Chapter 2: Acids, bases and salts

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Chapter 2

Acids, Bases & Salts

 

Introduction

• A substance that tastes sour in water, turns blue litmus red, and neutralises bases is known as an acid.
• If a substance’s aqueous solution tastes bitter, turns red litmus blue, or neutralises acids, it’s called a base.
• Salt is a neutral material that has no effect on litmus in aqueous solution.

Acids and Bases Reactions

A neutralisation reaction occurs when an acid reacts with a base. A salt and water are the end products of this reaction. An acid–base neutralisation reaction is formulated as a double-replacement reaction in this standard approach.

Reaction of Acid & Bases

a) Reaction of Acid & Bases with Metals

Acids, in general, react with metals to produce salt and hydrogen gas. Bases, in general, do not react with metals and do not produce hydrogen gas.
Acid + active metal → Salt + hydrogen + heat
2HCl + Mg → MgCl₂ + H₂ (↑)
Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen
Base + metal → Salt + hydrogen + heat
2NaOH + Zn → Na₂ZnO₂ + H₂ (↑)
Sodium hydroxide + Zinc → Sodium zincate + Hydrogen
A more reactive metal displaces the less reactive metal from its base.
2Na + Mg (OH) ₂ → 2NaOH + Mg
Sodium + Magnesium hydroxide → Sodium hydroxide + Magnesium

b) Reaction of Acids with metal carbonates and bicarbonates

Acids produce carbon dioxide, as well as metal salts and water, when they react with metal carbonates or metal bicarbonates. Sodium chloride, carbon dioxide, and water are formed when sodium carbonate interacts with hydrochloric acid. Allowing carbon dioxide gas to travel through lime water turns it milky.

Acid + metal carbonate or bicarbonate → salt + water + carbon dioxide.
2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂
H₂SO₄ + Mg (HCO₃)₂ → MgSO₄ + 2H₂O + 2CO₂
Effervescence indicates liberation of CO₂ gas.

c) Reaction of Acid with base

1. Reaction of metal oxides and hydroxides with acids
Metal oxides or metal hydroxides are basic in nature.
Acid + base → salt + water + heat

H₂SO₄ + MgO → MgSO₄ + H₂O
2HCl + Mg (OH) ₂ → MgCl₂ + 2H₂O

2. Reaction of non-metal oxides with bases

Non-metal oxides are acidic in nature
Base + Nonmetal oxide →  salt + water + heat

2NaOH + CO₂→ Na₂CO₃ + H₂O

3. Reaction of acids and base

A very common acid is hydrochloric acid. The reaction between strong acid says hydrochloric acid and strong base say sodium hydroxide forms salt and water. The complete chemical equation is shown below.
HCl (strong acid) + NaOH (strong base) → NaCl (salt) + H₂O (water)

Acid & bases in water Solution

Dilution of Acid and Base: The concentration of hydrogen ion in an acid and hydroxide ion in a base, per unit volume, shows the concentration of acid or base.
By mixing of acid to water, the concentration of hydrogen ion per unit volume decreases. Similarly, by addition of base to water, the concentration of hydroxide ion per unit volume decreases. This process of addition of acid or base to water is called Dilution and the acid or base is called Diluted.
The dilution of acid or base is exothermic. Thus, acid or base is always added to water and water is never added to acid or base. If water is added to a concentrated acid or base, a lot of heat is generated, which may cause splashing out of acid or base and may cause severe damage as concentrated acid and base are highly corrosive.
Strength of Acid and Base: Acids in which complete dissociation of hydrogen ion takes place are called Strong Acids. Similarly, bases in which complete dissociation of hydroxide ion takes place are called Strong Bases.
In mineral acid, such as hydrochloric acid, sulphuric acid, nitric acid, etc. hydrogen ion dissociates completely and hence, they are considered as strong acids. Since inorganic acids hydrogen ions do not dissociate completely, so they are weak acids.

Universal indicator

A universal indicator has a pH range from 0 to 14 that indicates the acidity or alkalinity of a solution.
A neutral solution has pH=7

Importance of pH in everyday life

1. pH sensitivity of plants and animals
Plants and animals are sensitive to pH. Crucial life processes such as digestion of food, functions of enzymes and hormones happen at a certain pH value.

2. pH of Soil

The pH of a soil optimal for the growth of plants or crops is 6.5 to 7.0.

3. pH in digestive system

The process of digestion happens at a specific pH in our stomach which is 1.5 to 4.
The pH of the interaction of enzymes, while food is being digested, is influenced by HCl in our stomach.

4. pH in tooth decay

Tooth decay happens when the teeth are exposed to an acidic environment of pH 5.5 and below.

5. pH of self-defense by animals and plants

Acidic substances are used by animals and plants as a self-defense mechanism. For example, bee and plants like nettle secrete a highly acidic substance for self-defense. These secreted acidic substances have a specific pH.

Characteristics of salt:

• Most of the salts are crystalline soild.
• Salts may be transparent or opaque.
• Most of the salts are soluble in water.
• Solution of the salts conducts electricity in their molten state also.
• The salt may be salty, sour, sweet, bitter and umami (savoury).
• Neutral salts are odourless.
• Salts can be colourless or coloured.

Family of Salt: Salts having common acidic or basic radicals are said to belong to the same family.
Example:
(i) Sodium chloride (NaCl) and Calcium chloride (CaCl₂) belongs to chloride family.
(ii) Calcium chloride (CaCl₂) and Calcium sulphate (CaSO₄) belongs to calcium family.
(iii) Zinc chloride (ZnCl₂) and Zinc sulphate (ZnSO₄) belongs to the zinc family.

Neutral, Acidic and Basic Salts:
(i) Neutral Salt: Salts produced because of reaction between a strong acid and strong base are neutral in nature. The pH value of such salts is equal to 7, i.e. neutral.
Example : Sodium chloride, Sodium sulphate. Postassium chloride, etc.
(ii) Acidic Salts: Salts which are formed after the reaction between a strong acid and weak base are called Acidic salts. The pH value of acidic salt is lower than 7. For example Ammonium sulphate, Ammonium chloride, etc.
Ammonium chloride is formed after reaction between hydrochloric acid (a strong acid) and ammonium hydroxide (a weak base).
(iii) Basic Salts: Salts which are formed after the reaction between a weak acid and strong base are called Basic Salts. For example; Sodium carbonate, Sodium acetate, etc.
Sodium carbonate is formed after the reaction between sodium hydroxide (a strong base) and carbonic acid (a weak acid).
Cause of formation of acidic, basic and neutral salts:

• When a strong acid reacts with a weak base, the base is unable to fully neutralize the acid. Due to this, an acidic salt is formed.
• When a strong base reacts with a weak acid, the acid is unable to fully neutralize the base. Due to this, a basic salt is formed.
• When equally strong acid and a base react, they fully neutralize each other. Due to this, a neutral salt is formed.
• Some Important Chemical Compounds

1. Common Salt (Sodium Chloride): Sodium chloride (NaCl) is also known as Common or Table Salt. It is formed after the reaction between sodium hydroxide and hydrochloric acid. It is a neutral salt. The pH value of sodium chloride is about 7. Sodium chloride is used to enhance the taste of food. Sodium chloride is used in the manufacturing of many chemicals.
Important chemical from sodium chloride Sodium Hydroxide (NaOH): 
Sodium hydroxide is a strong base. It is also known as caustic soda. It is obtained by the electrolytic decomposition of solution of sodium chloride (brine). In the process of electrolytic decomposition of brine (aqueous solution of sodium chloride), brine decomposes to form sodium hydroxide. In this process, chlorine is obtained at anode and hydrogen gas is obtained at cathode as by products. This whole process is known as Chlor – Alkali process.
Use of products after the electrolysis of brine:

• Hydrogen gas is used as fuel, margarine, in making of ammonia for fertilizer, etc.
• Chlorine gas is used in water treatment, manufacturing of PVC, disinfectants, CFC, pesticides. It is also used in the manufacturing of bleaching powder and hydrochloric acid.
• Sodium hydroxide is used for degreasing of metals, manufacturing of paper, soap, detergents, artificial fibres, bleach, etc.

2. Bleaching Powder (CaOCl₂): Bleaching powder is also known as chloride of lime. It is a solid and yellowish white in colour. Bleaching powder can be easily identified by the strong smell of chlorine.
When calcium hydroxide (slaked lime) reacts with chlorine, it gives calcium oxychloride (bleaching powder) and water is formed.
Aqueous solution of bleaching powder is basic in nature. The term bleach means removal of colour. Bleaching powder is often used as bleaching agent. It works because of oxidation. Chlorine in the bleaching powder is responsible for bleaching effect.
Use of Bleaching Powder:

• Bleaching powder is used as disinfectant to clean water, moss remover, weed killers, etc.
• Bleaching powder is used for bleaching of cotton in textile industry, bleaching of wood pulp in paper industry.
• Bleaching powder is used as oxidizing agent in many industries, such as textiles industry, paper industry, etc.

3. Baking Soda (NaHCO₃): Baking soda is another important product which can be obtained using by-products of chlor – alkali process. The chemical name of baking soda is sodium hydrogen carbonate (NaHCO₃) or sodium bicarbonate.
Properties of Sodium Bicarbonate:

• Sodium bicarbonate is white crystalline solid, but it appears as fine powder.
• Sodium hydrogen carbonate is amphoteric in nature.
• Sodium hydrogen carbonate is sparingly soluble in water.
• Thermal decomposition of sodium hydrogen carbonate (baking soda).
• When baking soda is heated, it decomposes into sodium carbonate, carbon dioxide and water.
  2NaHCO₃ + heat → Na₂CO₃ + CO₂ + H₂O
• Sodium carbonate formed after thermal decomposition of sodium hydrogen carbonate decomposes into sodium oxide and carbon dioxide on further heating.
  Na₂CO₃ → Na₂O + CO₂
  This reaction is known as Dehydration reaction.

Use of Baking Soda:

• Baking soda is used in making of baking powder, which is used in cooking as it produces carbon dioxide which makes the batter soft and spongy.
• Baking soda is used as an antacid.
• Baking soda is used in toothpaste which makes the teeth white and plaque free.
• Baking soda is used in cleansing of ornaments made of silver.
• Since sodium hydrogen carbonate gives carbon dioxide and sodium oxide on strong heating, thus, it, is used as a fire extinguisher.

Baking Powder: Baking powder produces carbon dioxide on heating, so it is used in cooking to make the batter spongy. Although, baking soda also produces carbon dioxide on heating, but it is not used in cooking because on heating, baking soda produces sodium carbonate along with carbon dioxide. The sodium carbonate, thus, produced, makes the taste bitter.
Baking powder is the mixture of baking soda and a mild edible acid. Generally, tartaric acid is mixed with baking soda to make baking powder.
When baking powder is heated, sodium hydrogen carbonate (NaHCO₃) decomposes to give CO₂ and sodium carbonate (Na₂CO₃). CO₂ causes bread and cake fluffy. Tartaric acid helps to remove bitter taste due to formation of Na₂CO₃.
4. Washing Soda (Sodium Carbonate)
Sodium carbonate is a crystalline solid and it is soluble in water when most of the carbonates are insoluble in water.
Use of sodium carbonate:

• It is used in the cleaning of cloths, especially in rural areas.
• In the making of detergent cake and powder.
• In removing the permanent hardness of water.
• It is used in glass and paper industries.

The water of Crystallization: Many salts contain water molecule and are known as Hydrated Salts. The water molecule present in salt is known as Water of crystallization.
Examples:
Copper sulphate pent hydrate (CuSO₄.5H₂O): Blue colour of copper sulphate is due to presence of 5 molecules of water. When copper sulphate is heated, it loses water molecules and turns: into grey – white colour, which is known as anhydrous copper sulphate. After adding water, anhydrous copper sulphate becomes blue again.
Plaster of Paris

Plaster of Paris is a widely used chemical compound that is extensively used in sculpting materials and gauze bandages. Plaster of Paris is a white powdery chemical compound that is hydrated calcium sulphate that is usually obtained by calcining gypsum. While we have seen many applications of this material in our everyday lives, if we try to understand its chemistry, we will find that it is a white powdery chemical compound that is hydrated calcium sulphate that is usually obtained by calcining gypsum. To put it another way, Plaster of Paris is often manufactured of heated gypsum at a high temperature.
Gypsum plaster is another name for plaster of Paris. Plaster of Paris is expressed as CaSO₄. ½ H₂O in chemical formula.
Gypsum, CaSO₄.2H₂O (s) on heating at 100°C (373K) gives CaSO₄. ½ H₂O and 3/2 H₂O
CaSO₄. ½ H₂O is plaster of paris.
CaSO₄. ½ H₂O means two formula units of CaSO₄ share one molecule of water.
Uses – cast for healing fractures.