weak base

(noun)

a proton acceptor that does not ionize fully in an aqueous solution

Related Terms

Examples of weak base in the following topics:

  • Weak Bases

    • In aqueous solution, a weak base reacts incompletely with water to yield hydroxide ions.
    • A weak base is a chemical base that does not ionize fully in an aqueous solution.
    • As Brønsted-Lowry bases are proton acceptors, a weak base may also be defined as a chemical base with incomplete protonation.
    • Like weak acids, weak bases have important applications in biochemical studies, chemistry reactions, and physiological purposes, particularly because of their role in buffer solutions.
    • A weak base, symbolized by B:, can catalyze enolate formation by acting as a proton acceptor.

  • 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.

  • Naming Acids and Bases

    • Acid names are based on the anion they form when dissolved in water; base names follow the rules for ionic, organic, or molecular compounds.
    • 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.

  • 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.
    • At the equivalence point, all of the weak acid is neutralized and converted to its conjugate base (the number of moles of H+ = added number of moles of OH-).
    • 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.

  • Base Dissociation Constant

    • The base dissociation constant, Kb, is a measure of basicity—the base's general strength.
    • The base dissociation constant can be expressed as follows:
    • The pH of a weak base in aqueous solution depends on the strength of the base (given by Kb) and the concentration of the base (the molarity, or moles of the base per liter of solution).
    • A convenient way to find the pH for a weak base in solution is to use an ICE table: ICE stands for "Initial," "Change," and"Equilibrium."
    • However, if the base is weak, then we can assume that x will be insignificant compared to [B]0, and the approximation [B]0– x ≈ [B]0 can be used.

  • 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.

  • 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.

  • Strong Bases

    • Strong bases are capable of deprotonating weak acids; very strong bases can deprotonate very weakly acidic C–H groups in the absence of water.
    • Generally, the alkali metal bases are stronger than the alkaline earth metal bases, which are less soluble.
    • When writing out the dissociation equation of a strong base, assume that the reverse reaction does not occur, because the conjugate acid of a strong base is very weak.
    • Group 1 salts of carbanions (such as butyllithium, LiC4H9, which dissociates into Li+ and the carbanion C4H9-), amides (NH2-), and hydrides (H-) tend to be even stronger bases due to the extreme weakness of their conjugate acids—stable hydrocarbons, amines, and hydrogen gas.
    • Unlike weak bases, which exist in equilibrium with their conjugate acids, the strong base reacts completely with water, and none of the original anion remains after the base is added to solution.

  • 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.

  • 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.
    • where HA is the undissociated species and A- is the conjugate base of the acid.
    • The Ka of weak acids varies between 1.8×10−16 and 55.5.
    • A common assumption is that x is small; we can justify assuming this for calculations involving weak acids and bases, because we know that these compounds only dissociate to a very small extent.