weak acid

(noun)

one that dissociates incompletely, donating only some of its hydrogen ions into solution

Related Terms

Examples of weak acid in the following topics:

  • Weak Acids

    • A weak acid is one that does not dissociate completely in solution; this means that a weak acid does not donate all of its hydrogen ions (H+) in a solution.
    • The majority of acids are weak.
    • Weak acids ionize in a water solution only to a very moderate extent.
    • The Ka of weak acids varies between 1.8×10−16 and 55.5.
    • Although it is only a weak acid, a concentrated enough solution of acetic acid can still be quite acidic.

  • Weak Acid-Strong Base Titrations

    • A weak acid will react with a strong base to form a basic (pH > 7) solution.
    • Titrations are reactions between specifically selected reactants—in this case, a strong base and a weak acid.
    • The initial pH of the solution at the beginning of the titration is approximately that of the weak acid in water.
    • This figure depicts the pH changes during a titration of a weak acid with a strong base.
    • Distinguish a weak acid-strong base titration from other types of titrations.

  • Calculating Percent Dissociation

    • We have already discussed quantifying the strength of a weak acid by relating it to its acid equilibrium constant Ka; now we will do so in terms of the acid's percent dissociation.
    • Strong acids have a value of α that is equal to or nearly 100%; for weak acids, however, α can vary, depending on the acid's strength.
    • As we would expect for a weak acid, the percent dissociation is quite small.
    • However, for some weak acids, the percent dissociation can be higher—upwards of 10% or more.
    • Calculate percent dissociation for weak acids from their Ka values and a given concentration.

  • Salts that Produce Basic Solutions

    • It is due to the fact that the anion in the salt is the conjugate base of a weak acid.
    • The bicarbonate ion is the conjugate base of carbonic acid, a weak acid.
    • Notice that for all of these examples, the anion is the conjugate base of a weak acid (carbonic acid, bisulfate (second dissociation step of sulfuric acid), acetic acid, hydrocyanic acid, hydrogen sulfide).
    • Keep in mind that a salt will only be basic if it contains the conjugate base of a weak acid.
    • Because the bicarbonate ion is the conjugate base of carbonic acid, a weak acid, sodium bicarbonate will yield a basic solution in water.

  • Acid Dissociation Constant (Ka)

    • The acid dissociation constant (Ka) is the measure of the strength of an acid in solution.
    • Acid dissociation constants are most often associated with weak acids, or acids that do not completely dissociate in solution.
    • A weak acid has a pKa value in the approximate range of -2 to 12 in water.
    • Acids with a pKa value of less than about -2 are said to be strong acids.
    • Acetic acid is a weak acid with an acid dissociation constant $K_a=1.8\times 10^{-5}$ .

  • Strong Acid-Weak Base Titrations

    • A strong acid will react with a weak base to form an acidic (pH < 7) solution.
    • An example of a strong acid-weak base titration is the reaction between ammonia (a weak base) and hydrochloric acid (a strong acid) in the aqueous phase:
    • In the case of titrating the acid into the base for a strong acid-weak base titration, the pH of the base will ordinarily start high and drop rapidly with the additions of acid.
    • In strong acid-weak base titrations, the pH at the equivalence point is not 7 but below it.
    • Recall that strong acid-weak base titrations can be performed with either serving as the titrant.

  • Acid-Base Titrations

    • strong acid-weak base titration: methyl orange indicator the base is off the scale (e.g., pH > 13.5) and the acid has pH > 5.5: alizarine yellow indicator
    • A strong acid will react with a weak base to form an acidic (pH < 7) solution.
    • A weak acid will react with a strong base to form a basic (pH > 7) solution.
    • When a weak acid reacts with a weak base, the equivalence point solution will be basic if the base is stronger and acidic if the acid is stronger; if both are of equal strength, then the equivalence pH will be neutral.
    • Weak acids are not often titrated against weak bases, however, because the color change is brief and therefore very difficult to observe.

  • Overview of the Acid-Base Properties of Salt

    • Basic salts form from the neutralization of a strong base and a weak acid; for instance, the reaction of sodium hydroxide (a strong base) with acetic acid (a weak acid) will yield water and sodium acetate.
    • Acid salts are the converse of basic salts; they are formed in the neutralization reaction between a strong acid and a weak base.
    • The conjugate acid of the weak base makes the salt acidic.
    • For instance, in the reaction of hydrochloric acid (a strong acid) with ammonia (a weak base), water is formed, along with ammonium chloride.
    • However, as we have already discussed, the ammonium ion acts as a weak acid in solution, while the bicarbonate ion acts as a weak base.

  • Strong Acids

    • The strength of an acid refers to the ease with which the acid loses a proton.
    • where HA is a protonated acid, H+ is the free acidic proton, and A- is the conjugate base.
    • Strong acids yield weak conjugate bases.
    • For sulfuric acid, which is diprotic, the "strong acid" designation refers only to the dissociation of the first proton:
    • p-Toluenesulfonic acid is an example of an organic soluble strong acid, with a pKa of -2.8.

  • Naming Acids and Bases

    • Simple acids, known as binary acids, have only one anion and one hydrogen.
    • Weak bases made of ionic compounds are also named using the ionic naming system.
    • Weak bases are also sometimes molecular compounds or organic compounds because they have covalent bonds.
    • For example, methyl amine (CH3NH2) is a weak base.
    • Some weak bases have "common" names.