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9.5 Industrial Chemistry: 4. Sodium hydroxide

Syllabus reference (October 2002 version)
4. The industrial production of sodium hydroxide requires the use of electrolysis	Students learn to: explain the difference between galvanic cells and electrolytic cells in terms of energy requirements outline the steps in industrial production of sodium hydroxide from sodium chloride solution and describe the reaction in terms of net ionic and full formula equations distinguish between the three electrolysis methods used to extract sodium hydroxide: mercury process diaphragm process membrane process by describing each process and analysing the technical and environmental difficulties involved in each process	Students: identify data, plan and perform a first-hand investigation to identify the products of the electrolysis of sodium chloride analyse information from secondary sources to predict and explain the different products of the electrolysis of aqueous and molten sodium chloride

Extract from Chemistry Stage 6 Syllabus (Amended October 2002. © Board of Studies, NSW.

[Edit: 25Jun08]

Prior Learning: Preliminary module: 8.3.2 , HSC Module: 9.2.4

Background: Make sure you can define the following terms from module 9.2.4: oxidation, reduction, redox, reductant, oxidant, electrolyte, galvanic cell, electrode, cathode, anode. If you aren’t sure of any of these go to Chemistry 9.2.4

This summary table may help:

reductant (reducing agent) causes reduction		oxidant (oxidising agent) causes oxidation
e.g. metals, non-metal ions	electrons (e–)	e.g. metal ions, non-metals
gets oxidized at the anode		gets reduced at the cathode

Know how to use the redox table.

Remember that the standard reduction potential for each equation shown in the redox table, is a measure of the relative tendency of a substance to gain one or more electrons compared to the standard hydrogen half cell.

explain the difference between galvanic cells and electrolytic cells in terms of energy requirements

• There are two types of electrochemical cells, galvanic and electrolytic cells.
  1. A galvanic cell is one in which a redox reaction takes place spontaneously to produce electricity. Chemical energy is changed to electrical energy.

     

  2. An electrolytic cell is one in which electrolysis occurs, a compound is decomposed by passing electricity through it. The reaction occurring is not spontaneous, it is forced by applying a voltage.

     An electrolytic cell is used to refine copper.

     

  The differences between galvanic and electrolytic cells can be summarised in a table.

  Galvanic/Voltaic Cells	Electrolytic Cells
  chemical energy electrical energy	electrical energy  chemical energy
  two half-cells with separate electrolytes and a salt bridge (or porous barrier).	electrodes usually in the same electrolyte
  chemical reaction is spontaneous Eo total is positive	chemical reaction is forced by applying a voltage - it is not spontaneous Eo total is negative
  anode - negative terminal cathode - positive terminal oxidation always occurs at the anode the galvanic cell is a battery, releasing electrons into the circuit from the anode so the anode is the negative terminal of the battery as it gives up electrons, the anode becomes more positive and attracts negative ions from the solution	anode - positive electrode Cathode - negative electrode oxidation occurs at the anode
  electrons flow from the negative terminal to the positive terminal	electrons flow from the negative battery terminal to the negative cathode
  uses batteries	uses extract Al from Al2O3 electroplating purifying copper

identify data, plan and perform a first-hand investigation to identify  the products of the electrolysis of sodium chloride

• You probably carried out an experiment on the electrolysis of water during the preliminary course and you should be able to use the same equipment, ie a voltameter.
  
  
• The results you get will be affected by the concentration of the sodium chloride solution, so you might like to try this experiment using a dilute solution and then repeat it with a more concentrated solution.
  
  
• Suitable solutions can be made as follows:
  • dilute solution - add crystals about the size of 2 or 3 grains of rice to 100 mL of water and dissolve.
  • concentrated solution - add 4 or 5 heaped teaspoonsful of sodium chloride to 100 mL of water and dissolve.
    
    
• Think about what you expect might happen. The solutions will contain Na⁺ ions, Cl^– ions and H₂O molecules. You would not expect any significant number of hydrogen or hydroxide ions to be present as water is neutral (pH=7). So, reactions taking place at the electrodes might include:
  
  2H₂O(l)   +   2e^–      H₂(g)   +   2OH^–(aq)
  Na⁺(aq)   +  e^–         Na(s)
  2Cl^–(aq)                  Cl₂ (g)   +    2e^–
  2H₂O(l)                   O₂(g)    +    4H⁺(aq)   +    4e^–     
  
  
• Look at the possible products. Think about tests you could use to detect oxygen, hydrogen, chlorine and sodium hydroxide. Decide how you can use these tests to identify which reaction is happening at each electrode.

outline  the steps in industrial production of sodium hydroxide from sodium chloride solution and describe  the reaction in terms of net ionic and full formula equations

• The electrolysis of sodium chloride to produce sodium hydroxide and chlorine can be carried out in 3 types of electrolytic cells - mercury, diaphragm and membrane cells. The diaphragm cell is used most, but the membrane cell is becoming more common as it is used in most new plants.
  
  In all three cells, the steps involved are:
  1. A saturated brine (sodium chloride) solution has impurities removed by precipitation. Water used to dissolve the salt and make the brine must be purified and softened.
     • Calcium ions are removed by adding sodium carbonate - forming insoluble CaCO_3.
     • Magnesium ions are removed by adding sodium hydroxide - forming insoluble Mg(OH)_2.
     • Iron ions are removed by the sodium carbonate and sodium hydroxide - forming FeCO₃ and Fe(OH)₂.
     • Sulfate ions are removed by adding calcium chloride - forming CaSO₄.
       
       
     You should be able to write equations for these reactions.
     
     These precipitates are removed as a sludge.
     
     An ion-exchange process may also be used to remove calcium ions.
     
     
  2. Electricity is passed through the brine solution.
     
     The electrolyte surrounding the cathode is called a catholyte, the electrolyte surrounding the anode is called an anolyte.
     
     
  3. Products are separated out - these include chlorine, sodium hydroxide, hydrogen and wastes.

analyse information from secondary sources to predict  and explain  the different products of the electrolysis of aqueous and molten sodium chloride

Information about the products formed by the electrolysis of sodium chloride, both molten and aqueous, is available in text books and on the Internet. Compare this information with the results from your first hand investigation.

• Electrolysis of molten sodium chloride
  
  The only ions present are sodium and chloride ions.  Work out which electrode these ions will move towards when molten and predict what will happen to them. Will they gain or lose electrons?
  
  
• Electrolysis of an aqueous sodium chloride solution
  
  When a solution is placed in an electrolytic cell, a number of reactions is possible. A sodium chloride solution contains sodium ions, chloride ions and also water, so there is more than one possible reaction at each electrode. Remember that the higher the reduction potential, the more easily the substance is reduced (and thus the greater its oxidising power). The reduction potential of water is higher than the reduction potential of the ions of active metals, so water is often reduced rather than the active metal.

distinguish between  the three electrolysis methods used to extract sodium hydroxide:

• mercury process
• diaphragm process
• membrane process

by describing  each process and analysing  the technical and environmental difficulties involved in each process

Mercury Cell

• A thin layer of mercury, flowing across steel, is the cathode. Sodium from the brine forms an amalgam with the mercury and is removed from the cell. The amalgam goes to a decomposer where it is decomposed by reaction with water.

  2Na/Hg(l)   +   2H₂O(l)       2NaOH(aq)   + H₂(g)   +   2Hg(l)

  This arrangement allows NaOH of high purity to be produced in a separate vessel, so brine hardly contaminates the NaOH. It also allows H₂ and Cl₂ to be produced in different areas of the plant. This allows for greater safety.

  Try one or both of the following websites. Use the information there to try to draw a flow diagram of the process.

  Chlorine Online Information Resource, Brussels, Belgium
  

  Compare the flow diagram you have drawn with the one below.

  

Diaphragm Cell

• In the diaphragm cell, two electrolyte solutions are separated by a diaphragm. Early diaphragms were made of asbestos, which caused environmental problems, however newer ones are made of polymers. 

  Scroll down to the information above and below the second diagram and to the diagram itself.

Membrane Cell

• The electrolyte solutions in a membrane cell are separated by a cation exchange membrane. This is really an improved diaphragm, allowing Na⁺ ions to move across, but not allowing OH^– ions to cross it. New membranes developed allow the electrodes to be very close to each other, on opposite sides of the membrane. This makes the cell very energy efficient, allowing considerable savings.

  Refer to: The Chemical Engineers’ Resource Page Midlothian, Virginia, USA.

• The following table compares the three types of electrolytic cells:

• 	Mercury Cell	Diaphragm Cell	Membrane Cell
  cathode	mercury flowing over steel sodium ions reduced	steel mesh water reduced	stainless steel or Ni water reduced
  cathode product	Na/Hg amalgam (Cl2 and brine are removed separately)	NaOH, NaCl, H2 (have to be separated)	NaOH, H2    (membrane will not let Cl– or OH– through)
  decomposer	NaOH & H2 made in a decomposer	o	o
  anode	titanium	titanium	titanium
  anode product	Cl2	Cl2	Cl2
  purity of NaOH	[NaOH] is about 50 % (produces purest NaOH)	[NaOH] is only about 11 %	[NaOH] is 30-40 %
  electrical energy	operating voltage 4-4.5 volts	operating voltage 4-5 volts	operating voltage 3-4 volts and may become lower
  cell temperature (°C)	90-95	75-85	88-90
  environmental problems	environmental problems with use of Hg & its disposal	older cells contain asbestos	o

  Technical difficulties/safety aspects

  Problem	Implications
  chlorine is a toxic gas	check for leaks, protective breathing apparatus must be carried at all times and showers easily accessible
  corrosive action of salt, chlorine and sodium hydroxide	maintenance is expensive metal pipes, bricks and aluminium surfaces corrode
  hydrogen will react explosively with chlorine or oxygen	check for leaks, especially in diaphragm and membrane cells
  mercury is toxic.	check for leaks, safe disposal of waste
  uses lots of electricity	electric currents create heat and magnetic effects. The size of the gap between electrodes must be controlled by means of computers. The smaller the gap, the lower the operating voltage and hence the lower the costs of production.
  quality control	check: concentration of NaOH (titration) concentration of metal ions in brine (AAS) moisture content of Cl2 (gravimetric analysis) impurities such as H2 and O2 in the chlorine (gas chromatography).