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Acids and Bases - conjugate pairs

Part I, Part II

Advance discussion of acids bases

A second part is devoted to the subject of conjugation of acids and bases. The relationship between the acidic constant Ka, basic constant Kb, and the constant of autoionization of water, Kw will be discussed. The relationship is useful for weak acids and bases.

Skills to develop

Acids and Bases

The primitive way to characterize a substance is by tasting. Acids taste soure, and bases taste bitter.

AcidFormula
AceticCH3COOH
HydrochloricHCl
SulfuricH2SO4
NitricHNO3
CarbonicH2CO3
BaseFormula
Ammonia
water
NH4OH
Sodium
hydroxide
NaOH
sodium
carbonate
Na2CO3
soapsVaries
At this point of your development, you already know some acids and bases. Are you familiar with the acids and bases listed here? If not, you probably will learn a lot here.

Acids and bases are also powerful concepts used to characterize substances. The concepts can and has been extended to much wider applications. For these reason, there are at least three definitions for each of acid and base. These definitions are given at different times to expand the concepts of acids and bases. This is why we do not even define acids and bases in this section.

Let us take a look how the concepts of acid and base are expanded over time. Acids and bases are related, and the relationship is called conjugation. An acid has a conjugate base and vice versa.

Evolution of the Acid-Base Concept

In 1884, Arrhenius noticed that all acids have H+ ions and bases have OH- ions. Thus, he considered all substances giving H+ and OH- ions are acids and bases respectively.

Less than 40 years later, in 1923, Bronsted, Lowry and Lewis wanted to expand the concepts of acids and bases so that a wider area of chemistry can be understood using the same principle.

  AcidBaseNeutraliztion
1884
Arrhenius
ionize
H+
ionize
OH-
H+ + OH-
-> H2O
1923
Bronsted-
Lowry
Proton
donor
Proton
acceptor
HA + B
-> A + HB
1923
Lewis
ElectrophilNucleophilE + :Nu -> E:Nu
Bronsted-Lowry's concept focused on the proton, and defined

  • Acids as proton donors,
  • Bases as proton acceptors,
    whereas Lewis focused on the electrons and defined
  • Acids as electronphils,
  • Bases as nucleophils.

    Lewis definitions are more general than Bronsted-Lowry's definitions, they are now widely accepted and practiced. The more general definitions for acids and bases allow us to discuss more chemical reactions as acid-base reactions.

    Conjugate Acid-Base Pairs

    Conjugate
    acid
    Conjugate
    base
    H3O+H2O
    H2O OH-
    H2SO4HSO4-
    HSO4-SO42-
    NH4+NH3
    NH3 NH2-
    H3PO4 H2PO4-
    H2PO4-HPO42-
    HPO42-PO43-
    CH3COOHCH3COO-
    CH3NH3+CH3NH2
    According to the Bronsted-Lowry theory of acids and bases, an acid is a proton donor and a base is a proton acceptor. Once, an acid has given up a proton, the remaining part can be a proton acceptor, and thus a base. In this regard, an acid and a base are closely related to one another. H+ + Base = Conjugate_acid of Base+
    Acid = H+ + Conjugate_base of Acid-
    For example: NH3 + H2O = NH4+ + OH-
    HAc = H+ + Ac-
    Thus, NH4+ and NH3 are a pair of conjugate acids and bases, as are HAc and Ac-.

    The table lists conjugate acid-base pairs for your reference so that you can figure out the strategy of identifying them. Furthermore, for an acid or base, you should be able to give its conjugate base or acid.

    Confidence Building Questions

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