THE CHLOR-ALKALI INDUSTRY QUESTIONS AND ANSWERS GRADE 12 As students embark on their journey through Grade 12 chemistry, one of the topics that often sparks curiosity and interest is the chlor-alkali industry. This vital sector plays a significant role in various industrial processes, and understanding its principles is crucial for budding scientists and engineers. In this page, we’ll delve into some common questions about the chlor-alkali industry, providing clear and concise answers to aid in your understanding.

Q1: What is the chlor-alkali industry, and what does it involve?

A: The chlor-alkali industry is a sector of chemical manufacturing that primarily deals with the production of chlorine (Cl2), sodium hydroxide (NaOH), and hydrogen gas (H2). These three substances are essential in numerous industrial applications, ranging from the production of plastics and detergents to water treatment and pharmaceuticals.

Q2: How are chlorine, sodium hydroxide, and hydrogen gas produced in the chlor-alkali industry?

A: The production of chlorine, sodium hydroxide, and hydrogen gas occurs through an electrochemical process known as electrolysis. This process involves passing an electric current through a solution of brine (sodium chloride dissolved in water), which results in the breakdown of the salt into its constituent elements: chlorine gas is liberated at the anode, hydrogen gas at the cathode, and sodium hydroxide remains in the solution.

Q3: What are the main applications of chlorine, sodium hydroxide, and hydrogen gas?

A: Chlorine is widely used in the production of PVC (polyvinyl chloride) for pipes and plastic materials, as well as in water disinfection and the manufacturing of solvents and pesticides. Sodium hydroxide, also known as caustic soda, is a versatile chemical used in the production of soaps, detergents, paper, textiles, and various industrial processes. Hydrogen gas finds applications in petroleum refining, ammonia production for fertilizers, and as a fuel for fuel cells and hydrogen-powered vehicles.

Q4: What are the environmental concerns associated with the chlor-alkali industry?

A: While the chlor-alkali industry plays a crucial role in various sectors, it also poses environmental challenges, particularly regarding the production of chlorine gas, which can be toxic and harmful if released into the atmosphere. Additionally, the disposal of brine waste generated during the electrolysis process can have detrimental effects on aquatic ecosystems if not properly managed.

Q5: How can the chlor-alkali industry mitigate its environmental impact?

A: The chlor-alkali industry has made significant strides in implementing cleaner production technologies and improving waste management practices to minimize its environmental footprint. These include the adoption of membrane cell technology, which reduces energy consumption and eliminates the generation of mercury waste associated with older production methods. Furthermore, advancements in brine treatment and recycling processes help minimize the discharge of waste into the environment.

Q6: What are some emerging trends in the chlor-alkali industry?

A: In recent years, there has been growing interest in hydrogen as a clean and renewable energy source, leading to increased research and development efforts in electrolysis technologies for hydrogen production within the chlor-alkali industry. Additionally, the demand for chlorine and sodium hydroxide continues to rise with the expanding global economy, driving innovation in process efficiency and sustainability.

Activity 1

The electrolysis of saturated sodium chloride can be illustrated as follows:
Hydrogen and chlorine bubble off at the electrodes.

1. Give an equation to show how chlorine bubbles are formed at the electrode. (2)
2.  At which electrode are the chlorine bubbles formed? (1)
3. At which electrode does hydrogen gas form? (1)
4. What is the name of the product which leaves the cell? (1)
5. Give TWO applications of the product formed in 2.4. (2)
6. What purpose does the porous diaphragm serve? (2)
7. The chlorine gas produced, dissolves in water to form chlorine water. Write down a balanced equation for the reaction that takes place. (2) [11]

Solutions

1. 2Cℓ^–(aq) → Cℓ₂(g) + 2e^–  (2)
2. Anode  (1)
3. Cathode  (1)
4. Sodium hydroxide  (1)
5. Making soap and detergents; paper; rayon and other fibres; dyeing textiles  (any 2) (2)
6. Stops chlorine passing through; helps to separate sodium hydroxide from NaCℓ(aq).  (2)
7. Cℓ₂ + H₂O → HCℓ + HOCℓ (2)  [11]

Activity 2

The simplified diagram of a cell used in the chlor-alkali industry is shown below.

1. Write down the CHEMICAL FORMULA of brine. (1)
2. At which electrode, X or Y is chlorine gas formed? (1)
3. Write down a half-reaction that explains the formation of hydrogen gas at one of the electrodes. (4)
4. The purity of the sodium hydroxide produced in the chlor-alkali industry depends on the extent to which it is separated from the chlorine gas produced by this cell. Briefly describe how chlorine gas and sodium hydroxide are prevented from mixing in this cell. (2)
5. Apart from the advantages and disadvantages of products produced, write down for this process:
   5.1 ONE positive impact on humans. (1)
   5.2 ONE negative impact on humans (1)  [10]

Solutions

1.  NaCℓ (aq)  (1)
2.  Y  (1)
3. 2H₂O + 2e^– →H₂+2OH ^– (4)
4. The membrane prevents chloride ions from moving to the cathode, only allows positive ions through. (2)
5. 5.1 Job creation resulting in more people having a better life.  (1)
   5.2 Uses huge amounts of electricity resulting in load shedding.
   OR
   Chemical plant uses a lot of space that could have been used for housing/gardens, etc.  (any one) (1)