Ammonia

What is ammonia?  

Ammonia is an incredibly important chemical used in:   

 

Explanation

• agricultural processes, especially in nitrogen-based fertilisers  
• household cleaning agents  
• explosives, for demolishing buildings and structures  

For large-scale industrial production, the Haber Process is used. 

In this post, we will be focusing on reversible reactions and the Haber Process. 

1. Reversible reactions

Many reactions that you have encountered proceed in a single direction towards completion. These are known as irreversible reactions and a forward-facing arrow is used in the chemical equation.

 

e.g. Titration:

KOH (aq) + HCl (aq) ®  KCl (aq) + H2O (l) 

• However, many reactions in the real world proceed in both directions. These are known as reversible reactions and a double arrow (⇌) is used instead.  

e.g. Thermal Decomposition of ammonia chloride:

NH4Cl (s) ⇌ NH3 (g) + HCl (g) 

These reactions do not proceed towards completion and instead reach an equilibrium.  

At equilibrium, the reaction does not stop, but instead the forward and backward reactions occur at the same rate. The ratio of products and reactants in the equilibrium mixture does not change. This equilibrium mixture depends on the reaction conditions (temperature, pressure, etc).  

The forward reaction is the reaction going from left to right. For example, when referring to the decomposition reaction above, the forward reaction is:  

NH4Cl (s) ® NH3 (g) + HCl (g) 

The backward reaction would be:  

NH3 (g) + HCl (g) ® NH4Cl (s) 

Another example of a reversible reaction is the Haber Process.  

 

2. The Harber Process

The Haber Process is the production of ammonia from nitrogen gas and hydrogen gas. 

Reagents required: 

• N2 (g), produced from the fractional distillation of liquified air,  

• H2 (g), produced from the cracking of crude oil. 

•  

Equation:   N2 (g) + 3H2 (g) ⇌ 2NH3 (g)  ∆H = -ve    Both nitrogen and hydrogen gas are relatively unreactive at room temperature, so a high temperature and pressure with finely divided iron catalyst is required. To ensure the highest yield with the lowest production cost, the temperature and pressure for the reaction are carefully controlled.    Diagram:   Process:

1. Nitrogen and hydrogen gas are pumped in through the pipes in a 1:3 volume ratio. 
2. The gases are compressed to around 200 atm in the compressor. 
3. The pressurised gases are pumped into the convertor and passed over an iron catalyst at 450 oC. 
4. As the reaction is reversible, a mixture of hydrogen, nitrogen and ammonia is formed. 
5. The mixture of gases then enters the condenser, where ammonia is liquified, as ammonia has a lower boiling point than hydrogen and nitrogen gas. 
6. Unreacted hydrogen and nitrogen are pumped back (recycled) into the convertor, which forms more ammonia.

Conditions:

  1) Temperature   The higher the temperature, the lower the yield of ammonia. Hence, to get a high yield, lower temperature should be used. However, with too low of a temperature, the reaction would proceed very slowly, and the rate of production of ammonia will be slow. Hence, an optimal temperature of 450°C is chosen.   2) Pressure   The higher the pressure, the higher the yield of ammonia. Hence, to get a high yield of ammonia, higher pressure should be used. However, with too high of a pressure, the production plant may be too dangerous, and the equipment required to maintain the high pressure is more expensive. Hence, an optimal pressure of 200 atm is chosen.   3) Catalyst   Even with the high temperature and pressure, the rate of reaction is slow. The catalyst provides an alternative reaction pathway with lower activation energy, resulting in a faster rate of formation of ammonia.   % Yield Graph:   The following % Yield Graph shows the change in the yield of ammonia depending on the pressure and temperature.  

 

Question 1 

A student says that since low temperatures produce a greater yield of ammonia, the reaction should be carried out at 50C instead of 450C.

Which statement best explains why the reaction should not be carried out at 50C?[2024 CGS Prelim P1]

Α.  Ammonia is unstable at 50C.

B. The reactants are unstable at 50C.

C. The reaction is too slow at 50C.

D. The reaction mixture is easily separated at higher temperatures.

Answer:

When the temperature is decreased, the reaction is too slow, and the production rate of ammonia decreases, causing the yield to decrease.

Answer: C

 

Question 2

Which of the following can increase the yield of ammonia in the Haber Process? [2024 AMK Prelim P1]

• Increase the pressure to 450 atm.
• Increase the temperature to 1000C.
• Use an equal volume of hydrogen and nitrogen.
• Use platinum as a catalyst.

Answers:

Option A: 

Increasing the pressure increases the yield of ammonia, and the equilibrium mixture contains more ammonia.

Option B:

Increasing the temperature to 1000C decreases the yield of ammonia, and the equilibrium mixture contains less ammonia.

Option C:

N2(g) + 3H2(g) 2NH3(g)

 

Nitrogen gas and hydrogen gas react in a 1:3 ratio. Reacting nitrogen and hydrogen in a 1:1 ratio means more nitrogen will remain unreacted, as hydrogen is the limiting reagent.

Option D:

Finely divided iron catalyst provides the best conditions to improve the rate of production of ammonia.

Answer: A

Question 3 

In the Haber Process,

• Ammonia is liquified after being formed
• Hydrogen is obtained from fractional distillation of air
• Some of the ammonia formed decomposes to form nitrogen and hydrogen
• The reaction involves the use of a compound of transition metal

Which of the statements are true?            [2018 Fuhua Prelim P1]

• 1 and 3
• 2 and 3
• 3 and 4
• 1,2 and 4

 

Answer:

Option 1:

Correct. Ammonia has a lower boiling point than hydrogen and nitrogen gas, causing it to become liquid at a lower temperature. It is liquified when it enters the condenser.

Option 2:

Incorrect. Hydrogen gas is obtained from the cracking of crude oil.

Option 3:

Correct. As the reaction is reversible, some of the ammonia formed decomposes to form nitrogen and hydrogen.

Option 4:

Incorrect. The catalyst used is finely divided iron metal, which is a transition element, not a compound.

Answer: A