5.1 The chloralkali industry

The chloralkali industry

1. Refer to the flow diagram below which shows the reactions that take place in the membrane cell, and then answer the questions that follow.

   

   1. What liquid is present in the cathode compartment at (a)?
   2. Identify the gas that is produced at (b).
   3. Explain one feature of this cell that allows the Na ${}^{+}$ and OH ${}^{-}$ ions to react at (c).
   4. Give a balanced equation for the reaction that takes place at (c).
2. Summarise what you have learnt about the three types of cells in the chloralkali industry by completing the table below:

   	Mercury cell	Diaphragm cell	Membrane cell
   Main raw material
   Mechanism of separating Cl ${}_2$ and NaOH
   Anode reaction
   Cathode reaction
   Purity of NaOH produced
   Energy consumption
   Environmental impact

Soaps and detergents

Another important part of the chloralkali industry is the production of soaps and detergents . You will remember from an earlier chapter, that water has the property of surface tension . This means that it tends to bead up on surfaces and this slows down the wetting process and makes cleaning difficult. You can observe this property of surface tension when a drop of water falls onto a table surface. The drop holds its shape and does not spread. When cleaning, this surface tension must be reduced so that the water can spread. Chemicals that are able to do this are called surfactants . Surfactants also loosen, disperse and hold particles in suspension, all of which are an important part of the cleaning process. Soap is an example of one of these surfactants. Detergents contain one or more surfactants. We will go on to look at these in more detail.

Surfactant

A surfactant is a wetting agent that lowers the surface tension of a liquid, allowing it to spread more easily.

1. Soaps In chapter [link] , a number of important biological macromolecules were discussed, including carbohydrates, proteins and nucleic acids. Fats are also biological macromolecules. A fat is made up of an alcohol called glycerol, attached to three fatty acids ( [link] ). Each fatty acid is made up of a carboxylic acid attached to a long hydrocarbon chain. An oil has the same structure as a fat, but is a liquid rather than a solid. Oils are found in plants (e.g. olive oil, sunflower oil) and fats are found in animals.

   

   To make soap, sodium hydroxide (NaOH) or potassium hydroxide (KOH) must be added to a fat or an oil. During this reaction, the glycerol is separated from the fatty acid in the fat, and is replaced by either potassium or sodium ions ( [link] ). Soaps are the water-soluble sodium or potassium salts of fatty acids.

   

   Interesting fact

   Soaps can be made from either fats or oils. Beef fat is a common source of fat, and vegetable oils such as palm oil are also commonly used.
   Fatty acids consist of two parts: a carboxylic acid group and a hydrocarbon chain. The hydrocarbon chain is hydrophobic , meaning that it is repelled by water. However, it is attracted to grease, oils and other dirt. The carboxylic acid is hydrophilic , meaning that it is attracted to water. Let's imagine that we have added soap to water in order to clean a dirty rugby jersey. The hydrocarbon chain will attach itself to the soil particles in the jersey, while the carboxylic acid will be attracted to the water. In this way, the soil is pulled free of the jersey and is suspended in the water. In a washing machine or with vigourous handwashing, this suspension can be rinsed off with clean water.

   Soap

   Soap is a surfactant that is used with water for washing and cleaning. Soap is made by reacting a fat with either sodium hydroxide (NaOH) or potassium hydroxide (KOH).

2. Detergents

   Detergent

   Detergents are compounds or mixtures of compounds that are used to assist cleaning. The term is often used to distinguish between soap and other chemical surfactants for cleaning.

   Detergents are also cleaning products, but are composed of one or more surfactants. Depending on the type of cleaning that is needed, detergents may contain one or more of the following:
   • Abrasives to scour a surface.
   • Oxidants for bleaching and disinfection.
   • Enzymes to digest proteins, fats or carbohydrates in stains. These are called biological detergents .

The choralkali industry

1. The diagram above shows the sequence of steps that take place in the mercury cell.
   1. Name the 'raw material' in step 1.
   2. Give the chemical equation for the reaction that produces chlorine in step 2.
   3. What other product is formed in step 2.
   4. Name the reactants in step 4.
2. Approximately 30 million tonnes of chlorine are used throughout the world annually. Chlorine is produced industrially by the electrolysis of brine. The diagram represents a membrane cell used in the production of Cl ${}_2$ gas.

   

   1. What ions are present in the electrolyte in the left hand compartment of the cell?
   2. Give the equation for the reaction that takes place at the anode.
   3. Give the equation for the reaction that takes place at the cathode.
   4. What ion passes through the membrane while these reactions are taking place? Chlorineis used to purify drinking water and swimming pool water. The substance responsible for this process is the weak acid, hypochlorous acid (HOCl).
   5. One way of putting HOCl into a pool is to bubble chlorine gas through the water. Give an equation showing how bubbling Cl ${}_2$ (g) through water produces HOCl.
   6. A common way of treating pool water is by adding 'granular chlorine'. Granular chlorine consists of the salt calcium hypochlorite, Ca(OCl) ${}_2$ . Give an equation showing how this salt dissolves in water. Indicate the phase of each substance in the equation.
   7. The OCl ${}^{-}$ ion undergoes hydrolysis , as shown by the following equation: ${\mathrm{OCl}}^{-}+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons \mathrm{HOCl}+{\mathrm{OH}}^{-}$ Will the addition of granular chlorine to pure water make the water acidic, basic or will it remain neutral? Briefly explain your answer.
   (IEB Paper 2, 2003)