Sodium Hydroxide and Bleaching Powder

Key Concepts

• Common Salts
• Preparation of NaOH from Common salt
• Preparation of CaOCl₂  from Common salt
• Properties of NaOH and CaOCl₂ 
• Applications of NaOH and CaOCl₂ 

Introduction

By now, we know that the combination of acid and base forms salt. Salts are formed as an effect of neuralization of acids and bases therefore, we can say that salts formed by neutralization reactions. Common salt (NaCl) can be formed by combining hydrochloric acid and sodium hydroxide solution. This common salt is very important for our body to maintain a balance of fluids and keep nerves and muscle running smoothly. NaCl is also widely used as food preservative and taste enhancer in food industry. The salt thus obtained is an important raw material for the substances that we use daily, such as washing soda, baking soda, bleaching powder, and many more. 

Let us learn more about how one substance, i.e., common salt (sodium chloride), is used to make all these different substances.  

COMMON SALT 

Common salt is a substance formed by the combination of hydrochloric acid and sodium hydroxide solution is called sodium chloride. 

HCl + NaOH → NaCl + water 

Generally, the product of neutralization reactions is salt and water. 

Sodium Hydroxide(NaOH) 

Sodium hydroxide (NaOH) is solid ionic compound. 

Sodium hydroxide is also called caustic soda and can be formed from sodium chloride. 

Preparation of Sodium Hydroxide 

1. Electrolysis of Brine: 

Brine is the aqueous solution of sodium chloride. This brine solution on electrolysis produces sodium hydroxide as the principal product. 

Electrolysis: It is process in which the electric current is passed through an aqueous solution of ionic compounds to decompose into its constituent elements. 

Commercially, NaOH is  manufactured by the electrolysis of brine (NaCl solution). 

1. When electric current is passed through an aqueous solution sodium chloride, it decomposes to form sodium hydroxide and chlorine. 

2. This process is called chlor-alkali process because the products obtained are (chlor for) chlorine and alkali. (Alkali as sodium hydroxide). 

Process of Electrolysis: 

On passing electricity through the brine solution, sodium chloride dissociates as Na⁺ and Cl^– ions: 

The chlorine gas is given off at anode and hydrogen gas at cathode. 

Anode :

Cathode:  

The three products formed in this reaction including sodium hydroxide, chlorine, and Hydrogen. All are very useful. 

2. By Heating Sodium Carbonate (Caustication Of Na₂CO₃) 

Sodium hydroxide can be also prepared by heating 10 % Na₂CO₃ solution with milk of lime. 

This process is also called as lime – caustic soda process. 

Physical Properties of Sodium Hydroxide 

1. Sodium hydroxide is a white crystalline translucent solid,  

deliquescent substance. 

2. It is highly soluble in water and gives silky touch like soap 

3. It is highly corrosive solid. 

Chemical Properties of Sodium Hydroxide 

1. Reaction with Acid and Acidic Oxides: 

Sodium hydroxide reacts with acid and acidic oxides to form salts. 

With Acid: 

When it reacts with acids like hydrochloric acids, the neutralization reaction takes place and sodium chloride is obtained due to neutralization. 

With Silica: Sodium hydroxide reacts with the Silica, i.e., silicon dioxide to give sodium silicate and water. 

Carbon dioxide:  reacts with carbon dioxide to give sodium carbonate and water. 

With Sulphur dioxide:  NaOH reacts with sulphur dioxide to give sodium sulphate as salt and water. 

2. Reaction with the metal salts 

Sodium hydroxide produces precipitate with some metal salts. 

With Ferric chloride:  

With Copper sulphate: 

With Manganese sulphate: 

With Ferrous sulphate 

Application/ Uses of sodium Hydroxide: 

1. Sodium hydroxide is used to manufacture soap, paper, artificial silk, and several chemicals. 

2. It is used in for the preparation of pure fats and oils. 

3. NaOH is used as laboratory reagent. 

4. It is used in petroleum refining. 

5. NaOH is also used in textile industries for mercerizing cotton fabrics. 

BLEACHING POWDER (CaOCl₂) 

The chemical name for bleaching powder is calcium hypochlorite or calcium oxychloride. 

The formula of bleaching powder is CaOCl_2. 

The chlorine that is obtained in the electrolysis process of brine solution, is used for manufacture of bleaching powder. 

Methods Of Preparation Of Bleaching Powder 

Action of Chlorine and Slaked Lime: 

Principle: Chlorine gas passing through slaked lime at high temperature (400°C) gives bleaching powder. 

Process: 

Bleaching powder can be manufactured in two types of plants, namely, Hasen Clever plant and Bachmann plant.  

Bachmann Plant: 

Bachmann plant is shaped like a vertical tower and made of cast iron. 

1. There are two inlets provided for the entry of chlorine gas and hot air into this tower. 

2. A hopper is also given at the top of the tower to enter slaked lime into the tower. 

3. The whole tower is made up of horizontal shelves required with rotating rakes. 

4. Dry slaked lime is introduced into the chlorinating tower through hopper. 

5. The chlorine gas and hot air are supplied through the inlet at the bottom. 

6. Due to the movement of rotating rakes, slaked lime moves downwards, whereas chlorine gas along with hot air moves upwards. 

7. This counter-current mechanism helps the thorough mixing up of chlorine gas with slaked lime which leads to completion of the reaction. 

Physical Properties of Bleaching Powder 

1. Bleaching powder is a pale-yellow powder. It has a strong smell of chlorine. 

2. It is soluble in water, but a clear solution is never formed due to the presence of impurities. 

Chemical Properties of Bleaching Powder 

On account of the formation of nascent oxygen, it shows oxidizing and bleaching  properties. 

1. Oxidizing Properties 

When excess dilute acid like HCl, H₂SO₄, H₂S is added to the bleaching powder, chlorine gas will be liberated. 

 CaOCl₂ + H₂S → CaCl₂ + H₂O + S 

Thereby, the chlorine produced because of dilute acid’s action on bleaching powder acts as a bleaching agent. Therefore, chlorine is the bleaching agent in bleaching powder.  

2. Decomposition: 

Bleaching powder slowly decomposes by itself to form Calcium Chloride and Calcium Chlorate. 

3. Bleaching action 

Uses of Bleaching Powder 

1. As bleaching agent: 

Bleaching powder is mainly used as a bleaching agent for bleaching cotton and linen in the textile industry, for wood pulp in paper factories . 

2. As a disinfectant and germicide  

Bleaching powder is very much used as disinfectant and germicide to sterilise the domestic water supply water. 

3. It is also used to manufacture chloroform given as anesthesia by doctors to perform the surgical operations. 

4. As an oxidizing agent in chemical industries. 

5. It can be used for making clothes of wools unshrinkable. 

Note: However, delicate articles like straw, silk, ivory are not bleached by bleaching powder. 

Summary:

• Sodium chloride is very important raw material for the manufacture of many substances
• that used in daily life like sodium hydroxide and bleaching powder. Sodium hydroxide can be prepared by electrolysis of brine solution (aq. Sodium chloride)
• and by heating sodium carbonate along with lime.
• Sodium hydroxide reacts with acid and acidic oxides to form salts.
• Sodium hydroxide used in manufacture of soap, paper, artificial silk, and several chemȧls and fat and oils.
• Bleaching powder is pale-yellow powder formed by chlorine gas and slaked Bleaching powder reacts with excess dilute acids like HSO4, HCI to give chlorine gas. The
• lime.
• chlorine that is produced here acts as a bleaching agent. Bleaching powder is mainly used as a bleaching agent for cotton, linen, and wood pulp an as Disinfectant and Germicide.