Manufacture of Ammonia

Ammonia is manufactured by Haber Process. Nitrogen and hydrogen are reacted together to produce ammonia in reversible reaction:

N_2(g)  + H_{2(g) -->} 2 NH_3(g)                ΔH = negative.

The nitrogen is obtained from air and the hydrogen is obtained from methane.

Conditions for the reaction:

• Iron catalyst (to make reaction very fast)
• High pressure of 200 atm
• Moderately high temperature of 450⁰ C

But only 15% of the N₂ and H₂ react when passed over the iron catalyst, so the ammonia is removed and unreacted N₂ and H₂ are passed over the catalyst again. Most ammonia is used to manufacture nitrogen fertilisers such as : ammonium nitrate (NH₄NO₃) and ammonium sulphate ((NH₄)₂SO₄).

We look the questions from O Level past examination about manufacture of ammonia.

Problem 1 (N2000 P2)

Ammonia is manufactured in the Haber Process. The following graph shows the amount of ammonia present in the equilibrium mixture under different conditions of temperature and pressure.

a. Use the graph to deduce the effect of increasing the pressure on the
percentage of ammonia in the equilibrium mixture.

b. An industrial process uses conditions of 450⁰ and 200 atm.

     (i)   Use the graph to find the percentage of ammonia in the
equilibrium mixture.

    (ii)  Suggest one advantage and one disadvantage of using 450⁰C rather than
350⁰ C as the working temperature

c. A catalyst is used in the Haber process. Give one other industrial process
which uses a catalyst, and name the catalyst. Explain why expensive metals
are economical to use a catalyst.

d. A student has three tubes of gas. One tube contains ammonia, one contains
hydrogen and one contain nitrogen. Describe test the students could do to
identify the three gases.

 Solutions

a. As pressure increases, percentage of ammonia also increases
b. (i) From the graph, the percentage of ammonia in the equilibrium mixture
about 27% - 28%

      (ii) The advantage of using 450⁰C rather than 350⁰C is the reaction
achieved  in a shorter time but the disadvantage the percentage of yield
of ammonia is smaller.

c. Manufacture of sulphuric acid using vanadium(V) oxide as catalyst.
Expensive metals are economical to use a catalyst because a small amount of
the metal is needed and if not poisoned, the metal can be used indefinitely.

d. Ammonia gas will turn red litmus paper blue

     Hydrogen gas will extinguish a lighted splinter with a ‘pop’ sound
Nitrogen gas will extinguish a lighted splinter.

Problem 2 (N2002 P2)

Ammonia is used to manufacture nitric acid, by a two stage process.

Stage 1: the ammonia is converted to nitrogen(II)oxide.

               4NH_3(g) + 5O_2(g) --> 4NO_(g) + 6H₂O_(g)  ΔH = - 950kJ/mol

a. State and explain how the rate changes when the pressure is increased. Use
ideas about colliding particles.
b. During the reaction, the ammonia and oxygen are passed through a
powdered catalyst

     (i) Explain why the catalyst becomes hot during the reaction

    (ii) Explain why the catalyst is used in the form of a powder

c. It is possible to find out wheter this reaction has finished by following pH
changes during the reaction. Samples of gas are taken from the reaction
vessel at intervals and bubbled through water to form a solution. The pH of
each solution is measured. Explain why the measured pH changes during the
reaction.

Stage 2: the nitrogen dioxide is converted to nitric acid

               4NO_(g) + 2H₂O_(g) + 3O_2(g) --> 4HNO_3(aq)

d. Calculate the maximum mass of nitric acid which can be made from 720
dm³ of nitrogen(II) oxide, NO, at room temperature and pressure.
e. Use the two equations to construct an overall equation for the conversion of
ammonia to nitric acid.

Solutions

a. When the pressure is increased, the gaseous molecules are packed closer
together. It makes the reacting molecules will collide more frequently,
increasing the rate of reaction.

b. (i) The catalyst becomes hot because the reaction is exothermic, exothermic
reaction giving out heat.

      (ii) The catalyst is used in the form of powder because the surface area is
bigger when the catalyst in form of powder. Hence the rate of
reaction is increased

c. Ammonia dissolve in water to form an alkali solution. As the reaction
progresses, the pH of the mixture in solution will decrease until pH 7 when
all the ammonia is used up.
d. From the equation given 1 mole of NO forms 1 mole of HNO₃

mole of NO = 720/24 = 30 moles
mole of HNO₃ = 30 moles
mass of HNO₃ = 30 x 63 = 1890 grams

e. 4NH_3(g) + 5 O_2(g)                    --> 4NO_(g) + 6H₂O_(g)
4NO_(g) + 2H₂O_(g) + 3O_2(g) --> 4HNO_3(aq)
____________________________________ +
4NH_3(g) + 8 O_2(g) -->  4HNO_3(aq) + 4H₂O_(g)
_________________________________ divide by 4
NH_3(g) + O_2(g) -->   HNO_3(aq) + H₂O_(g)