conjugate acid

(noun)

The species created when a base accepts a proton.

Related Terms

(noun)

the species formed after a base accepts a proton; typically a weak acid

Related Terms

Examples of conjugate acid in the following topics:

  • Brønsted Acids and Bases

    • The conjugate base is the ion or molecule that remains after the acid has donated its proton, and the conjugate acid is the species created after the base accepts the proton.
    • Consider the following reactions of acetic acid (CH3COOH), the organic acid that gives vinegar its characteristic taste:
    • The acetate ion CH3CO2- is the conjugate base of acetic acid, and the hydronium ion H3O+ is the conjugate acid of the base, water:
    • The hydroxide ion is the conjugate base of water, which acts as an acid, and the ammonium ion is the conjugate acid of the base, ammonia.
    • Identify the Brønsted acid, Brønsted base, conjugate acid, and conjugate base in an acid-base reaction.

  • The Brønsted-Lowry Definition of Acids and Bases

    • Here, a conjugate base is the species that is left over after the Brønsted acid donates its proton.
    • The conjugate acid is the species that is formed when the Brønsted base accepts a proton from the Brønsted acid.
    • Therefore, according to the Brønsted-Lowry definition, an acid-base reaction is one in which a conjugate base and a conjugate acid are formed (note how this is different from the Arrhenius definition of an acid-base reaction, which is limited to the reaction of H+ with OH- to produce water).
    • The products include the acetate ion, which is the conjugate base formed in the reaction, as well as hydronium ion, which is the conjugate acid formed.
    • The conjugate acid formed in the reaction is the ammonium ion, and the conjugate base formed is hydroxide.

  • Absolute Concentrations of the Acid and Conjugate Base

    • A buffer is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid.
    • Therefore, it is very important to be able to identify acid and conjugate base pairs.
    • The conjugate acid is created by accepting (adding) a proton (H+) donated by the conjugate base.
    • 8.1.3 Deduce the formula of the conjugate acid/base of any Brønsted-Lowry base/acid IB Chemistry SL - YouTube
    • Remember: A conjugate ACID is made by ADDING a proton (H+).

  • Buffer Range and Capacity

    • A buffer solution usually contains a weak acid and its conjugate base.
    • Each conjugate acid-base pair has a characteristic pH range where it works as an effective buffer.
    • The buffering region is about 1 pH unit on either side of the pKa of the conjugate acid.
    • In other words, the pH of the equimolar solution of acid (e.g., when the ratio of the concentration of acid and conjugate base is 1:1) is equal to the pKa.
    • Discuss correlation between the pKa of the conjugate acid of a buffer solution and the effective range of the corresponding buffer.

  • Preparing a Buffer Solution with a Specific pH

    • A buffer is a solution of weak acid and conjugate base or weak base and conjugate acid used to resist pH change with added solute.
    • A buffer is an aqueous solution containing a weak acid and its conjugate base or a weak base and its conjugate acid.
    • When some strong acid (more H+) is added to an equilibrium mixture of the weak acid and its conjugate base, the equilibrium is shifted to the left, in accordance with Le Chatelier's principle.
    • If the buffer is made with a base and its conjugate acid, the pH can be adjusted using a strong acid like HCl.
    • where pH is the concentration of [H+], pKa is the acid dissociation constant, and [A-] and [HA] are concentrations of the conjugate base and starting acid.

  • Acidity of Amines

    • pKa is being used as a measure of the acidity of the amine itself rather than its conjugate acid, as in the previous section.
    • The same factors that decreased the basicity of amines increase their acidity.
    • This is illustrated by the following examples, which are shown in order of increasing acidity.
    • The acids shown here may be converted to their conjugate bases by reaction with bases derived from weaker acids (stronger bases).
    • For complete conversion to the conjugate base, as shown, a reagent base roughly a million times stronger is required.

  • Relative Amounts of Acid and Base

    • A buffer is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid.
    • You can use one of these acid/conjugate base pairs:
    • Extrapolating further from this, a buffer is most effective when the concentrations of acid and conjugate base (or base and conjugate acid) are approximately equal—in other words, when the log [base]/[acid] equals 0 and the pH equals the pKa.
    • The change is minimized if the concentrations of acid and conjugate base are equal.
    • Calculate the relative amounts of a weak acid and its conjugate base that must be used to generate a buffer solution of desired pH.

  • 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.

  • Strong Acids

    • In water, strong acids completely dissociate into free protons and their conjugate base.
    • 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.
    • p-Toluenesulfonic acid is an example of an organic soluble strong acid, with a pKa of -2.8.

  • Diprotic and Polyprotic Acids

    • As their name suggests, polyprotic acids contain more than one acidic proton.
    • Two common examples are carbonic acid (H2CO3, which has two acidic protons and is therefore a diprotic acid) and phosphoric acid (H3PO4, which has three acidic protons and is therefore a triprotic acid).
    • Although the subsequent loss of each hydrogen ion is less favorable, all of a polyprotic acid's conjugate bases are present to some extent in solution.
    • The fractional concentration is defined as the concentration of a particular conjugate base of interest, divided by the sum of all species' concentrations.
    • Identify the key features that distinguish polyprotic acids from monoprotic acids.