Chapter 9: Unit 1. Acids and Bases Character and Concepts

Acids and BasesCharacter and Concepts

Acids are characterized by their sour taste and stingy character and they tend to turn blue litmus paper red. Bases on the other hand are characterized by their bitter taste and slippery and soapy character and they tend to turn red litmus paper blue.

There are three acids and bases concepts:

1.Arrhenius Concept of Acids and Bases:

Arrhenius concept of acids and bases states that the acids any chemical compounds that form hydrogen ions in aqueous solution and that the bases any chemical compounds that form hydroxide ions in aqueous solution.

In other words, acids are chemicals that can release hydrogen ions into a solution (mainly water) and bases are chemical compounds that can release hydroxide ions into a solution (mainly water).

Examples:

H 2 SO 4 (aq) + 2 H 2 O(l) ßà 2 H 3 O (aq) + SO 4 2–(aq) [ H 2 SO 4 (aq) is an acid ]

Ca(OH) 2 (aq) + H 2 O(l) ßà Ca 2+ (aq) + 2 OH–(aq) [ Ca(OH) 2 (aq) is a base ]

Arrhenius concept of acids and bases was a great contribution and it did replace Lavoisier’s old concept of the acids that acids are compounds that have oxygen.

On the other hand, Arrhenius concept has several disadvantages:

a. It did not take any consideration of solvent effect and role.

b. It did not cover the concept of acids and bases in other solvent than water such as toluene which is not dissociating (no ionization).

The figures below summarized the Arrhenius Acids – Bases Concepts:

2.Bronsted –Lowry Conceptof Acids and Bases:

Bronsted – Lowry concept of acid and bases replaces Arrhenius concept of acids and bases. It introduces the concept of Donor – Acceptor character of the acids and bases.

An acid according to Bronsted – Lowry is a chemical compound that produces hydronium ion when dissociating in water and furthermore they can donate this hydronium ion to another chemical compound which can accept this donated hydronium ion.

The acid is said to be hydronium ion donor and the base is said to be hydronium ion acceptor.

Example:

HNO 3 (aq) + H 2 O(l) ßà H 3 O + (aq) + NO 3–(aq), [ Producing H 3 O + (aq) and Donating it, HNO 3 is an acid ]

H 3 O + (aq) + KOH à 2 H 2 O(l) + K + (aq) [ Accepting H 3 O + (aq), KOH is a base ]

The figure below summarized the Bronsted – Lowry Acids – Bases Concepts:

3.Lewis Concept of Acids and Bases:

Lewis concept of acids and bases is based on the Electron Pair (Lone Pair Electron) Donor –Acceptor. Bases have excess of lone pairs can donate their lone pair electrons and the bases are said to be Electron Pair Donor. The acids can accept these Lone PairElectrons and said to be Electron Pair Acceptor.

The two examples taken from the reference below illustrate this Electron Pair Donor –Acceptor features of Acids and Bases.

http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch11/lewis.php

The first example:

The lone pair electrons on nitrogen atom of ammonia NH 3 are donated to vacant site of Boron in BF 3

NH 3 is said to be Lewis Base (Electron Pair Donor) and BF 3 is to be Lewis Acid (Electron Pair Acceptor)

The second example:

CO 2 (g) + H 2 O(l) ⇔ H 2 CO 3 (aq)

The lone pair electrons on oxygen atom are donated to vacant site of Carbon in CO 2

H 2 O is said to be Lewis Base (Electron Pair Donor) and CO 2 is to be Lewis Acid (Electron Pair Acceptor)