You often use acids and bases in all areas of life. For example, vinegar, aspirin, lemon juice, cola drinks, apple, tomato and toilet cleaners contain acids. Substances containing bases such as drain cleaner, antacid tablets, baking powder, soda, etc. You eat and drink certain acids and bases and your body produces them. From "acid Indigestion" to "acid rain," the word acid appears frequently in news and advertisements. What is acid rain? This chapter will help you understand which substances are called acids and which are bases. How are they classified? What happens when an acid reacts with a base? Why do we use lemon juice on fish? In this chapter, you will learn about the chemistry of acids and bases. This will help you better understand these important classes of compounds. What do we mean by the pH of a solution like acid rain? Acids are widely used in the manufacture of fertilizers and in the food industry.
Acids and bases are generally recognized by their characteristic properties.
| Sr. No. | Property | Acid | Base |
|---|---|---|---|
| 1 | Taste | Sour | Bitter |
| 2 | Effect on blue Litmus | Turns red | No effect |
| 3 | Effect on red litmus | No effect | Turns blue |
| 4 | Effect on skin | Corrosive | Corrosive |
| 5 | Electrical conductivity | Aqueous solutions conduct electricity | Aqueous solutions conduct electricity |
In 1887, a Swedish chemist Svante Arrhenius proposed the first successful theory of acids and bases. According to this theory:
An acid is a substance that ionizes in water to produce H+ ions and a base is a substance that ionizes in water to produce OH- ions.
For example:
HCl → H+(aq) + Cl-(aq)
NaOH → Na+(aq) + OH-(aq)
Which substances in the following reactions are acids or bases?
8.3 HNOHNO
H₂SO₄(l)
110 K(aq) + OH-(aq)
11.0 NH₂OH(aq) → NH₂+(aq) + OH-(aq)
Table 10.2 shows some common acids and Table 10.3 shows some common bases.
| Name | Formula | Common use |
|---|---|---|
| Hydrochloric acid | HCl | Cleaning of metals, bricks and removing scale from boilers |
| Nitric acid | HNO₃ | Manufacture of fertilizers, explosives |
| Sulphuric acid | H₂SO₄ | Manufacture of many chemicals, drugs, dyes, paints and explosives. |
| Phosphoric acid | H₃PO₄ | Manufacture of fertilizers, acidulant for food |
| Name | Formula | Common use |
|---|---|---|
| Sodium hydroxide | NaOH | Antacid, laxative |
| Potassium hydroxide | KOH | Soap making, drain cleaners |
| Calcium hydroxide | Ca(OH)₂ | Making mortar, plasters, cement |
| Magnesium hydroxide | Mg(OH)₂ | Making liquid soap, shaving cream |
The Arrhenius theory has its limitations. This applies to aqueous solutions. This does not explain why compounds like CO₂, SO₃, etc. are acidic. Why are substances like NH₃ bases? There is no H in CO₂ and no O in NH₃.
In 1923, J. N. Bronsted and T. M. Lowery independently proposed another theory to overcome the shortcomings of the Arrhenius theory. This theory is known as the Bronsted-Lowery theory. According to this theory, an acid is a proton donor and a base is a proton acceptor.
Consider the following example:
Acid + H⁺ → H₃O⁺
Hydranium ion
Chloride ion
Unit
1. Which substance is donating proton?
2. Which substance is accepting proton?
3. Which substance is acid?
4. Which substance is base?
Where does OH⁻ come from when ionizing bases such as ammonia? The Arrhenius theory is not sufficient to answer this question, but the Bronsted-Lowry theory explains how ammonia acts as a base in water. Ammonia is a gas at room temperature. When it is dissolved in water, the following reaction occurs:
NH₃ + H₂O → NH₄⁺ + OH⁻
Which substance is donating proton, NH₃ or H₂O? Which substance is proton acceptor? All the acids included in the Arrhenius Theory are also acids in the Bronsted-Lowry Theory. However, all the bases included in the Bronsted-Lowry Theory except OH⁻ are not Arrhenius bases. Consider the above two examples. In one example, a water molecule accepts a proton and in the other water donates a proton. This means water behaves like an acid as well as a base. It is amphoteric in nature. Substances that react with both acids and bases are called amphoteric substances.
Example 10.1: Classify substances as acids or bases or as proton donor or proton acceptor. Identify Bronsted-Lowry acids or bases in the following reactions.
HCl + H₂O → H₃O⁺ + Cl⁻
H₂O + NH₃ → NH₄⁺ + OH⁻
Problem solving strategy:
An acid is a proton donor. After donating a proton, an acid forms a negative ion.
A base is a proton acceptor. After accepting a proton from an acid, it forms a positive ion.
Solution:
1. Because HCl is converted to Cl⁻ by donating a proton, HCl is an acid.
2. Because H₂O accepts the proton that HCl donates and forms H₃O⁺, water is a base.
3. H₂O is converted to OH⁻ by donating a proton, so H₂O is an acid. Because NH₃ accepts the proton and forms NH₄⁺, it is a base.
Identify Bronsted acids and Bronsted bases in the following reactions.
1. H₂SO₄ + H₂O → H₃O⁺ + HSO₄⁻
2. CH₃COOH + H₂O → CH₃COO⁻ + H₃O⁺
3. HS⁻ + NH₃ → NH₂⁺ + HS⁻