Conjugate Base: Definition & Overview

Lesson Transcript
Instructor: Elizabeth (Nikki) Wyman

Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.

In this lesson, you'll learn about conjugate bases in chemistry and where they come from. You'll also learn about their relationship to acids and see how acid-base reactions work.

Conjugate Bases

If we approach conjugate bases like experienced detectives, the first place to investigate them is the last place we would expect them to be: in the world of acids. Chemically speaking, acids are the opposite of bases. In general, acids are substances that break apart in water to produce hydrogen ions (H+). They have low pH values, taste, and smell sour, and can be corrosive.

Conjugate bases are intimately related to acids. A conjugate base is the particle that is left over after the acid loses its hydrogen ion. Most of the time, conjugate bases are negatively charged and have high pH values.

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  • 0:01 Conjugate Bases
  • 0:45 Types of Conjugate Bases
  • 2:23 Acid-Base Reactions
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Types of Conjugate Bases

There are as many conjugate bases as there are acids. When an acid loses a hydrogen ion, its conjugate base is produced. An acid and its conjugate base have the same formula but with one minor difference: an acid has one more hydrogen ion than the conjugate base does.

Let's take hydrochloric acid (HCl), a fairly common acid. Since HCl is an acid, when put into water it produces hydrogen ions (H+). After the acid loses its hydrogen ion, the negatively charged chloride ion (Cl-) is left over. The chloride ion (Cl-) is the conjugate base of HCl.

Take a look at this chart of acids and their conjugate bases. Do you notice any patterns that describe the difference between an acid and its conjugate base?

For example, did you notice how the conjugate bases resemble the acids they were formed from (except that they are missing a hydrogen ion (H+), making them negatively charged)?

We can show the relationship between an acid and its conjugate base using this expression: HA --> H+ + A-, where HA is the acid, H+ is the hydrogen ion, and A- is the conjugate base of the acid.

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