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Conjugate acids and bases

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What are we learning and why are we learning this? Content, procedures, or skills.

Vocabulary objective

Tier II: High frequency words used across content areas. Key to understanding directions & relationships, and for making inferences.

Tier III: Low frequency, domain specific terms.

Building on what we already know

Make connections to prior knowledge. This is where we build from.


Introduction: Brønsted–Lowry acid–base theory

A conjugate acid is a species formed by the reception of a proton (H+), by a base
(the base with a hydrogen ion added to it.)

A conjugate base is what is left after an acid has donated a proton.

(a species formed by the removal of a proton from an acid.)

Acid + Base ⇔ Conjugate Base + Conjugate Acid

In an acid-base reaction, an acid plus a base reacts to form a conjugate base plus a conjugate acid:
Conjugates are formed when an acid loses a hydrogen proton or a base gains a hydrogen proton.

“Conjugate base reaction” by Schlenk – Chem Draw. Licensed under CC BY-SA 3.0 via Wikimedia Commons – http://commons.wikimedia.org/wiki/File:Conjugate_base_reaction.jpg#/media/File:Conjugate_base_reaction.jpg


Acetic acid example


Now let’s look at some details. Adapted from “Saskatchewan Evergreen Curriculum for Chemistry 30.” Let’s look at ammonia:

ammonia reacts with water to form ammonium

Consider how NH3 changes to: NH4+            NH3 → NH4+

The formulas differ by a single hydrogen; NH3 gains an H+ to become NH4+

Consider how H2O (or HOH) changes to OH–:               HOH → OH–

Again the formulas differ only by a single hydrogen; H2O lost a H+ forming OH–

Now consider these two changes as reversible reactions. What if the reaction proceeds in the opposite direction:

NH4+ can change back to NH3:                NH4+ → NH3

OH– can change back into H2O:              OH– → HOH

Putting these observations together we see that

* ammonia acts as a base because it can combine with a hydrogen ion. It’s partner ammonium is now an acid, for it has a hydrogen ion that it can give up; once it does it is converted back into ammonia.

* water acts as an acid because it gives away a hydrogen ion to ammonia. Once it has lost the hydrogen ion and becomes hydroxide, the hydroxide in turn can act as a base and accept a hydrogen ion from ammonium.

What we have here are conjugate acid-base pairs

Conjugate acid-base pairs differ from each other by the presence or absence of a single hydrogen ion (proton).

Every acid has a conjugate base, and every base has a conjugate acid.

The conjugates will always be listed on the product side of the reaction.

from https://sites.prairiesouth.ca/legacy/chemistry/chem30/5_acids_bases/acids1_5.htm


Related readings:

1.1 What are acids and bases?

1.2 Arrhenius Theory of Acids & Bases

1.3 Ionization & Dissociation

1.4 Brønsted-Lowry Theory of Acids & Bases

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