Chemistry 112

Acid/Base Chemistry


Acids and bases are classifications of chemical compounds based on reactivity properties. The goal is to be able to predict the reaction products when chemical species are brought together.


The nature of an acid/base reaction is definition dependent: we define what an acid is, what a base is, and what the acid/base reaction is. Then, working within the given definition, we hope to be able to identify an acid and a base and, if they are mixed together, we want to be able to predict the products of the reaction and the extent of reactivity.


We will discuss three different acid/base definitions: Arrhenius, Brønsted–Lowry, and Lewis.


Arrhenius Acids and Bases

(A quick review from Chapter 2)

Acid: a substance that increases the concentration of H+ when dissolved in water.

Base: a substance that increases the concentration of OH when dissolved in water.

Acid/base reaction: an acid reacts with a base to create water and/or a salt.

The acid/base reaction is often called a neutralization reaction.

This is a limited definition: requires water as a solvent; liquid phase; and depends on only two ions.

Why H+ and OH?

This is related to the chemistry of water.

Pure water exhibits a small amount of electrical conductivity attributable to ions:

H2O(l) H+(aq) + OH(aq)

This is called the autoionization of water.

Autoionization is when a single species (usually a solvent) reacts with itself to give ions.

In Arrhenius theory, acids and bases are defined by the ions present in water. Essentially, then, acids and bases are just modifying the chemistry of water by changing the concentrations of the H+ or OH ions.

Most of the common acid/base reactions encountered in CHM101 can be classified as Arrhenius A/B reactions:

HCl(aq)

+

NaOH(aq)

H2O(l)

+

Na+(aq)

+

Cl(aq)

Acid

 

Base

Salt


Brønsted–Lowry Acid and Bases

Acid: a substance that can donate a hydrogen ion (H+ donor).

Base: a substance that can accept a hydrogen ion (H+ acceptor).

Acid/Base reaction: transfer of a hydrogen ion (H+ transfer).

This is a broader definition: not solvent restricted; not phase restricted; however, still restricted by the H+ ion.

In aqueous solution, always use the Brønsted–Lowry definition.


The autoionization of water is a Brønsted–Lowry acid/base reaction:


Water acts as both the acid and the base.

Our goal in the course is to considerably improve our understanding of Brønsted–Lowry acids and bases.