Conjugate Acid

7/27/2019 Conjugate Acid

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Conjugate Acid-Base Pairs

In an acid-baseneutralization, an acid and a base react to form water and salt. In order for the reaction to carry out,

there must be the transfer of protons between acids and bases. Proton acceptors and proton donors are the basis for

these reactions, and are also referred to as conjugate bases and acids.

Essential Background Information

ReadAcids and Basesbefore proceeding with this one.

React with metals

Have a low pH

When phenolphthalein (a common indicator) is added to an acidic solution during atitration, the solution stays clear.

Arrhenius Acid: An Arrhenius acid produceshydronium ions[H3O+] when it dissociates in water.

Bronsted-Lowry Acid: Proton donor; donates hydrogen ions [H+] to the base.
http://chemwiki.ucdavis.edu/Physical_Chemistry/Acids_and_Bases/PH_Scale/Neutralizationhttp://chemwiki.ucdavis.edu/Physical_Chemistry/Acids_and_Bases/PH_Scale/Neutralizationhttp://chemwiki.ucdavis.edu/Physical_Chemistry/Acids_and_Bases/PH_Scale/Neutralizationhttp://chemwiki.ucdavis.edu/Physical_Chemistry/Acids_and_Baseshttp://chemwiki.ucdavis.edu/Physical_Chemistry/Acids_and_Baseshttp://chemwiki.ucdavis.edu/Physical_Chemistry/Acids_and_Baseshttp://chemwiki.ucdavis.edu/Analytical_Chemistry/Quantitative_Analysis/Titrationhttp://chemwiki.ucdavis.edu/Analytical_Chemistry/Quantitative_Analysis/Titrationhttp://chemwiki.ucdavis.edu/Analytical_Chemistry/Quantitative_Analysis/Titrationhttp://chemwiki.ucdavis.edu/Physical_Chemistry/Acids_and_Bases/The_hydronium_Ionhttp://chemwiki.ucdavis.edu/Physical_Chemistry/Acids_and_Bases/The_hydronium_Ionhttp://chemwiki.ucdavis.edu/Physical_Chemistry/Acids_and_Bases/The_hydronium_Ionhttp://chemwiki.ucdavis.edu/Physical_Chemistry/Acids_and_Bases/The_hydronium_Ionhttp://chemwiki.ucdavis.edu/Analytical_Chemistry/Quantitative_Analysis/Titrationhttp://chemwiki.ucdavis.edu/Physical_Chemistry/Acids_and_Baseshttp://chemwiki.ucdavis.edu/Physical_Chemistry/Acids_and_Bases/PH_Scale/Neutralization


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What makes an acid strong or weak?

A strong acid is one that can dissociate easily to form hydrogen ions. A weak acid is one that can

not dissociate as easily, therefore cannot form hydrogen ions as readily.

Example of a strong acid in solution
http://upload.wikimedia.org/wikipedia/commons/d/de/AcidBase%E2%80%93pfe.png


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Acetic Acid Dissociation

Examples of Weak Acids: Acetic acid (HC2H3O2), hydrofluoric acid(HF), Nitrous Acid (HNO2)

Examples of Strong Acids: Hydrochloric Acid (HCl), Sulfuric Acid (H2SO4), Nitric Acid (HNO3)

Bases
http://upload.wikimedia.org/wikipedia/commons/9/96/Acetic-acid-dissociation-3D-balls.pnghttp://upload.wikimedia.org/wikipedia/commons/e/ef/ProtonPairs1.jpghttp://upload.wikimedia.org/wikipedia/commons/9/96/Acetic-acid-dissociation-3D-balls.pnghttp://upload.wikimedia.org/wikipedia/commons/e/ef/ProtonPairs1.jpg


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Is an electrolyte

Has a high pH level

When phenolphtalein is added to a basic solution during atitration, the solution turns pink.

Arrhenius Base: An Arrhenius Base produces hydroxide ions [OH-] when it dissociates in water.

Bronsted-Lowry Base: Proton acceptor; accepts hydrogen ions [H+] from the acid.

What makes a base strong or weak?

Like a strong acid, a strong base dissociates easily to form not hydronium, but hydroxide ions. Aweak base cannot dissociate as easily, therefore cannot form hydroxide ions as readily.

Examples of Weak Bases:Ammonia (NH3), Phosphate Ion (PO43-

), Water (H2O)

Examples of Strong Bases: Sodium Hydroxide (NaOH), Potassium Hydroxide (KOH), Calcium Oxide (CaO)

Acid/Base Reactions

Example of Proton Transfer
http://chemwiki.ucdavis.edu/Analytical_Chemistry/Quantitative_Analysis/Titrationhttp://chemwiki.ucdavis.edu/Analytical_Chemistry/Quantitative_Analysis/Titrationhttp://chemwiki.ucdavis.edu/Analytical_Chemistry/Quantitative_Analysis/Titrationhttp://upload.wikimedia.org/wikipedia/commons/c/cd/ProtonTransfer.pnghttp://upload.wikimedia.org/wikipedia/commons/9/91/Reaction_acide_base_conjuguee_acide_carbox.svghttp://upload.wikimedia.org/wikipedia/commons/c/cd/ProtonTransfer.pnghttp://upload.wikimedia.org/wikipedia/commons/9/91/Reaction_acide_base_conjuguee_acide_carbox.svghttp://upload.wikimedia.org/wikipedia/commons/c/cd/ProtonTransfer.pnghttp://upload.wikimedia.org/wikipedia/commons/9/91/Reaction_acide_base_conjuguee_acide_carbox.svghttp://chemwiki.ucdavis.edu/Analytical_Chemistry/Quantitative_Analysis/Titration


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Conjugate Acid-Base Pairs

A conjugate pair refers to acids and bases with common features. These common features are the

equal loss/gain of protons between the pairs. Conjugate acids and conjugate bases are

characterized as the acids and bases that lose or gain protons. In an acid-base reaction, and acid

plus a base reacts to form a conjugate base plus a conjugate acid.Acid + Base Conjugate Base + Conjugate Acid

The conjugate acid of a base is formed when the base gains a proton. Refer to the following equation:

NH3(g)+H2O(l)NH+4(aq)+OH(aq

We say that NH4+

is the conjugate acid to the base NH3, because NH3 gained a hydrogen ion toform NH4

+, the conjugate acid. Theconjugate base of an acid is formed when the acid donates a

proton. In the equation, OH-is the conjugate base to the acid H2O, because H2O donates a

hydrogen ion to form OH-, the conjugate base. Note: The stronger the acid or base, the weaker

the conjugate. The weaker the acid or base, the stronger the conjugate.

Finding Conjugates of Acid- Base PairsIn the table below, the acidand conjugate bases are conjugate pairs and the base and conjugate

acids are conjugate pairs. When finding a conjugate acid or base, make sure you look at the reactants. The

reactants are the acids and bases, and the acidcorresponds to the conjugate base on the product side of the

equation. This goes for the base too; the base corresponds to the conjugate acidon the product side of the equation.

To identify the conjugate acid, look for the pair of compounds that are related. The acid-base reaction can be view in

a before and after sense. The before is the reactant side of the equation, the after is the product side of the equation.

The conjugate acid in the after side of an equation gains a hydrogen ion, so in the before side of the equation the

compound that has one less hydrogen ion of the conjugate acid is the base. The conjugate base in the after side of

the equation lost a hydrogen ion, so in the before side of the equation the compound that has one more hydrogen ion

of the conjugate base is the acid.

Example of a Bronsted-Lowry Reaction


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How to identify Conjugate Pairs

1. HNO3 Is an acidbecause it donates a proton to water and its conjugate base is NO3-.An easy way to identify

the conjugate base is that it differs from the acid by one proton.

2. H2O is a base because it accepts a proton from HNO3 and its conjugate acidis H3O+

. Again to identify

the conjugate acid(or any conjugate pair) is that it differs from the base by one proton.

The following table is an example of conjugate acid and base pairs in a given equation:

Equation Acid BaseConjugate

Base

Conjugate

Acid

HClO2 H2O ClO2-

H3O+
http://upload.wikimedia.org/wikipedia/commons/1/12/Bronsted-lowry_diagram.pnghttp://upload.wikimedia.org/wikipedia/commons/1/12/Bronsted-lowry_diagram.pnghttp://upload.wikimedia.org/wikipedia/commons/1/12/Bronsted-lowry_diagram.png


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H2O OCl-

OH-

HOCl

HCl H2PO4-

Cl-

H3PO4

Practice Problems

Identify the acid, base, conjugate acid and conjugate base for the following reaction:

1. HF+H2OF+H3O+2. HSO4+NH3SO24+NH+4

3. C2H3O2+HClHC2H3O2+Cl

4. HNO2+H2OH3O++NO25. HCN+H2OH3O++CN

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Conjugate Acid