The chlor-alkali electrolysis process is used in the manufacture of chlorine, hydrogen and sodium hydroxide solution. Of these three, the primary product is chlorine.
In each one, a salt solution is electrolyzed by action of direct electric current that converts chloride ions to elemental chlorine .
Anode and cathode reactions in the membrane cell
A membrane allows positive ions such as Na+ (aq), to pass through it, but not negative ions, like Cl- (aq), or water molecules.
On one side circulating brine is in contact with a titanium anode and chloride ions are discharged producing chlorine gas . (Figure 1)
Figure 1: The membrane cell 
Redox reactions involve electron transfer. Redox reactions can be split into two half-reactions, one producing electrons, and one accepting them . If the ions in the electrolyte solution are positively charged, they will flow towards the cathode and be reduced. If the ions are negatively charged, they will flow to the anode and be oxidised. The extraction of chlorine is an example of such reaction:
2NaCl + 2H2O Cl2 + H2 + 2NaOH
Anode: 2Cl- (aq) Cl2 (g) + 2e-
Cathode: 2 H+ (aq) + 2e- H2 (g)
Redox reactions in the membrane cell
If chlorine is produced in the presence of sodium hydroxide solution it will combine with it to produce ClO- and Cl- ions, which will result in production of sodium chlorate ( | ), NaClO. To overcome this problem, chlorine and sodium hydroxide in a mercury cathode cell are produced in two different containers. Titanium anodes, coated with an oxide are in the top of the container where chlorine ions are discharged and Cl2 is formed.
2Cl- (aq) Cl2 (g) + 2e-
A layer of mercury flowing along the base of the container is the cathode.The bottom of this container is called the decomposer. There, sodium ions are reduced to sodium and sodium and the sodium dissolves in the mercury.
Whereas in a membrane cell, the products are kept apart because a membrane allows positive ions like Na+ (aq), to pass through it, but not negative ions, like Cl- (aq), or water molecules . Therefore the products cannot react again.
Uses of chlorine, sodium hydroxide and hydrogen in present-day industry
Chlorine (Figure 2), sodium hydroxide (Figure 3) and hydrogen (Figure 4) are among the top ten chemicals produced in the world and are involved in the manufacturing of a wide variety of products used in day-to-day life. For example:
Figure 2: A pie chart showing the uses of chlorine 
Figure 3: A pie chart showing the uses of sodium hydroxide 
A process of the electrolysis of an aqueous sodium chloride solution involves passing electric current through the said solution and separating the products in an anode and cathode chamber separated by a membrane. In a membrane cell, chlorine is formed at the anode and hydrogen is formed at the cathode.
-Extract from: Ullman’s Encyclopaedia of Industrial Chemistry, UCH Publishers, New York, 1989, page 473.
- Extract from: The essential chemical Industry by Bill Fox, Chemistry review, 1996, Volume 6, Number 2.
- http://www.cellchem.com/docs/products-services/chlorine caustic.htm, informative web-site; 27 February.
- Extract from: Redox reactions and electrode potentials, Chemical Ideas, Heinemann, page 208.
- http://electrochem.cwru.edu/ed/encycl/, electrochemistry encyclopaedia web-site; 28 February.
ADVANCED SUBSIDIARY GCE
Skills for Chemistry: Open-Book Paper
Primary pollutants are formed by oxidation under high temperatures in vehicle engines during combustion:
N2 + 2O2 –> 2NO2
N2 + O2 –> 2NO
NOx emissions are formed from fuels that contain nitrogen compounds and are oxidised, but most importantly NOx is formed from nitrogen and oxygen atoms in the air. NOx is proven to be hazardous to plant and animal health. 
When carbon compounds in the fuel are oxidised, carbon monoxide is formed.
3O2 + 2CH4 –> 2CO + 4H2O
Carbon monoxide is harmful because it reduces oxygen delivery to the body’s organs and tissues. It is most harmful to those who suffer from heart and respiratory disease. High CO pollution levels also affect healthy people. 
Hydrocarbon pollution results when unburned or partially burned fuel is emitted from the engine exhaust, and also when fuel evaporates directly into the atmosphere.
C2H6 + 1.5O2 –> CO2 + H2O + CH4
Hydrocarbons include many toxic compounds that cause cancer and other adverse health effects. 
In sunlight, NOx (a primary pollutant) reacts with hydrocarbons to produce a variety of harmful substances called photochemical oxidants. These include ?-oxoethylperoxylnitrate (Peroxyacetyl nitrate, or PAN – Figure 1) and ozone (O3) (secondary pollutants). Ozone is a pollutant gas in the lower atmosphere because it is an irritant, is toxic, and causes health problems. It is also involved in formation of photochemical smogs.
Figure 1: Peroxyacetyl nitrate
Ozone is formed by the photochemical reaction of NOx with oxygen.
Step 1 NO2 –> NO + O
Step2 O + O2 –> O3 
There are three main types of car engine: the conventional engine, the ‘lean-burn’ and the diesel engine. One difference between them is the ratio of air : fuel that they use.
Conventional engine produces a lot of nitrogen oxide, due to the mixture having a comparatively high level of fuel, resulting oxygen being used up during the combustion of fuel and generating high internal temperatures.
‘Lean burn’ engines use more air which leaves extra oxygen for the combustion process. Less NOx is emitted; however when the concentration of fuel becomes too low for combustion, the left-over fuel goes down the exhaust pipe, resulting in higher hydrocarbon emissions.
The diesel engine is also a ‘lean-burn’ engine and gives the cleanest gaseous emissions; however it gives high emissions of smoke and particulates. 
Acid rain occurs when sulfur dioxide and nitrogen oxides are emitted into the atmosphere, undergo chemical transformations and are absorbed by water droplets in clouds. The droplets then fall to earth as rain, snow, mist, dry dust, hail, or sleet. This increases the acidity of the soil, and affect the chemical balance of lakes and streams. 
NO emissions in the lower atmosphere are eventually oxidised to NO2:
2NO + O2 –> 2NO2
NO + O3 –> NO2 + O2
The NO2 can then react with OH radicals from water to form nitric acid (HNO3) which falls to Earth as acid rain:
NO2 + OH –> HNO3 
The term heterogeneous catalysis describes the catalysis where catalyst is in a different state (i.e. solid, liquid, and gas) to the reactants.
Heterogeneous catalysts provide a surface for the chemical reaction to occur, which means one or more of the reactants diffuses to the catalyst surface and absorbs onto it. After the reaction, the products must be desorbed from the surface and diffuse away from the solid surface.
Titanium dioxide (Figure 2) is a photocatalyst. Using sunlight it absorbs and renders oxides of nitrogen (NO and NO2, or NOx) given out by moving vehicles – a principal cause of air pollution. This reaction is assumed to be working in dry and wet conditions and will be used to try and reduce pollution levels. 
Figure 2: 3D model of Titanium Dioxide
When titanium dioxide is exposed to ultra-violet radiation in sunlight, it absorbs the radiation and electrons are excited to a higher energy level. The following reactions then occur on the surface of the titanium dioxide crystals (Figure 4):
O2 + e- –> O2-
H2O –> H+ + OH + e-
The overall reaction:
H2O + O2 –> H+ + O2- + OH
The hydroxyl radical is a powerful oxidising agent and can oxidise nitrogen dioxide to nitrate ions:
NO2 + OH –> H+ + NO3-
The superoxide ion can form nitrate ions from nitrogen monoxide:
NO + O2- –> NO3-
The nitrate is then washed away by rain or it soaks into the block to form stable compounds within the concrete. 
Figure 4: A noxer block
Three-way catalytic converters
Three-way catalytic converters oxidise CO, CxHy and reduce NO emissions. They only work if the air:petrol mixture is controlled so that it’s exactly the stoichiometric mixture for the fuel. If the mixture is too rich, there will not be enough oxygen in the exhaust fumes to remove CO and CxHy. This means that cars that have three-way catalytic converters need to have oxygen sensors in the exhaust gases, linked back to electronically controlled fuel injection systems. (Figure 5)
Figure 5: A three-way catalytic converter
Problems yet to be solved
Ground level ozone is a product of NOx gases, VOC’s and oxygen in the presence of visible/ultraviolet radiation from the sun. It is destroyed by the reaction of combustion products of internal combustion engines. It is not wholly a road transport problem. Improvements could be made by bringing out ‘oxygenated fuels’ that contain methyl tertiary butyl ether which is believed will lead to cleaner emissions.