Definition of Brønsted and Lowry Theory and Acid-Basic Force

Arrhenius defined the H protons⁺ as isolated species, although today it is known that in solution they have high attraction with the molecules of Water and are found forming the hydronium ions (H₃OR⁺). Based on these concepts we illustrate from the acid-base reaction referring to vinegar, with acetic acid diluted in water:

C₂ H₃ OR_2(ac)+ H₂ OR_(l) ↔ C₂ H₃ OR₂^-_(ac)+ H₃ OR⁺_(ac)

In this case, acetic acid is the one who donates an acidic hydrogen while water acts as a base taking the donated proton. In turn, two new ionic species are formed, which are the conjugated acids and bases of the acids and bases from which they come. In this case, the

species C₂ H₃ OR₂^- is the conjugate base of acetic acid while H₃ OR⁺ it is the conjugated acid of water. Therefore, the conjugated acid-base pair differ only in the presence of an acidic hydrogen and, furthermore, the premise that every acid has its conjugate base and vice versa is fulfilled.

Now let's review the following reaction:

NH_3(ac)+ H₂ OR_(l)↔NH₄⁺_(ac)+ OH^-_(ac)

In this case, we have a conjugated acid-base pair that is water and hydroxyl ion respectively, and a base, ammonia, with its conjugated pair, the acidic species NH₄⁺.

Now, you may wonder how it is that water acts both as an acid and as a base, that ability it is known as amphotericism. That is, a substance that it can act in both ways depending on who it is combined with is an amphoteric substance.

Just as we define conjugate pairs, they have a peculiar characteristic: the more strength acid has the acid of the pair, the lower basic force will have its conjugate base, and it is analogous for the case of the bases, the greater the basicity force the base has, its conjugate pair will decrease the force of the acid. You may wonder what force are we talking about?

Well, when an acid is strong we are talking about a species that is capable of completely donating acidic hydrogen, transferring all its protons to water and completely dissociating. Otherwise, it is the weak acids, which are partially ionized in aqueous solution, this implies that part of the acid will be found as dissociated species and part will retain its structure. Let's look at the following typical examples:

HCl_(g)+ H₂ OR_(l)→ Cl^-_(ac)+ H₃ OR⁺_(ac)

This is a strong acid, since it dissociates completely, and similarly occurs with sodium hydroxide, which is a strong base:

NaOH_(s)→ Na⁺_(ac)+ OH^-_(ac)

If we remember the reaction of acetic acid in aqueous solution, we note that there is a Balance between species, since the dissociation is not complete and, therefore, there is an acidity constant thermodynamics that governs the process, which is expressed in terms of the activities of the species; However, in dilute solutions, it can be estimated through the molar concentrations:

Ka = C₂ H₃ OR₂^-H₃ OR⁺/HC₂ H₃ OR₂

While for the case of weak bases we can describe the degree to which said base is ionized if we talk about its thermodynamic constant of basicity, such is the case of ammonia:

Kb = NH₄⁺Oh^-/NH₃

These constants are tabulated at reference temperatures, while there is also a bibliography that indicates the level of acidity or basicity of certain compounds.

Finally, we will refer to the autoionization of water, as we have already seen, water has both a base and a conjugated acid, being able to describe this phenomenon in its ionization reaction:

2H₂ OR_(l)↔ OH^-_(ac)+ H₃ OR⁺_(ac)

We could define this process as we did previously through the constant involved, which would be:

Kc = H₃ OR⁺Oh^-/ H₂ OR²

Using a mathematical arrangement we could express the ionic product of water as the following constant:

Kw = H₃ OR⁺Oh^-

Whose value at 25ºC is constant and is: 1x10-14, which implies that, if the solution is neutral, that is to say the same amount of acid than base, each of the ionic species concentrations will be: 1x10-7 mol / L.

Topics in Brønsted and Lowry Theory and Acid-Basic Force