IB Docs (2) Team Ammonium nitrate & IB Chemistry
Ammonium nitrate has been very much in the news recently after 2750 tonnes of the salt which were being stored in the port area of Beirut caused a massive explosion on 4 August 2020 killing and injuring many people. After a brief introduction this page uses the chemistry of ammonium nitrate holistically to test students understanding of content found in eight different core/AHL topics on the IB Diploma chemistry syllabus. Most of the questions can be answered by both SL and HL students although a few are only for HL.
Introduction
The main use of ammonium nitrate is as a fertilizer. It is relatively easy to mIB Docs (2) Teamfacture from simple raw materials and has a high nitrogen content. It is also used as one of the components in explosive mixtures (e.g. ANFO made from 94% ammonium nitrate + 6% fuel oil) used in quarrying and mining. Because it is readily available it has been used in IEDs (improvised explosive devices), e.g. the Oklahoma City Bombing on 19 April 1995. There have been several major accidental explosions resulting from fires near stored ammonium nitrate. Several of these occurred during the First World War including the Great Explosion at Faversham in Kent, UK in April 1916. The latest, and the one involving by far the greatest amount of ammonium nitrate, took place on 4 August 2020 in Beirut, Lebanon. The explosion involved 2750 tonnes of ammonium nitrate which was being stored in a warehouse in the port area. It caused many deaths and injuries and a huge amount of damage making an estimated quarter of a million people homeless.
Since many, if not all, of your students will have heard of ammonium nitrate following the explosion in Beirut it provides a newsworthy subject upon which to base questions covering many of the core/AHL topics covered by the IB chemistry syllabus. The questions cover stoichiometric relationships, chemical bonding and structure, energetics, kinetics, equilibrium, acids and bases, oxidation and reduction and the Nature of Science. They are not designed to be exactly similar to IB examination questions but are given to demonstrate how the syllabus can be applied holistically to a topic of current interest to stimulate your students and to increase their understanding of IB chemistry.
Although several of the questions cover the AHL programme, standard level students should be able to answer all of questions a, b, c and f and part iii) of question d but not question e as this requires a knowledge of entropy and Gibbs energy. Disproportionation and the Ostwald process are not specifically on the syllabus but from the information given both SL and HL students should be able to deduce the answers.
Questions on ammonium nitrate
Ammonium nitrate, NH4NO3 is a white crystalline solid which melts at 169.6 oC.
Ammonium nitrate is normally mIB Docs (2) Teamfactured from ammonia and nitric acid. Ammonia can be obtained by the reaction between nitrogen and hydrogen in the Haber process and nitric acid is obtained from the oxidation of ammonia in the Ostwald process.
a) Haber process:
N2(g) + 3H2(g) ⇌ 2NH3(g) ΔH⦵ = – 92 kJ
The reaction is carried out at a pressure of about 200 atmospheres, a temperature of about 450 oC and uses finely divided iron as a catalyst.
i) Write the equilibrium expression for the reaction.
Kc = ![fraction numerator text [NH end text subscript text 3 end text end subscript text (g)] end text to the power of text 2 end text end exponent over denominator text [H end text subscript text 2 end text end subscript text (g)] end text to the power of text 3 end text end exponent text x [N end text subscript text 2 end text end subscript text (g)] end text end fraction](../../../ckeditor/plugins/wiris/integration/showimage-18.php?formula=81817e64b20c44ea650c28b5c971b2c0.png)
ii) Explain why a high pressure is used.
There are four moles of reactants and two moles of products. Avogadro's law equates moles of gas with volume. Increasing the pressure will reduce the volume so making the position of equilibrium move to the side with less volume, i.e. increase the yield of ammonia.
iii) Suggest why the iron is finely divided.
It makes the catalyst more efficient by increasing the surface area.
iv) Explain what effect increasing the temperature will have on the value of the equilibrium constant.
As the reaction is exothermic heat is evolved (i.e. heat is a product), so increasing the temperature (adding heat) will lower the value of Kc to move the position of equilibrium to the reactant side..
b) Ostwald Process:
Ammonia is first oxidized to nitrogen monoxide and steam by heating with oxygen in the presence of a platinum catalyst.
(1) 4NH3(g) + 5O2(g) ⇌ 4NO(g) + 6H2O(g)
The nitrogen monoxide produced is then further oxidized to nitrogen dioxide which is then reacted with water to produce nitric acid.
(2) 2NO(g) + O2(g) → 2NO2(g)
(3) 3NO2(g) + H2O(l) → 2HNO3(aq) + NO(g)
The nitric acid is concentrated to the desired strength by distillation and the nitrogen monoxide is recycled.
i) Disproportionation occurs when the same substance is oxidized and reduced simultaneously.
Use oxidation states to identify which, if any, of the three reactions in the Ostwald process is a disproportionation reaction.
3NO2(g) + H2O(l) → 2HNO3(aq) + NO(g) shows disproportionation as the oxidation state of N in NO2 is +4 and it has been oxidized to form HNO3 (oxidation state of N = +5) and reduced to form NO (oxidation state of N = +2). In the other two reactions both NH3 and NO are oxidized but not also reduced.
ii) Explain why nitrogen monoxide is an example of a free radical.
The electron configuration of N is 1s22s22p3 and the electron configuration of O is 1s22s22p4. This means that NO contains an odd number of electrons, one of which must be unpaired, so it is a free radical.
iii) The enthalpy change for the oxidation of ammonia shown in the first equation is – 905 kJ.
Use the bond enthalpies given in Section 11 of the IB data booklet to calculate the average bond enthalpy for the nitrogen to oxygen bond in nitrogen monoxide.
Energy in: 12 x N−H = 12 x 391 = 4692 kJ
+ 5 x O=O = 5 x 498 = 2490 kJ
Total energy required to break bonds = + 7182 kJ
Energy out: 4 x NO
+ 12 x O−H = 12 x 463 = 5556 kJ
Total energy released when bonds are formed = (5556 + (4 x NO)) kJ
ΔH = – 905 kJ so more energy is released than taken in
(5556 + 4NO) − 7182 = 905
4 x NO = 2531 kJ so NO bond enthalpy = 2531/4 = 633 kJ
iv) Suggest why the bond enthalpy obtained for the NO bond enthalpy in iii) is not the same as the value of 587 kJ mol−1 given for the N=O bond in Section 11 of the data booklet.
N has five valence electrons and O has six valence electrons. This means that the bond between N and O is not a simple double bond as although the O atoms would then achieve its octet the N atom only has a share in 7 electrons. Alternatively if there is a triple bond N achieves its octet but O would have a share in 9 electrons. This means that the 'double bond' is actually stronger than a normal double bond, so the bond enthalpy is greater than the N=O bond.
c)
i) State the equation for the formation of ammonium nitrate from ammonia and nitric acid.
NH3 + HNO3 → NH4NO3
ii) Assuming all the reactions give the theoretical yield of 100% and all the nitrogen monoxide produced in reaction (3) is recycled, calculate the mass of ammonia that needs to be mIB Docs (2) Teamfactured to produce 2750 tonnes of ammonium nitrate using nitrogen, hydrogen, oxygen and water as the starting materials.
1 tonne = 106 g; Mr(NH4NO3) = (2 x 14.01) + (4 x 1.01) + (3 x 16.00) = 80.06
Amount of ammonium nitrate in 2750 tonnes = (2.750 x 109) ÷ 80.06 = 3.435 x 107 mol
1 mol of NH3 required to produce 1 mol of NH4+ and 1 mol of NH3 required to produce NO3− (assuming all the NO in the third step of the Ostwald process is recycled) so amount of NH3 required = 2 x 3.435 x 107 mol
Mr(NH3) = 14.01 + 3.03 = 17.04 so mass of ammonia required = 17.04 x 2 x 3.435 x 107 = 1.171 x 109 g = 1171 tonnes.
iii) Calculate the mass of ammonia required to produce 2750 tonnes of ammonium nitrate if all the reactions give the theoretical yield of 100% but none of the nitrogen monoxide produced in reaction (3) is recycled?
1 mol of NH3 required to produce 1 mol of NH4+
From equation (3): 1.5 mol of NO2 required to produce 1 mol of HNO3
From equation (2): 1.5 mol of NO required to produce 1.5 mol of NO2
From equation (1): 1.5 mol of NH3 required to produce 1.5 mol of NO
so 1.5 mol of NH3 required to produce 1 mol of NO3−
Total amount of NH3 required to produce 2750 tonnes (3.435 x 107 mol) of NH4NO3 = 2.5 x 3.435 x 107 mol
Mass of ammonia required = 2.5 x 17.04 x 3.435 x 107 = 1.463 x 109 g = 1463 tonnes.
d) Ammonium nitrate is very soluble in water. As it dissolves in water the temperature of the solution formed becomes considerably lower than the temperature of the original water.
i) Explain why dissolving ammonium nitrate in water is an endothermic process.
More energy is required to overcome the attraction between the NH4+ and NO3− ions in the crystal lattice than is given out when the ions become hydrated. The extra energy required to dissolve the salt is taken from the water so the temperature goes down.
ii) State and explain whether a solution of ammonium nitrate is acidic, basic or neutral.
It is acidic as the salt is made from a strong acid and a weak base. OH− ions will recombine with NH4+ ions to form undissociated NH3(aq) and H2O leaving excess H+(aq) ions in solution.
iii) State the conjugate acid and the conjugate base of ammonia.
Conjugate acid : NH4+
Conjugate base: NH2−
e) Solid ammonium nitrate decomposes when heated.
Below around 300 °C, mainly nitrogen(I) oxide, N2O and water are formed.
NH4NO3 → N2O + 2H2O
Above 300 oC, nitrogen, oxygen and steam are mainly produced. It is this reaction which probably predominated during the explosion in Beirut.
2NH4NO3(l) → 2N2(g) + O2(g) + 4H2O(g) ΔH⦵= – 413 kJ
i) Deduce whether the entropy change for this reaction above 300 oC will be positive or negative.
It will be positive as two moles of liquid (molten ammonium nitrate) become seven moles of gaseous products.
ii) Deduce whether this reaction above 300 oC will be spontaneous or non-spontaneous at all temperatures or whether its spontaneity will change at a particular temperature.
ΔG⦵ = ΔH⦵ –TΔS⦵. Since both ΔH⦵ and –TΔS⦵ have negative values, ΔG⦵ will be negative for all values of T and so the reaction will be spontaneous at all temperatures.
iii) Explain why ammonium nitrate is potentially explosive.
Once sufficient energy has been provided (by a fire or through friction) to provide the activation energy, the fast rate of the spontaneous reaction, the huge change in volume that takes place and the large amount of Gibbs energy released will all combine together to cause an explosion.
f)
i) Suggest why no major accidental explosions involving ammonium nitrate occurred before the First World War.
Fritz Haber only discovered the way to fix nitrogen from the air with hydrogen industrially in 1913 so the large scale mIB Docs (2) Teamfacture of ammonium nitrate was not possible before that date. (Some historians think that World War 1 would have not lasted so long if this discovery had not been made as the supply of nitrates from S. America required to make bombs and ammunition could have been blockaded).
ii) During the Beirut explosion a brown gas was produced which may have added to the number of people killed and injured as a result of the explosion. Suggest the identity of the gas.
Nitrogen dioxide, NO2, a brown poisonous gas (which smells like chlorine) formed by the combination of NO(g) and O2(g) at high temperatures.
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