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Conjugate Acid-Base Pairs

In an acid-base neutralization, 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

Acids

Read Acids and Bases before proceeding with this one.

  • React with metals

  • Have a low pH
  • When phenolphthalein (a common indicator) is added to an acidic solution during a titration, the solution stays clear.

     

  • Arrhenius Acid: An Arrhenius acid produces hydronium ions [H3O+] when it dissociates in water.
  • Brønsted-Lowry Acid: Proton donor; donates hydrogen ions [H+] to the base.

 

File:AcidBase–pfe.png

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 and, therefore, cannot form hydrogen ions as readily.

Example of a strong acid in solution

File:ProtonPairs1.jpg

Acetic Acid Dissociation

File:Acetic-acid-dissociation-3D-balls.png

  • 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)

Base

  • Is an electrolyte
  • Has a high pH level
  • When phenolphtalein is added to a basic solution during a titration, the solution turns pink.
  • Arrhenius Base: An Arrhenius Base produces hydroxide ions [OH-] when it dissociates in water.
  • Brønsted-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. A weak base cannot dissociate as easily and, 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

File:Reaction acide base conjuguee acide carbox.svg

Example of Proton Transfer

File:ProtonTransfer.png

Conjugate Acid-Base Pairs

A conjugate pair refers to acids and bases with common features. These common features include the equal loss/gain of protons between the pairs. Conjugate acids and conjugate bases are characterized as the acids and bases that gain or lose protons, respectively.  In an acid-base reaction, an 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: 

\[ NH_{3}(g)\; +\; H_{2}O(l) \rightarrow NH_4^+ (aq) \; + \; OH^-(aq) \]

We say that NH4+ is the conjugate acid to the base NH3 because NH3 gained a hydrogen ion to form NH4+, the conjugate acid. The conjugate 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 Pairs

In the table below, the acids and conjugate bases are conjugate pairs and the bases 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 acid corresponds to the conjugate base on the product side of the equation. This goes for the base as well; the base corresponds to the conjugate acid on 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 viewed in a before and after sense. The before is the reactant side of the equation and the after is the product side of the equation. The conjugate acid in the after side of an equation gains a hydrogen ion. So, on the before side of the equation, the compound that has one less hydrogen ion than the conjugate acid is the base. The conjugate base on the after side of the equation lost a hydrogen ion. Therefore, on the before side of the equation, the compound that has one more hydrogen ion than the conjugate base is the acid.

Example of a Brønsted-Lowry Reaction

File:Bronsted-lowry diagram.png

How to identify Conjugate Pairs

  1. HNO3  is an acid because 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 acid is H3O+ . Again, to identify the conjugate acid (or any conjugate pair) is to see 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 Base Conjugate Base Conjugate Acid
HClO2 H2O ClO2- H3O+
H2O OCl- OH- HOCl
HCl H2PO4- Cl- H3PO4

Practice Problems

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

  1. \( HF + H_2O \rightarrow F^- + H_3O^+ \)
  2. \( HSO_4^-+ NH_3 \rightarrow SO_4^{2-}+NH_4^+   \)
  3. \( C_2H_3O_2^-+HCl \rightarrow HC_2H_3O_2+Cl^- \)
  4. \( HNO_2+ H_2O \rightarrow  H_3O^++ NO_2^- \)
  5. \( HCN+ H_2O \rightarrow H_3O++ CN^-        \)

Answers

1.

Acid: HF

Base: H2O

Conjugate Acid: H3O+

Conjugate Base: F-

2.

Acid: HSO4-

Base: NH3

Conjugate Acid: NH4+

Conjugate Base: SO42-

3.

Acid: HCl

Base: C2H3O2-

Conjugate Acid: HC2H3O2

Conjugate Base: Cl-

4.

Acid: HNO2

Base: H2O

Conjugate Acid: H3O+

Conjugate Base: NO2-

5.

Acid: HCN

Base: H2O

Conjugate Acid: H3O+

Conjugate Base: CN-

 

References

  1. "Acids and Bases." Journal of Chemical Education (2005): n. pag. Web. 1 Mar 2010. <http://www.jce.divched.org/JCEDLib/Q...ions/AcidBase/>.
  2. Petrucci, Ralph H., William S. Harwood, F. Geoffrey Herring, and Jeffrey D. Madura. General Chemistry: Princples and Modern Applications. 9th. Upper Saddle River, New Jersey: Pearson Prentice Hall, 2007. 664-669. Print.

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Contributors

  • Sana Akhtar, Emily Pong, Harjeet Bassi (UCD)