEWORK AND CONTEXT OF THE STUDY
3.3 CHEMISTRY CONTEXT
3.3.2 The Arrhenius Model
In the 19th century, Arrhenius suggested in his PhD thesis that ions formed when salts dissolved in water rather than, as previously believed, only once a current was passed through the solution (Kolb, 1978). From this proposal, a new explanatory paradigm arose, wherein acids or bases were substances, which dissociated in aqueous solution to produce hydrogen (H+) and
hydroxide (OH–) ions respectively (for example Arrhenius, 1903; 1912). In this model, the particular acid or base is considered irrelevant as all neutralization reactions are fundamentally the same; hydrogen ions from the acid react with hydroxide ions from the base and the primary product is water. It follows that the Arrhenius model does not consider formation of a specific salt, although one could be isolated by evaporation of the resultant solution. An ionic equation may be used to represent the reaction, in either a complete or net ionic form (Drechsler &
Schmidt, 2005a).
(H+ + Cl–) + (Na+ + OH–) (Na++ Cl–) + H2O or H+ + OH– H2O Equations with single arrows as shown above would indicate the reaction goes to completion.
In this model, water molecules dissociate partially, so the equation below shows the reversibility of the equilibrium system: H2O H+ + OH–
The ion-product constant for water is given by: KW =[H+][OH−]where square brackets [ ], represent concentration of the indicated species, in this case at equilibrium. This infers that in an equilibrium system a higher concentration of hydrogen ion infers a lower concentration of hydroxide ions, and vice versa.
3.3.2.1 Acid-base strength in the Arrhenius model
Being based on electrolytic theory, the Arrhenius model treats acids and bases as electrolytes;
those that are fully dissociated into ions are strong, while those that are not fully dissociated are weak. Typical equations representing the dissociation process for strong acids and bases are:
HCl H+ + Cl– and NaOH Na+ + OH–
Concentrations may be obtained from electrical conductivity of solutions, to give values for corresponding equilibrium constants Ka and Kb, also known as dissociation constants.
] HCl [
] Cl ].[
H KaHCl [
−
= + and
] NaOH [
] OH ].[
Na KbNaOH [
−
= +
The model is limited to aqueous solutions, so differences in strength between acids and bases that are 100% dissociated will not be detected. Dissociation of a weak acid could be represented as a reversible system such as:
CH3COOH CH3COO– + H+ The corresponding dissociation constant for the equilibrium is
] 3COOH CH [
] 3COO CH ].[
H [ KaCHCOOH
3
− +
=
Ka for HCl will be much greater for than for CH3COOH (Bell, 1969, pp 13, 16). Consequently, for the same bulk concentration of monoprotic acids, such as HCl and CH3COOH, the solution of a stronger acid will have a higher concentration of ions.
Polyprotic acids dissociate in two or more stages, thus for diprotic sulfuric acid:
H2SO4 HSO4
– + H+ and HSO4
– SO4 2– + H+
Consequently, a polyprotic acid may have a higher concentration of hydrogen ions than monoprotic acids of similar strength.
3.3.2.2 Aspects of the protective belt for the Arrhenius model
Some ways in which challenges from empirical observations have been accommodated by adjusting the protective belt of the Arrhenius model are discussed next. The Arrhenius model accommodates the first challenge presented by the basic nature of a solution of ammonia (NH3) which has no hydroxide group, through postulating formation of molecular ammonium hydroxide, which could dissociate partially in solution (e.g. Kobe & Markov, 1941; Tuttle,
1991), thus: NH4OH NH4
+ + OH–
However, modern chemists have challenged the existence of ammonium hydroxide (e.g. Laing
& Laing, 1988; Yoke, 1989). In particular, Davis (1953) maintains: “Nothing is gained in clarity or understanding by continuing the fiction of the reality of the ammonium hydroxide molecule”.
A further challenge to the Arrhenius model arises concerning the phenomenon of substances that do not themselves dissociate into hydrogen or hydroxide ions (so not fitting definitions of acids or bases) but still have acidic or basic aqueous solutions (Rayner-Canham, 1994). In each case the salt is first presumed to dissociate – which in itself may not be completely true (Hawkes, 1996a). The acidic nature of an ammonium chloride solution may be explained by production of excess hydrogen ions depicted as follows:
NH4Cl(s) NH4+(aq) + Cl–(aq) followed by NH4
+(aq) NH3(aq) + H+(aq)
To explain these empirical observations concerning salts such as sodium ethanoate (acetate) or sodium carbonate, which have basic aqueous solutions, Arrhenius acid-base theory includes a notion of these ionic species being hydrolysed, or reacting with water, whereby ions from weak acids produce the original weak acid (un-dissociated) and excess hydroxide ions. For sodium ethanoate, excess hydroxide ions can be produced according to the equations:
CH3COONa(s) CH3COO–(aq) + Na+(aq) and CH3COO–(aq) + H2O(l) CH3COOH + OH–(aq)
A similar process is shown by the following equations for sodium carbonate:
Na2CO3(s) 2Na+(aq) + CO32–(aq) and CO3
2–(aq) + 2H2O(l) H2CO3(aq) + 2OH–(aq) As the equations above show, the aspect of the protective belt needed to explain the phenomenon of acidic or basic solutions also relies on the existence of carbonic acid (H2CO3) which is again merely postulated. The phenomenon of basic solutions for salts is also explained much more simply by the Brønsted model, as will be shown below.
3.3.2.3 Terminology: dissociation and ionization in the Arrhenius model.
The terms ionization and dissociation appear to have been used interchangeably to indicate the process whereby electrolytes provide ions in solution. For example “According to this theory strong acids and bases, as well as salts, are in extreme dilution completely dissociated”
Arrhenius, 1903, p51) and “ionization of sodium chloride...” (Arrhenius, 1912). Even with modern knowledge of bonding, de Vos and Pilot (2001) use ionization in relation to acids and bases in solution. For clarity I have used dissociation for all these processes concerning the Arrhenius model.