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.

  Beirut: Explosion explained

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.

ii) Explain why a high pressure is used.

iii) Suggest why the iron is finely divided.

iv) Explain what effect increasing the temperature will have on the value of the equilibrium constant.

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.

ii) Explain why nitrogen monoxide is an example of 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.

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.

c)

i) State the equation for the formation of ammonium nitrate from ammonia and nitric acid.

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.

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?

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.

ii) State and explain whether a solution of ammonium nitrate is acidic, basic or neutral.

iii) State the conjugate acid and the conjugate base of ammonia.

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.

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.

iii) Explain why ammonium nitrate is potentially explosive.

f)

i) Suggest why no major accidental explosions involving ammonium nitrate occurred before the First World War.

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.

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