Ways of Understanding Acids and Bases

Chemists understanding of acids and bases evolved over an extended period of time. Several scientists proposed ways of looking at acids and bases that became accepted practice.

Svante Arrhenius Arrhenius began thinking about acids and bases in graduate school. His work was not highly regarded by his thesis committee; they gave him a really hard time. He later got the last laugh when he won the Nobel Prize for his work in 1903.

According to Arrhenius, Acids are compounds containing hydrogen that ionize to produce hydrogen ions in aqueous solution. Bases are compounds that ionize to yield hydroxide ions in aqueous solution.

Here’s another way of putting it: Acid - any substance that delivers hydrogen ion (H+) to the solution.

Arrhenius Bases Base - any substance that delivers hydroxide ion (OH¯) to the solution.

Bronsted-Lowry Acids and Bases In 1923, Johannes Bronsted and Thomas Lowry proposed a new theory of acids and bases: 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. Remember: proton, hydrogen ion and H+ all mean the same thing.

Limits of Arrhenius’ Theory After a few decades, it became obvious that Arrhenius’ definition of an acid did not go far enough. Some compounds that behaved as bases could not be explained using his definition – NH3 and Na2CO3 in particular. Dissolving HCl(g) in benzene, for example, did not produce an acidic solution, so can HCl really be considered an acid?

Another way of putting this: An acid is a "proton donor." A base is a "proton acceptor."

All Arrhenius acids and bases are also acids and bases under Bronsted-Lowry. B-L also accounts for acids and bases that Arrhenius could not explain:

Ammonia is a weak base: the forward reaction happens occasionally. The reverse reaction also occurs, and is favored According to B-L, NH4+ is an acid (proton donor) and OH- is a base (proton acceptor). These are called conjugate acids and bases.

Which substance can act as an Arrhenius base in an aqueous solution? LiCl LiNO3 LiBr LiOH

An Arrhenius acid has only hydroxide ions in solution only hydrogen ions in solution hydrogen ions as the only positive ions in solution hydrogen ions as the only negative ions in solution

Hydrogen chloride, HCl, is classified as an Arrhenius acid because it produces H+ ions in aqueous solution Cl- ions in aqueous solution OH- ions in aqueous solution NH4+ ions in aqueous solution

According to the Brönsted-Lowry theory, an acid is a proton donor only a proton acceptor only a proton donor and a proton acceptor neither a proton donor nor a proton acceptor

A sample of Ca(OH)2 is considered to be an Arrhenius base because it dissolves in water to yield Ca2+ ions as the only positive ions in solution H3O+ ions as the only positive ions in solution OH- ions as the only negative ions in solution H- ions as the only negative ions in solution

In the reaction HNO3 + H2O ↔ H3O+ + NO3- the two Brönsted acids are H2O and HNO3 H2O and NO3- H2O and H3O+ HNO3 and H3O+

Given the reaction: NH3 + HCl → NH4Cl In this reaction, ammonia molecules (NH3) act as a base because they accept hydrogen ions (H+) accept hydroxide ions (OH−) donate hydrogen ions (H+) donate hydroxide ions (OH−)

One acid-base theory states that an acid is an electron donor a neutron donor an H+ donor an OH- donor