Conjugate pairs

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A conjugate pair is an acid-base pair that differs by one proton in their formulas (remember: proton and hydrogen ion mean the same thing).

A conjugate pair is always one acid and one base.

Example #1: Here is a chemical reaction between an acid and a base:

HCl + H₂O ⇌ H₃O⁺ + Cl¯

Solution:

1) Here is one conjugate pair from the reaction:

HCl and Cl¯

Usually, HCl is called an acid and Cl¯ is called its conjugate base, but that can be reversed if the context calls for it. So, we can correctly speak of Cl¯ as a base and HCl as its conjugate acid.

Notice that the word conjugate is used with one of the pair and the conjugate is not the primary focus of the context, it is the secondary.

2) The other conjugate pair is:

H₂O and H₃O⁺

Water is the base, since it is minus a proton compared to H₃O⁺, which is the conjugate acid to water.

Remember conjugate pairs differ by only one proton. If you take away the proton (or add it), you get the other formula.

Example #2: Concentrated sulfuric acid (98% H₂SO₄) is widely-used as a solvent in chemical industry. Nitric acid will dissolve in this solvent, reacting as follows:

HNO₃ + H₂SO₄ ⇌ H₂NO₃⁺ + HSO₄¯

Using the Brønsted-Lowry definition of acid and base, identify (a) the acid(s) and base(s) in the above chemical equation and (b) the conjugate acid-base pairs.

Solution:

1) Let us state the appropriate definitions:

in a moden way:

An acid is a substance from which a proton can be removed.
A base is a substance that can remove a proton from an acid.

in the words of Brønsted himself:

". . . acids and bases are substances that are capable of splitting off or taking up hydrogen ions, respectively."

2) Let us consider only the forward reaction:

HNO₃ + H₂SO₄ ---> H₂NO₃⁺ + HSO₄¯

The reactant H₂SO₄ has had a proton removed, becoming the product HSO₄¯.

The reactant HNO₃ has removed that proton from sulfuric acid, becoming the reactant H₂NO₃⁺.

When considering only the reactants of the forward reaction, H₂SO₄ is the acid and HNO₃ is the base.

3) Let us consider only the reverse reaction:

HNO₃ + H₂SO₄ <--- H₂NO₃⁺ + HSO₄¯ (Note the direction of the arrow, reactants are on the right side.)

The reactant H₂NO₃⁺ has had a proton removed, becoming the product HNO₃

The reactant HSO₄¯ has removed that proton from H₂NO₃⁺, becoming the product H₂SO₄.

When considering only the reactants of the reverse reaction, H₂NO₃⁺ is the acid and HSO₄¯ is the base.

4) When considering an acid-base reaction in Brønsted-Lowry, the 'conjugate pair' concept is used. First, the definition:

An acid-base conjugate pair is composed of an acid and a base, the formulas of which differ by only one proton.

The acid is the formula with the proton, the base is the formula without the proton.

5) The conjugate acid-base pairs for the reaction in this problem are (acid first, base second):

H₂SO₄, HSO₄¯

and

H₂NO₃⁺, HNO₃

Note that the acid of the pair has the proton and the base of the pair does not. The formulas of conjugate acid-base pairs differ by only one proton.

Here are some more conjugate acid-base pairs to look for:

H₂O and OH¯

HCO₃¯ and CO₃²¯

H₂PO₄¯ and HPO₄²¯

HSO₄¯ and SO₄²¯

NH₄⁺ and NH₃

CH₃NH₃⁺ and CH₃NH₂

HC₂H₃O₂ and C₂H₃O₂¯

This last one is special. Because it is used so often, it has an abbreviation:

Acetic acid's (HC₂H₃O₂) abbreviation is HAc and the acetate ion's (C₂H₃O₂¯) is Ac¯.

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