# Acids - JEE Important Topic

## Acids: A Proton Donor

What are acids? Acids are the chemicals which have a sour taste and can alter the color of blue litmus to red. Acids contain hydrogen ions or ions and they can donate this hydrogen ion (proton) to another substance. Acids react with some metals such as iron, and release hydrogen. Acids also act as catalysts and promote certain chemical reactions. Some examples of acids include sulfuric, hydrochloric, and phosphoric acids. Bases are bitter in taste and they can change the color of the red litmus to blue. A base is a substance which is able to accept a hydrogen ion from an acid.

## Acid-Base Theory

### Arrhenius Theory

Swedish chemist Svante Arrhenius proposed the Arrhenius theory of acids and bases in

1884. This classification is based on the type of ions formed when acid or base is added to water. An Arrhenius acid is a chemical, which in aqueous solution, increases the concentration of H+ or protons. For example, when we mix hydrochloric acid and water, we get an aqueous solution of HCl. The hydrochloric acid then dissociates into H+ ions and Cl- ions. This results in an increase in H+ or proton concentration.

$HCL(aq.)\rightarrow H^+(aq.)+Cl^-(aq.)$

Arrhenius base is a substance which in aqueous solution causes the increase in concentration of OH- ions. When we make an aqueous solution of Sodium hydroxide, it dissociates and forms OH- and Na+ ions and the concentration of OH- ions increases in solution.

$NaOH~(aq.)\rightarrow Na^+~(aq.)+OH^-~(aq.)$

### Lewis Theory

According to Lewis theory of acid-base reactions, during reaction acids accept pairs of electrons and bases give away pairs of electrons. Thus, we can say that a Lewis acid is an electron-pair acceptor such as H+ ion. A Lewis base is known as electron-pair donor. This theory says that acids react with bases and share a pair of electrons.

### Br $\phi$ nsted-Lowry theory

Br $\phi$ nsted-Lowry theory states that any substance which can transfer proton or H+ to another substance is an acid, and the substance which accepts protons, is a base. The acidic substance when loses a proton, it forms a base and it is called the conjugate base of an acid. When a base gains a proton, it forms an acid called conjugate acid of a base. For example, the reaction between hydrochloric acid (acid) and ammonia (base), is represented by:

$HCl+NH_3\rightleftharpoons NH_4^+ +Cl^-$

Here, ammonium ion (NH4+) is the acid conjugate to the base ammonia and the chloride ion (Cl-) is the base conjugate to the hydrochloric acid.

### Neutralization ReactionA chemical reaction in which acid and base react to form salt and water is called a neutralization reaction of acid and base reaction. The H+ ions from acids combine with OH- ions off base to form water. For example, hydrochloric acid and sodium hydroxide react to form the corresponding salt sodium chloride and water:

${Acid}+{Alkali}(base)\rightarrow {Salt}+{Water}$

$HCl+NaOH\rightarrow NaCl+H_2O$

### $pH$ of Acids and Bases

Solutions can be classified into acids or bases according to hydrogen ion concentration relative to water. pH is defined as the negative $\log$ of hydrogen ion concentration in solution.

$pH=-\log_{10}\left[H^+\right]$

The hydrogen ion concentration in water is $1\times10^{-7}$. Therefore, the pH of water is 7.0 and it is known as neutral pH. The pH scale ranges from 0 to 14. The pH value of a solution below 7.0 means the solution is acidic. If the pH of the solution is above 7.0, then it is alkaline, or basic.

## Chemical Properties of Acids and Bases

### Properties of Acid

• Since acidic solutions have a high concentration of hydrogen ions, they taste tart or sour.

• Acids are generally rough to the touch.

• Acids react with metal and form salt and hydrogen gas.

• Acids are corrosive in nature.

• Strong acids are good conductors of electricity.

### Properties of Bases

• Bases taste bitter or sour.

• Base solution feels soapy or slippery. This happens because when we touch, the bases dissolve the fatty-acid oils from our skin and form soap.

• Bases generally do not react with metals.

• Strong bases are good electrolytes.

## Differentiate Between Acids and Bases

 Sr.No Acids Bases 1. Generate H+ ions in aqueous solution. Generate OH- ions in aqueous solution. 2. Change Blue litmus paper to red. Change red litmus paper to blue. 3. pH varies from 0 to 7.0 pH varies from 7.0 to 14 4. Reacts with metal and forms hydrogen gas. Does not react with metals. 5. Examples are HCl, H2SO4 etc. Examples are NaOH, KOH, CaOH etc.

## Types of Acids

Acids are divided into inorganic and organic acids.

• Inorganic acids are also known as mineral acids. Acids that contain inorganic substances are called inorganic acids. They are formed when an inorganic anhydride combines with water. Mineral acids do not have carbon and are strong acids.

E.g., Sulphuric acid, Hydrochloric acid, Nitric acid etc.

• Organic acids are obtained from organic materials and contain carbon. Generally, organic acids are weak acids. Hence, they do not dissociate completely in water.

E.g., Acetic acid, formic acid, oxalic acid, uric acid etc.

There are also Natural Acids which are obtained from natural sources like animal or plant products, fruits etc. Examples are lactic acid, citric acid, formic acid.

## Uses of Acids

### Uses of acid in Industries

• Nitric acid and sulphuric acids are used in fertilizer industries.

• Acids are used in the pickling process where they react with metal to liberate hydrogen and thus clean the metal surface.

• Sulphuric acid is used as electrolyte in batteries.

• For synthesis esters, carboxylic acids are used

### Uses of acid in Medicine

• Acetylsalicylic acid (Aspirin) is used as a painkiller.

• Scurvy disease is treated with Vitamin C (ascorbic acid).

### Uses of Acids in our body

• Hydrochloric acid present in the gastric juices in the stomach helps to break down food during digestion.

• Genetic material in our body is made up of Nucleic acids like RNA and DNA.

• Fat in our body contains Fatty acids.

## Conclusion

A substance is said to be acidic due to the formation of H+ ions in aqueous solution. Formation of OH- ions in aqueous solution results in the basic nature of a substance. An acid reacts with a metal to form hydrogen gas and salt. Aqueous solutions of strong acids and bases conduct electricity.  A neutral solution has a pH = 7. An acidic solution has a pH less than 7 and a basic solution has a pH greater than 7. Acids and bases react to form corresponding salts and water. Acids have numerous uses in our homes, industries, organic synthesis, and so on. Some of the most important acids used are hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, carbonic acid, citric acid, and so on.

Competitive Exams after 12th Science

## FAQs on Acids - JEE Important Topic

1. Why are acids important?

Acids play a significant role in production of chemicals used in our daily life. They are used in medicine, pharmaceutical industry, food industry etc. The hydrochloric acid present in the stomach helps in breaking of complex food molecules. DNA and RNA present in our body contain nucleic acid.

Amino acids help in growth and tissue repair. The hydrochloric acid present  in the stomach helps digestion by breaking down large and complex food molecules. Fatty acids are the major component of cell structure. Ascorbic acid, or vitamin C,  keeps our immune system strong. Vinegar, like diluted acids, are used to dress salads and make pickles, giving food a distinctive taste.

2. What is the Pickling of metals?

Pickling is a surface treatment technique used to remove rust impurities from metal surfaces by treating it with acids during this a layer of the surface is removed. In industrial equipment, sometimes metal surfaces get coated with impurities with time, which may affect the purity of the product or further processing like plating with metal or painting.

Strong acids, such as hydrochloric acid and sulfuric acid are commonly used as pickling agents. These solutions usually also contain additives such as wetting agents and corrosion inhibitors. Pickling is sometimes called acid cleaning. Acid cleaning causes corrosion of metals, and it is not applicable to all steels. Hydrogen embrittlement becomes a problem for some alloys and high-carbon steels.

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