The History of the Haber Process
Today's industries use the Haber process to synthesize ammonia, but it wasn't invented until the early 20th century.
Back in 1898, Sir William Cook, a British chemist, predicted that part of humanity could be faced with the prospect of starving to death because of lack of nitrogen fertilizer for plant growth. At that time, almost all nitrogen in the soil, as nitrate, was fixed over hundreds of years by microorganisms, and those rich organic soils were rapidly depleting. Already, industrialized countries, like Germany, were relying on animal manure, like guano from South America, to make nitrogen-based fertilizers.
Fortunately, in 1908, a German chemist named Fritz Haber developed a chemical method for producing large amounts of ammonia, using a process that is now known as the Haber process. The ammonia thus produced could be easily converted to nitrogen-based fertilizers. Ammonia is also used to make explosives and cleaning products.
The Haber Process
The Haber process is an industrial process that uses nitrogen gas and hydrogen gas to synthesize ammonia. The equation that represents the Haber process is given by:
So, how does the process obtain it's hydrogen?
1. Electrolysis of water
You will recall that the passage of an electric current through water using inert electrodes produces hydrogen at the cathode and oxygen at the anode.
2H2 O + Electrical energy --> 2H2 + O2.
It is good to note that this method of obtaining hydrogen for the Haber process was used for the production of ammonia in the 20th century.
2. Reaction of methane and steam
With time, chemists found a comparatively cheaper and more efficient way of producing hydrogen. The method involves using a methane source, such as natural gas, which is reacted with high-temperature steam at 700°C-1,000°C and under 3-25 bar pressure. Alongside hydrogen, carbon monoxide and a relatively small amount of carbon dioxide is formed.
CH4 + H2 O + heat --> CO + 3H2 (+ small amount of CO2)
Subsequently, the carbon monoxide and steam are reacted using nickel as a catalyst to produce carbon dioxide and more hydrogen.
CO + H2 O --> CO2 + H2
Finally, carbon dioxide and other impurities are removed from the gas stream by using a method known as the pressure-swing adsorption. This leaves essentially pure hydrogen behind.
Okay, now how does the process obtain nitrogen?
1. Fractional distillation of liquid air
Nitrogen can be obtained through the fractional distillation of the air around us. Air is compressed down to 200o C. Trace gases, carbon dioxide and water are removed by filtering. Oxygen and nitrogen liquefy at -183o C and -196o C respectively. This liquefied mixture, comprised of oxygen and nitrogen, is then separated by fractional distillation. Since liquid nitrogen boils at a lower temperature than oxygen, it will rise up the fractionating column while liquid oxygen remains at the bottom. However, this method of obtaining nitrogen is too expensive.
2. Reaction of methane and air
Recall that air is made up of around 78% nitrogen, with nearly all the rest being oxygen. If we burn methane in air, the oxygen reacts with the methane to produce carbon dioxide and water. You can understand that the nitrogen present in the air remains unreacted. If we remove the carbon dioxide and water, we obtain nitrogen.
Now that nitrogen and hydrogen have been obtained, they can be reacted to produce ammonia. These gases are then compressed and delivered to the reactor, where one nitrogen gas molecule reacts with three hydrogen gas molecules over finely divided iron as a catalyst to produce two ammonia molecules. A catalyst is a substance that speeds up a chemical reaction, without being consumed by the reaction.
The Haber process is a reversible exothermic equilibrium reaction. Any exothermic reaction releases energy to its surroundings. The production of each molecule of ammonia during the Haber process releases 46KJ of heat energy.
The Haber process uses Le Chatelier's principle to maximize ammonia while keeping operating and production costs in mind. Le Chatelier's principle states that if a disturbance is made to a reaction at equilibrium, the position of the equilibrium will shift in such a way so as to counteract the disturbance.
Le Chatelier's principle tells us that increasing pressure will favor the side with fewer moles. It would therefore make sense to conduct the reaction at a very high pressure, but we must also remember that it is expensive to build and operate a plant that can withstand such high pressures. Therefore, a compromise pressure of around 150-200 atmospheres is used.
Now let us consider the best temperature required for the process to maximize the yield of ammonia. Since the forward reaction is exothermic (remember that 46KJ of heat is released for each mole of ammonia produced), it would be sensible to conduct the reaction at a low temperature. However, a compromise temperature of around 450-500o C ensures that the reaction proceeds with sufficient yield. This temperature is neither too low nor too high. If a lower temperature is used, the rate of reaction would be slow and would take a long time for the reaction to reach equilibrium.
The yield of ammonia by the Haber process under the stated conditions of pressure and temperature is around 15-20%. Any unused nitrogen and hydrogen are recycled backed into the reactor.
Ammonia is industrially prepared by the Haber process, a chemical method that uses nitrogen gas and hydrogen gas to synthesize ammonia. One nitrogen gas molecule reacts with three hydrogen gas molecules over finely divided iron as a catalyst to produce two ammonia molecules.
The nitrogen gas can be obtained from air by fractional distillation of liquid air. Alternatively, if oxygen found in air is reacted with natural gas, the nitrogen that remains unreacted can be separated. On the other hand, hydrogen can either be obtained by electrolysis of water or by reacting natural gas with steam.
Both hydrogen and nitrogen react at 450-500o C and at a pressure of 150-200 atmospheres to yield around 15-20% ammonia.
To unlock this lesson you must be a Study.com Member.
Create your account
Modified True or False Exercise on the Synthesis of Ammonia
Check your knowledge in the lesson about the process and reaction in the synthesis of ammonia by determining whether the following statements are true or false. For this activity, print or copy this page on a piece of paper. Write TRUE if the statement is valid and FALSE if otherwise on the blank space provided. If the statement is FALSE, write down the word or phrase that makes it wrong.
_____ 1. A comparatively cheaper and more efficient way of producing hydrogen is by using natural gas.
_____ 2. In a slow temperature, the rate of synthesizing ammonia would be slow and reaching equilibrium requires a much longer time.
_____ 3. In the synthesis of ammonia, oxygen as well as hydrogen is used.
_____ 4. Before the Haber process was developed, industrialized countries like Germany relied on animal manure as fertilizers.
_____ 5. In the Haber process, any unused nitrogen and hydrogen are removed from the reactor.
_____ 6. Pressure-swing adsorption is a method used to remove hydrogen and other impurities from a gas stream.
_____ 7. A catalyst, like nickel, increases the rate of a physical reaction without being consumed by the process.
_____ 8. Le Chatelier's principle states that increasing temperature will favor the side with fewer moles.
_____ 9. The reaction of methane and steam produces carbon dioxide and water.
_____ 10. Ammonia produced through the Haber process could be easily converted to nitrogen-based fertilizers.
For False answers, the correct word or phrase is written in italics next to the incorrect word for comparison.
3. FALSE, oxygen, nitrogen
5. FALSE, removed from, recycled into
6. FALSE, hydrogen, carbon dioxide
7. FALSE, physical, chemical
8. FALSE, temperature, pressure
Register to view this lesson
Unlock Your Education
See for yourself why 30 million people use Study.com
Become a Study.com member and start learning now.Become a Member
Already a member? Log InBack