Haber process

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The Haber process is a process used to produce the useful substance ammonia from nitrogen and hydrogen.

Sources of gases

Hydrogen

Hydrogen is commonly produced on an industrial large scale by the catalytic reforming of methane (natural gas). Although hydrogen can also be produced by catalytically reforming methanol or by the electrolysis of water, neither of those processes are currently practiced on a large scale.

Reforming methane
For more information, see: Ammonia production.


Methane is reacted with water to form carbon dioxide and hydrogen: [1]

CH4 (g) + H2O (l) → CO + H2 (g) + H2 (g)

Just as with reforming methanol, the carbon monoxide produced reacts with water to form carbon monoxide and more hydrogen: [1]

CO (g) + H2O → CO2 (g) + H2O (l)

So, overall:

CH4 (g) + 2H2O (l) → CO2 (g) + 4H2O (l)
Reforming methanol

The reforming of methanol involves mixing liquid methanol with water, and then using a catalyst to help break down the methanol molecules into carbon monoxide and hydrogen. The water than reacts with the carbon monoxide to produce carbon dioxide and more hydrogen: [2]

CH3OH (l) → CO (g) + 2H2 (g)
CO (g) + H2O (g) → CO2 (g) + H2 (g)

So, overall:

CH3OH (l) + H2O (l) → CO2 (g) + 3H2

Electrolysis of water

Pure water is a poor conductor of electricity, so often a soluble ionic compound is added, such as an acid, base or salt to provide hydrogen ions. Sulphuric acid (H2SO4) is often used because it is fully dissociated when dissolved in water, and is difficult to oxidise, so oxygen gas will form at the anode.

In a sample of pure water, some water molecules form ions, and are thus aqueous in water. Due to the hydrogen bonding in water, it splits into hydronium (H3O+) and a hydroxide ion. The actual picture is more complex since any charged water molecule is surrounded by a cage of at least 9 and on average 16 water molecules that effectively share the proton charge over all the water molecules in the complex. The same complex exists for the hydroxyl-ion. The effective forming of cages to dissolve solvents in their "caves" is both strong and effective in water due to its highly structured nature. For symplicity, in this explanation, water is assumed to split into a hydrogen and a hydroxide ion for simplicity. Due to the lack of ions, water is a very poor conductor of electricity.

H2O (l) → H+ + OH- [3]

Sulphuric acid, on the other hand, is fully ionised when dissolved in water:

H2SO4 (aq) → 2H+ + SO42- [4]

Once electrolysis has begun, the hydrogen ions move towards the cathode where they are reduced to form hydrogen gas:

2H+ + 2e- → H2 (g) [5]

At the anode, each water splits into an oxygen ion and 2 hydrogen ions. Every pair of oxygen ions forms a covalent bond, forming a molecule of oxygen gas, which bubbles off. [5]

H2O → O2- + 2H+ + 2e-
2O2- → O2 (g)

For every two electrons passed, 2 hydrogen ions form a molecule of hydrogen gas at the cathode, but another 2 hydrogen ions are formed at the anode. The sulphate ions stay in solution throughout the reaction, meaning that overall, the amount of sulphuric acid remains constant, and it is the water that is electrolysed: [5]

4H+ + 2H2O (l) → 2H2 (g) + O2 (g) + 4H+

Or, more simply:

2H2O (l) → 2H2 (g) + O2 (g)

Nitrogen

Nitrogen is by far the most abundant gas in the Earth's atmosphere, making up 78.084% of the air we breathe[6]. It is from the air that nitrogen is normally collected.

Reaction

The reaction between nitrogen and hydgrogen gases is reversible [7], meaning that some ammonium will be formed, but not all with react. The yield of ammonia depends upon the conditions: temperature, pressure and the presence of a catalyst. [7]

N2 (g) + 3H2 (g) ↔ 2NH3 (g)

Each of the reactants and the products is gaseous at the conditions used in ammonia production plants. One mole of any gas uses the same volume (24L at room termperature and pressure), so the total volume of gas decreases as the reaction goes to the right.

Le Chatelier's principle explains the effects of changing the temperature and pressure on a reversible reaction, as well as showing the effects of a catalyst.

Temperature

Increasing the temperature breaks bonds apart, so increasing the temperature will force the equilibrium to the side with more molecules, thus decreasing the yield of ammonia. Furthermore, the higher the temperature, the higher the cost and also the higher the danger, so factory owners may not wish to make the temperature too high for economic and safety considerations. However, increasing the temperature will mean that the particles have more energy, so the rate of reaction will increase, so the ammonia will be made more quickly. In industry, the Haber process is usually carried out at a "compromise temperature" of between 400°C and 450°C. [8]

Pressure

Increasing the pressure will cause the particles to be compressed together more. This means that the equilibrium will be forced to the side with fewer molecules, so the yield will increase. Higher pressures will also increase the rate of reaction, so the ammonia will be produced quicker. However, creating and maintaining a high pressure is very expensive, thus factory owners must find a "compromise pressure". In industry, this is about 200 atmospheres. [8]

Catalyst

A catalyst is a substance that lowers the activation energy required for a reaction to take place. In a reversible reaction, a catalyst will have no effect on the direction of the reaction, but instead makes it reach equilibrium more quickly. Catalysts are used as they do not get used up, thus they only need to be bought once, and allow more ammonia to be produced in a fixed period of time, increasing the efficiency of the factory.

Industry

The Haber process is still used today to make ammonia. Ammonia is one of the most abundantly-produced inorganic chemicals. There are literally dozens of large-scale ammonia production plants throughout the industrial world, some of which produce as much as 2,000 to 3,000 tons per day of ammonia in liquid form. The worldwide production in 2006 was 122,000,000 metric tons.[9] China produced 32.0% of the worldwide production followed by India with 8.9%, Russia with 8.2%, and the United States with 6.5%. Without such massive production, our agriculturally-dependent civilization would face serious challenges.

Ammonia prices are expected to increase due to the rising prices of methane, which is used to produce the hydrogen required in the process.[10]

Uses of ammonia

  • Adding to soil to stimulate plant growth due to presence of nitrogen [11]
  • Producing fertilizers [11]
  • Manufacture of nitric acid [11]
  • pH control [11]
  • Cleaners and detergents [11]


References

  1. ↑ 1.0 1.1 http://www.getenergysmart.org/Files/HydrogenEducation/6HydrogenProductionSteamMethaneReforming.pdf
  2. ↑ http://www.howstuffworks.com/fuel-processor2.htm
  3. ↑ http://www.atmosphere.mpg.de/enid/3v8.html
  4. ↑ http://encarta.msn.com/encyclopedia_761566936/Sulfuric_Acid.html
  5. ↑ 5.0 5.1 5.2 http://www.ucc.ie/academic/chem/dolchem/html/comp/h2so4.html
  6. ↑ http://www.physlink.com/reference/AirComposition.cfm
  7. ↑ 7.0 7.1 http://scifun.chem.wisc.edu/chemweek/Ammonia/AMMONIA.html
  8. ↑ 8.0 8.1 http://www.chemguide.co.uk/physical/equilibria/haber.html
  9. ↑ United States Geological Survey publication
  10. ↑ http://www.farmgate.uiuc.edu/archive/2006/11/are_you_booking.html
  11. ↑ 11.0 11.1 11.2 11.3 11.4 http://www.rmtech.net/uses_of_ammonia.htm
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