Acid and Base Reaction

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Introduction to Acid-Base Reaction

Acid and base reaction play a major role in both industrial chemistry and biochemistry. Many substances we come across in our homes, supermarkets, and the pharmacy are either acids or bases.  Eg: aspirin and vinegar are acids and antacids are bases. Lemon is acidic in nature and Kiwi is alkaline. The taste in the food is also due to the presence of acids and bases in them. Before we discuss the characteristics of how do acids and bases react with each other and what is the reaction between acid and base,  let’s first understand What are Acids and Bases?


What are Acids and Bases?

We can define acids as substances that dissolve in water to produce H+ ions, it is capable of donating a proton (hydrogen ion) to another substance, whereas bases are defined as substances that dissolve in water to produce OH- ions and is a molecule or ion able to accept a hydrogen ion from an acid.

The general properties of acids and bases were known to people for more than a thousand years, The definitions of acid and base have changed dramatically as scientists have learned more about them.

Acids are usually identified by their sour taste. An acid is basically a molecule that can donate H+ ions and can remain favorable even after losing H+ion, Acids turn blue litmus red. On the other hand, A base is any substance that had a bitter taste, felt slippery to the touch, and caused color changes of red litmus paper to blue.

We encounter reactions between acid and base in our everyday lives. The orange, lemon, or grapefruit juice we drink contains citric acid. When milk is left for sometimes it turns sour, it contains lactic acid. The vinegar used contains acetic acid. According to this, a chemical bond is made by an acid-base combination. The properties of a molecule can be understood more easily by classifying them into acid and base fragments.


Theories of Acids and Bases

Three different theories define acids and bases. These different theories include the Arrhenius theory, the Bronsted-Lowry theory, and the Lewis concepts of acids and bases. A brief description of each of these theories with acid and base reaction examples are described below:


The Arrhenius Definition of Acids and Bases: 

  • The first person to give detail about acids and bases was the Swedish chemist Arrhenius. According to his definition, An acid is a substance that dissolves in water to produce H+ ions and a base is a substance that dissolves in water to produce hydroxide (OH-) ions. 

HCl(g)  →  H+(aq) + Cl- (aq)     (HCl is Arrhenius acid)

NaOH(s)  → Na+ (aq) + OH- (aq)     (NaOH is Arrhenius base)

  • Although Arrhenius’s ideas were widely accepted and it successfully explains the reaction between acids and bases that yield salts and water.

  • The limitation in Arrhenius’s definitions of acids and bases is that it fails to explain how substances lacking hydroxide ions form basic solutions when dissolved in water, Eg: NO2-- and F--.

  • The reaction between ammonia (a base) with gaseous HCl (an acid) to give ammonium chloride is not an acid base reaction examples because it does not involve  H+  and  OH-- :

NH3(g) + HCl(g) → NH4Cl(s)


The Bronsted–Lowry Definition of Acids and Bases

  • Because of some limitations in the Arrhenius definition, a more general definition of acids and bases was required. According to Bronsted and Lowry, A Bronsted acids undergo dissociation to yield protons and therefore increase the concentration of H+ ions in the solution, and A base is defined as a proton acceptor (or H+ ion acceptor) by this theory. The Bronsted–Lowry definition of an acid is the same as the Arrhenius definition, except that it is not restricted to aqueous solutions only. The Bronsted–Lowry definition of a base, is far more general because the hydroxide ion is just one of many substances that can accept a proton. An advantage of the Bronsted-Lowry definition of acids and bases is its ability to explain the acidic or basic nature of an ionic species.

  • The main limitation of  Bronsted–Lowry theory is that it fails to explain how compounds lacking hydrogen exhibit acidic properties, Eg: BF3 and AlCl3.


Lewis Concept of Acids and Bases

  • According to Lewis’s concept of an acid, it states that it is a species that has a vacant orbital and therefore, has the ability to accept an electron pair, and A Lewis base can hold a lone pair of electrons and can act as an electron-pair donor.

  • Lewis acids are electrophilic in nature and Lewis Bases are nucleophilic in nature.

  • A Lewis acid can accept an electron pair from a Lewis base and forms a coordinate covalent bond in the process. The theory does not involve the hydrogen and hydroxide atoms in its definition of acids and bases.

  • Lewis acids: Cu2+, BF3, Fe2+ ,Fe3+, Lewis bases: F-, NH3,AlCl3 ,C2H4.

  • The major advantage of this concept is that many compounds can be defined as acids and bases. However, it does not focus on the strength of acids and bases.

  • The major drawback of this theory is that it fails to explain the acid and base reaction that does not involve the formation of a coordinate covalent bond.


Difference Between Acid and Bases

Acids

Bases

Acid gives hydrogen ions in an aqueous solution.

Bases give hydroxide ions in an aqueous solution.

Acid turns blue litmus paper into the red.

The base turns red litmus paper into blue.

Its pH value ranges from 1 to 7.

Its pH value ranges from 7 to 14.

It has a sour taste.

It has a bitter taste.

Eg: HNO3, CH3COOH, HCl, H2SO4

Eg: KOH, NaOH, Mg(OH)2


The pH of Acids and Bases

  • It is clear from the above discussion that the nature of the solution (acidic, basic, or neutral) can be represented in terms of either hydrogen ion concentration or hydroxide ion concentration, but it is convenient to express the acidity or basicity of a solution by referring to the concentration of hydrogen ions only.

  •  pH measurement is done using pH paper and pH meter, The hydrogen ion concentration is expressed in terms of the numerical value of negative power to which 10 must be raised. This numerical value of negative power is termed pH, pH= -log [H+]  .

  • The pH of a solution is defined as the negative logarithm of the concentration (in mol per liter) of hydrogen ions which it contains or the pH of the solution can be defined as the logarithm of the reciprocal of H+ ion concentration. Acids have a pH value less than 7, bases have a pH value greater than 7 and neutral solutions have a pH equals to 7.


Importance of pH in Everyday Life

  • When the pH of rainwater is less than 5.6, it is acid rain.

  • Our body works well within a pH range of 7.0 to 7.8.

  • Our stomach produces HCl that helps in the digestion of food without harming the stomach. Sometimes excess acid is produced in the stomach that causes indigestion. To get rid of this, antacids (bases) are used.

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Natural Indicators

Natural Indicators can be found naturally and they help in determining whether the substance is acidic or basic in nature. Turmeric, grape juice, Red cabbage, curry powder, cherries, beetroots, onion, etc are some of the examples of natural indicators.

Some flowers like hydrangeas can also help in determining the acidity or basicity of the soil. These flowers change color to blue if the soil is acidic in nature, purple if the soil is neutral, and pink if the soil is basic in nature. The intensity of the color is dependent on the amount of acid or base present in the soil. Soil that is highly acidic has deep blue flowers, whereas soil which is highly basic yields deep pink flowers.


Uses of Acids 

Sulphuric Acid: It Is Called as King of Chemicals. Some of Its Major Uses Are:

  • It is used in manufacturing paints, drugs, dyes, and to produce fertilizers.

  • It is used in car batteries.

  • Used in the manufacturing of hydrochloric acid and alum.

Hydrochloric Acid: 

  • It is also used for cleaning sinks and sanitary ware.

  • It is used in various industries that use heating applications.

  • It is applied to remove deposits from the boilers.

Acetic Acid:

  • It is used as a cleansing agent in products like cleaning windows, floors, utensils, etc.

  • It is used for enhancing the flavor of food. Acetic acid is commonly known as vinegar.

  • This acid helps to remove stains on woodwork such as furniture and carpets.

Phosphoric Acid:

  • Used in fertilizer and detergent industries.

  • It is a key ingredient in many soft drinks.

Citric Acid: 

  • Used as a food preservative.

  • Used as a flavoring agent.

Ascorbic Acid:

  • It is mostly used in the process of treatment of bone marrow and scurvy diseases.

Boric Acid:

  • It is widely used in detergents.

  • Used in the manufacturing of glass, leather, paper, adhesives, and explosives.


Aqua Regia: It is a mixture of concentrated nitric acid and hydrochloric acid. It is highly corrosive and is able to attack gold and other resistant substances. It can be used to clean glassware, It is also used to remove noble metals such as gold, platinum, and palladium from the substrate.


Uses of Bases

Magnesium Hydroxide :

  • It is used as an antacid. It helps to correct the excess amount of acidity in the stomach.

Calcium Hydroxide:

  • It is an important ingredient in whitewash and mortar.

  • It is also used in the preparation of dry mixes for painting and decorating.

  • Used to neutralize the acidity in soils.

  • Used to make bleaching powder.

 Sodium Hydroxide:

  • It is occasionally used to unblock drains in the home.

  • It is the key ingredient in the manufacture of soaps and detergents.

  • It is also used in the manufacture of paper, textiles, and detergents.

  • It is used in the refining of petroleum.

Ammonium Hydroxide:

  • It is used to make fertilizers, rayon, plastics, and dyes.

  • It is used as a reagent in a chemical laboratory.


Acid and Base Reaction

When an acid is reacted with a base, salt and water are produced. The reaction between an acid and a base is termed a neutralization reaction. We will look at a few acid base reaction equations:

  • Hydrochloric acid reacts with sodium hydroxide to give sodium chloride (a salt) and water as a byproduct:

HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)

  • Hydrogen bromide reacts with potassium hydroxide to give potassium bromide (a salt) and water as byproducts.

HBr (aq) + KOH (aq) → KBr (aq)  + H2O (l) 

  • Hydrochloric acid reacts with ammonia and forms ammonium chloride as a product. NH3 acts as a base, despite not having OH-- ions

HCl (aq) + NH3(aq)  → NH4Cl (aq)


Did You Know?

Bee stings are acidic in nature and have a pH level between 5 and 5.5. They can be soothed by using substances like bicarbonate of soda and milk of magnesia(Bases). Both bases help to neutralize the acidic bee sting and relieve the itchiness.

FAQ (Frequently Asked Questions)

1. Write some properties of Acid, Base, and Neutral Substances.

Properties of Acids

  • They are corrosive in nature and sour in taste.

  • pH value is less than 7.

  • Acid turns blue litmus paper into the red.

  • In an aqueous solution, they act as good conductors of electricity.

Properties of Bases

  • They have a bitter taste and soapy texture when touched.

  • They act as good conductors of electricity in an aqueous solution.

  • Bases turn blue litmus paper into the red.

  • They release hydroxide ions (OH-- ions) when dissolved in water.

  • The pH values of bases are always greater than 7.

Properties of Neutral Solution/Salts

  • The pH value of salt is around 7.

  • In neutral substances, there is no change in the color of litmus paper.

  • Eg:  Water, Common salt (NaCl)

2. How strong acids and weak acids can be differentiated?

An acid that dissociates completely into ions is termed a strong acid. If it does not dissociate completely in the solution it is a weak acid. Acid solutions contain hydrogen ions. Higher the concentration of hydrogen ions, lower the pH, and vice versa. Sulphuric acid is a strong acid and vinegar is a weak acid. Strong acids can be fully ionized but weak acids can be ionized partly in the solution. At the same concentration, strong acids have a higher concentration of hydrogen ions compared to weak acids.

3. What is a buffer solution?

A buffer solution consists of an acid and a base. This solution is made by taking weak acid and then adding it to its conjugate base. Another way to form is by combining a weak base with its conjugate acid. The use of conjugate solutions is important in buffer solutions. We use buffer solutions to keep pH at a somewhat constant value.

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