3.2.3 Equilibria - Importance of equilibria in industrial processes
Students should:
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The Haber process
N2(g) + 3H2(g) 
2NH3(g) ΔH = -92.4 kJ mol-1
There are more moles of gas on the left than the right, so a greater yield will be produced at high pressure. (The equilibrium position will lie further to the right)
The reaction is exothermic, therefore it will give a greater yield at low temperatures. (The equilibrium position lies further to the right)
In practice, if low temperatures are used the time taken for the reaction to attain equilibrium becomes unfeasably long. An intermediate temperature is chosen (450ºC) which allows the reaction to get to an equilibrium in a reasonable time and still has enough of the products in the equilibrium mixture.
A catalyst of finely divided iron is also used to help speed the reaction (finely divided to maximise the surface area).
To make the process more efficient the ammonia produced at equilibrium is removed by first cooling the mixture when the ammonia turns into a liquid which can be tapped off. The unreacted gases in the process are then mixed with fresh reactants and returned to the reaction chamber to reestablish the equilibrium again and the cycle is repeated continuously.
Manufacture of ethanol
The reaction between ethene and steam in the presence of a phosphoric acid catalyst at 300ºC and 60 atmospheres pressure is used in industry to manufacture ethanol.
C2H4(g) + H2O(g)
C2H5OH(g)
This is a reversible reaction - alcohols can be dehydrated to alkenes - so to encourage the forward reaction high pressure is used.
You can see that in the equation there are two moles of gas on the left hand side and only one mole of gas on the right hand side. According to Le Chatelier the increased pressure pushes the reaction towards the side of fewer moles of gas, in this case the products.
Manufacture of methanol
Synthesis gas, a mixture of carbon monoxide, carbon dioxide and hydrogen, is first produced in a reformer. This is carried out by passing a mixture of a hydrocarbon feedstock and steam through a heated tubular reformer. The ratio of hydrogen and carbon oxides in the syngas may need to be adjusted by purging excess hydrogen or adding carbon dioxide.
H2O(g) + C(g)
CO(g) + H2(g)
The syngas is cooled and then compressed before being fed to the methanol converter. The methanol synthesis takes place in the presence of copper-based catalysts at 250-260ºC. The crude methanol is recovered and purified by distillation.
2H2(g) + CO(g)
CH3OH(g)
The reaction is reversible. The reaction is exothermic, so the temperature is kept to a minimum, high enough to ensure rapid reaction but low enough to ensure a high percentage of product at equilibrium. High pressure is also used to move the equilibrium to the side of fewer gaseous moles.
Alcohols as fuels
Both methanol and ethanol are important fuels, as they burn with a very clean flame and produce a large amount of energy.
CH3OH(g) + 1½O2(g)
CO2(g) + 2H2O(g)
Cars in many countries, for example, are adapted to run their engines on a mixture of gasoline and ethanol (gasohol).
Ethanol fuel mixtures have "E" numbers which describe the percentage of ethanol fuel in the mixture by volume, for example, E85 is 85% anhydrous ethanol and 15% gasoline. Low ethanol blends, from E5 to E25, are also known as gasohol, though internationally the most common use of the term gasohol refers to the E10 blend.
Blends of E10 or less are used in more than twenty countries around the world by 2011, led by the United States, where almost all retail gasoline sold in 2010 was blended with 10% of ethanol. Blends from E20 to E25 have been used in Brazil since the late 1970s. E85 is commonly used in the U.S. and Europe for flexible-fuel vehicles. Hydrous ethanol or E100 is used in Brazilian 'neat ethanol' vehicles and flex-fuel light vehicles and in hydrous E15 called hE15 for modern petrol cars in the Netherlands.