5.3 Electrochemistry and batteries

(DoE Exemplar Paper 2, 2007)

Summary

• The growth of South Africa's chemical industry was largely because of the mines, which needed explosives for their operations. One of South Africa's major chemical companies is Sasol . Other important chemical industries in the country are the chloralkali and fertiliser industries.
• All countries need energy resources such as oil and natural gas. Since South Africa doesn't have either of these resources, Sasol technology has developed to convert coal into liquid fuels.
• Sasol has three main operation focus areas: Firstly, the conversion of coal to liquid fuel , secondly the production and refinement of crude oil which has been imported, and thirdly the production of liquid fuels from natural gas .
• The conversion of coal to liquid fuels involves a Sasol/Lurgi gasification process, followed by the conversion of this synthesis gas into a range of hydrocarbons, using the Fischer-Tropsch technology in SAS reactors .
• Heavy hydrocarbons can be converted into light hydrcarbons through a process called cracking . Common forms of cracking are hydrocracking and steam cracking .
• With regard to crude oil, Sasol imports crude oil from Gabon and then refines this at the Natref refinery .
• Gas from Mozambique can be used to produce liquid fuels, through two processes: First, the gas must pass through an autothermal reactor to produce a synthesis gas. Secondly, this synthesis gas is passed through a Sasol Slurry Phase Distillate process to convert the gas to hydrocarbons.
• All industries have an impact on the environment through the consumption of natural resources such as water, and through the production of pollution gases such as carbon dioxide, hydrogen sulfides, nitrogen oxides and others.
• The chloralkali industry produces chlorine and sodium hydroxide . The main raw material is brine (NaCl).
• In industry, electrolytic cells are used to split the sodium chloride into its component ions to produce chlorine and sodium hydroxide. One of the challenges in this process is to keep the products of the electrolytic reaction (i.e. the chlorine and the sodium hydroxide) separate so that they don't react with each other. Specially designed electrolytic cells are needed to do this.
• There are three types of electrolytic cells that are used in this process: mercury cell , the diaphragm cell and the membrane cell .
• The mercury cell consists of two reaction vessels. The first reaction vessel contains a mercury cathode and a carbon anode. An electric current passed through the brine produces Cl ${}^{-}$ and Na ${}^{+}$ ions. The Cl ${}^{-}$ ions are oxidised to form chlorine gas at the anode. Na ${}^{+}$ ions combine with the mercury cathode to form a sodium-mercury amalgam. The sodium-mercury amalgam passes into the second reaction vessel containing water, where the Na ${}^{+}$ ions react with hydroxide ions from the water. Sodium hydroxide is the product of this reaction.
• One of the environmental impacts of using this type of cell, is the use of mercury , which is highly toxic.
• In the diaphragm cell , a porous diaphragm separates the anode and the cathode compartments. Chloride ions are oxidised to chlorine gas at the anode, while sodium ions produced at the cathode react with water to produce sodium hydroxide.
• The membrane cell is very similar to the diaphragm cell, except that the anode and cathode compartments are separated by an ion-selective membrane rather than by a diaphragm. Brine is only pumped into the anode compartment. Positive sodium ions pass through the membrane into the cathode compartment, which contains water. As with the other two cells, chlorine gas is produced at the anode and sodium hydroxide at the cathode.
• One use of sodium hydroxide is in the production of soaps and detergents , and so this is another important part of the chloralkali industry.
• To make soap, sodium hydroxide or potassium hydroxide react with a fat or an oil. In the reaction, the sodium or potassium ions replace the alcohol in the fat or oil. The product, a sodium or potassium salt of a fatty acid , is what soap is made of.
• The fatty acids in soap have a hydrophilic and a hydrophobic part in each molecule, and this helps to loosen dirt and clean items.
• Detergents are also cleaning products, but are made up of a mixture of compounds. They may also have other components added to them to give certain characteristics. Some of these additives may be abrasives, oxidants or enzymes.
• The fertiliser industry is another important chemical industry.
• All plants need certain macronutrients (e.g. carbon, hydrogen, oxygen, potassium, nitrogen and phosphorus) and micronutrients (e.g. iron, chlorine, copper and zinc) in order to survive. Fertilisers provide these nutrients.
• In plants, most nutrients are obtained from the atmosphere or from the soil.
• Animals also need similar nutrients, but they obtain most of these directly from plants or plant products. They may also obtain them from other animals, whcih may have fed on plants during their life.
• The fertiliser industry is very important in ensuring that plants and crops receive the correct nutrients in the correct quantities to ensure maximum growth.
• Nitrogen fertilisers can be produced industrially using a number of chemical processes: The Haber process reacts nitrogen and hydrogen to produce ammonia ; the Ostwald process reacts oxygen and ammonia to produce nitric acid ; the nitrophosphate process reacts nitric acid with phosphate rock to produce compound fertilisers.
• Phosphate fertilisers are also produced through a series of reactions. The contact process produces sulfuric acid . Sulfuric acid then reacts with phosphate rock to produce phosphoric acid , after which phosphoric acid reacts with ground phosphate rock to produce fertilisers such as triple superphosphate .
• Potassium is obtained from potash .
• Fertilisers can have a damaging effect on the environment when they are present in high quantities in ecosystems. They can lead to eutrophication . A number of preventative actions can be taken to reduce these impacts.
• Another important part of the chemical industry is the production of batteries .
• A battery is a device that changes chemical energy into electrical energy.
• A battery consists of one or more voltaic cells , each of which is made up of two half cells that are connected in series by a conductive electrolyte. Each half cell has a net electromotive force (emf) or voltage. The net voltage of the battery is the difference between the voltages of the half-cells. This potential difference between the two half cells is what causes an electric current to flow.
• A primary battery cannot be recharged, but a secondary battery can be recharged.
• The capacity of a battery depends on the chemical reactions in the cells, the quantity of electrolyte and electrode material in the cell, and the discharge conditions of the battery.
• The relationship between the current, discharge time and capacity of a battery is expressed by Peukert's law :
  $$\begin{array}{c}\hfill C_p=I^{k}t\end{array}$$
  In the equation, 'C ${}_p$ ' represents the battery's capacity (Ah), I is the discharge current (A), k is the Peukert constant and t is the time of discharge (hours).
• Two common types of batteries are lead-acid batteries and the zinc-carbon dry cell .
• In a lead-acid battery , each cell consists of electrodes of lead (Pb) and lead (IV) oxide (PbO ${}_2$ ) in an electrolyte of sulfuric acid (H ${}_2$ SO ${}_4$ ). When the battery discharges, both electrodes turn into lead (II) sulphate (PbSO ${}_4$ ) and the electrolyte loses sulfuric acid to become mostly water.
• A zinc-carbon cell is made up of an outer zinc container, which acts as the anode . The cathode is the central carbon rod, surrounded by a mixture of carbon and manganese (IV) oxide (MnO ${}_2$ ). The electrolyte is a paste of ammonium chloride (NH ${}_4$ Cl). A fibrous fabric separates the two electrodes, and a brass pin in the centre of the cell conducts electricity to the outside circuit.
• Despite their many advantages, batteries are made of potentially toxic materials and can be damaging to the environment .

Summary exercise

1. Give one word or term for each of the following descriptions:
   1. A solid organic compound that can be used to produce liquid fuels.
   2. The process used to convert heavy hydrocarbons into light hydrocarbons.
   3. The process of separating nitrogen from liquid air.
   4. The main raw material in the chloralkali industry.
   5. A compound given to a plant to promote growth.
   6. An electrolyte used in lead-acid batteries.
2. Indicate whether each of the following statements is true or false. If the statement is false, rewrite the statement correctly.
   1. The longer the hydrocarbon chain in an organic compound, the more likely it is to be a solid at room temperature.
   2. The main elements used in fertilisers are nitrogen, phosphorus and potassium.
   3. A soap molecule is composed of an alcohol molecule and three fatty acids.
   4. During the industrial preparation of chlorine and sodium hydroxide, chemical energy is converted to electrical energy.
3. For each of the following questions, choose the one correct answer from the list provided.
   1. The sequence of processes that best describes the conversion of coal to liquid fuel is:
      1. coal $\to$ gas purification $\to$ SAS reactor $\to$ liquid hydrocarbon
      2. coal $\to$ autothermal reactor $\to$ Sasol slurry phase F-T reactor $\to$ liquid hydrocarbon
      3. coal $\to$ coal purification $\to$ synthesis gas $\to$ oil
      4. coal $\to$ coal gasification $\to$ gas purification $\to$ SAS reactor $\to$ liquid hydrocarbons
   2. The half-reaction that takes place at the cathode of a mercury cell in the chloralkali industry is:
      1. $2{\mathrm{Cl}}^{-}\to {\mathrm{Cl}}_2+2{\mathrm{e}}^{-}$
      2. $2{\mathrm{Na}}^{+}+2{\mathrm{e}}^{-}\to 2\mathrm{Na}$
      3. $2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2$
      4. $\mathrm{NaCl}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{NaOH}+\mathrm{HCl}$
   3. In a zinc-carbon dry cell...
      1. the electrolyte is manganese (IV) oxide
      2. zinc is oxidised to produce electrons
      3. zinc is reduced to produce electrons
      4. manganese (IV) dioxide acts as a reducing agent
4. Chloralkali manufacturing process The chloralkali (also called 'chlorine-caustic') industry is one of the largest electrochemical technologies in the world. Chlorine is produced using three types of electrolytic cells. The simplified diagram below shows a membrane cell.

   

   1. Give two reasons why the membrane cell is the preferred cell for the preparation of chlorine.
   2. Why do you think it is advisable to use inert electrodes in this process?
   3. Write down the equation for the half-reaction taking place at electrode M.
   4. Which gas is chlorine gas? Write down only Gas A or Gas B.
   5. Briefly explain how sodium hydroxide forms in this cell.
   (DoE Exemplar Paper 2,2007)
5. The production of nitric acid is very important in the manufacture of fertilisers. Look at the diagram below, which shows part of the fertiliser production process, and then answer the questions that follow.

   

   1. Name the process at (1).
   2. Name the gas at (2).
   3. Name the process at (3) that produces gas (2).
   4. Name the product at (4).
   5. Name two fertilisers that can be produced from nitric acid.
6. A lead-acid battery has a number of different components. Match the description in Column A with the correct word or phrase in Column B. All the descriptions in Column A relate to lead-acid batteries.

   Column A	Column B
   The electrode metal	Lead sulphate
   Electrolyte	Mercury
   A product of the overall cell reaction	Electrolytic
   An oxidising agent in the cathode half-reaction	Lead
   Type of cells in a lead-acid battery	Sulfuric acid
   	Ammonium chloride
   	Lead oxide
   	Galvanic