Explosive Medicine

You’ve probably heard of Alfred Nobel, the inventor of dynamite and gelignite. The commercialisation of these explosives enabled him to accumulate great wealth during his lifetime which, by the terms of his will signed in 1895, was used to establish the Nobel Prize. In later life he was prescribed Trinitrin, a medication used to relieve a heart condition known as angina.

What do dynamite, gelignite and Trinitrin all have in common?

They all contain nitroglycerin!

This is not a story about the life of Alfred Nobel.

It is a story about the fascinating chemistry of nitroglycerin…

Read more in the June 2018 issue of AUS-e-NEWS, AUS-e-TUTE's free quarterly newsletter for chemistry students and teachers.

Subscribe to AUS-e-NEWS at https://www.ausetute.com.au/ausenews.html

Airbag Recalls

In 2008, a few thousand cars were recalled in the USA to have "faulty" Takata airbags replaced.
By 2016, tens of millions of cars world-wide are being recalled to have these "faulty" airbags replaced.
What is causing these particular airbags to rupture violently, sending lethal metal fragments flying through the car's interior?

If you subscribe to AUS-e-NEWS, AUS-e-TUTE's free quarterly newsletter for chemistry students and teachers, you will recall the issue in which we discussed the use of sodium azide, a compound that undergoes explosive thermal decomposition at temperatures well above ambient, to produce nitrogen gas to inflate the airbags used in cars.

What if you wanted to produce a cheaper airbag? Is there another, readily available, inexpensive, compound that can undergo explosive decomposition and produce lots of gas to fill an airbag? Yes there is, ammonium nitrate, NH₄NO₃, which is used in fertilizers, and, in explosives for mining and quarrying.

When anhydrous ammonium nitrate is heated above its melting point of 169.6°C, liquid ammonium nitrate decomposes to gaseous nitrous oxide (N₂O) and water vapor (H₂O) as shown in the balanced chemical equation below:
NH₄NO_3(l) → N₂O(g) + 2H₂O(g)
But this reaction is not explosive and therefore not suitable for use in airbags which must inflate incredibly quickly.
In order to produce an explosive decomposition of ammonium nitrate, you need to detonate it, which is the process that occurs if you use it in an airbag. The detonation of ammonium nitrate produces nitrogen gas, oxygen gas and water vapor, as shown in the balanced chemical equation below:
NH₄NO₃ → N_2(g) + ½O_2(g) + 2H₂O(g)

At first sight, this reaction looks like a much better, that is, safer choice, than the sodium azide reaction. But what happens if moisture gets into your airbag?
Ammonium nitrate readily dissolves in water, but if there is only a small amount of moisture present, the ammonium nitrate will "clump" together. At around 32°C (the temperature inside a pretty warm car) there is a phase transition in which the new phase occupies a greater volume than the previous phase which results in "cracking" of the "clumped" ammonium nitrate grains. These cracks can lead to catastrophic behaviour when the ammonium nitrate is detonated in the airbag, that is, the thermal decomposition of cracked ammonium nitrate grains is far more forceful and uncotrolled than that of anhydrous ammonium nitrate.

In order to safely use ammonium nitrate as the airbag propellant, you need to incorporate two safety features:

1.  a drying agent or desiccant to remove moisture and prevent "clumping" the ammonium nitrate
2.  a phase stabilizer to stabilize the phases to prevent "cracking"

Other airbag manufacturers do not use ammonium nitrate. In 2014, Key Safety Systems was reported to be using guanidine nitrate

and tetrazole

in the manufacture of its airbags.

Further Reading:
Definitions of a Mole
Mass-mole Calculations 
Molar Gas Volume Calculations 
Reaction Calculations: Mass and Moles

Suggested Study Questions:

1. What does the term "anhydrous" mean?
2. Why is a desiccant used in a desiccator?
3. Give the name and chemical formula for two commonly used desiccants in the school laboratory.
4. Write the molecular formula for each of the following compounds:
   • ammonium nitrate
   • nitrogen gas
   • oxygen gas
   • water vapor
   • nitrous oxide
   • guanidine nitrate
   • tetrazole
5. Calculate the molar mass of each of the following compounds:
   • ammonium nitrate
   • nitrogen gas
   • oxygen gas
   • water vapor 
   • nitrous oxide
6. Consider the chemical equation for the thermal decomposition of ammonium nitrate. If 2 moles of ammonium nitrate decomposed, how many moles of each of the following products would be produced?
   • nitrous oxide
   • water
7.  Consider the chemical equation for the thermal decomposition of ammonium nitrate. If 2.00 g of ammonium nitrate decomposed, calculate the mass of each of the following products in grams:
   • nitrous oxide
   • water
8. Consider the chemical equation for the detonation of ammonium nitrate in an airbag. If 2 moles of ammonium nitrate were detonated, how many moles of each of the following compounds would be produced?
   • nitrogen gas
   • oxygen gas
   • water vapor
9. Consider the chemical equation for the detonation of ammonium nitrate in an airbag. If 2.00 g of ammonium nitrate were detonated, what mass in grams of each of the following compounds would be produced?
   • nitrogen gas
   • oxygen gas
   • water vapor
10. Consider the chemical equation for the detonation of ammonium nitrate in an airbag. Calculate the mass of ammonium nitrate that would be required in order to fill a 70 L airbag at 25°C

Airbags

"BANG! I hit a hard pillow! Then there's white powder everywhere!", Sam graphically described a recent car accident.
The hard pillow is the inflated airbag, a safety device incorporated into cars which is designed to reduce the risk of serious injury during a collision, as it did in this case.

Where does the air in the airbag come from? And what is all that white powder?

Read all about the chemistry of airbags in the June 2017 edition of AUS-e-NEWS.

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Sulfuric Acid and Sugar

November 28, 2014, The Canberra Times reported that, "Students were looking on as a teacher conducted an experiment involving sugar and sulfuric acid inside a cabinet when the glass container holding the acid exploded."
The experiment referred to is probably similar to the one shown in the YouTube video below in which concentrated sulfuric acid from a container is poured over sugar in a beaker.

Safety Notes

Sulfuric acid will cause permanent damage if it comes into contact with the eyes or skin. Concentrated solutions of sulfuric acid are extremely corrosive. When sulfuric acid is dissolved in water enough heat is released to make water boil! Carbon monoxide and carbon dioxide are both toxic gases. Sulfur dioxide gas is toxic in high concentration and is a severe respiratory irritant at lower concentration.Some people, especially those prone to asthma, may be especially sensitive to sulfur dioxide. In the presence of moisture, sulfur dioxide forms an acidic, corrosive solution, which in contact with the skin or eyes may lead to burns.

You will notice that the reaction seems to proceed slowly at first. The reaction mixture turns yellow as the reaction begins. This reaction releases heat, it is said to be an exothermic reaction. The heat produced by the reaction then speeds up the rate of further reactions, and, in the video, this is also accelerated by stirring the mixture.

Table sugar is made up sucrose, molecular formula C₁₂H₂₂O₁₁ and structural formula as shown below:

The reaction between sucrose and sulfuric acid in which solid carbon, water vapour and heat are produced is known as a dehydration reaction  :
C₁₂H₂₂O₁₁(s) → 12C(s) + 11H₂O(g)     ΔH = -918.9 kJ mol^-1 Since the enthalpy change for this reaction is negative, the reaction is exothermic, the reaction gives off heat.
Sulfuric acid molecules have a great affinity for water, that is, sulfuric acid will readily and spontaneously dissolve in water. The water produced by the dehydration of sucrose will then be used to dilute the sulfuric acid that is present. This reaction is also exothermic.
H₂SO₄(l) → H₂SO₄(aq)     ΔH = - kJ mol^-1 The heat produced by this reaction also speeds up the rate of the dehydration reaction and subsequent dilution reactions.
Solid carbon is black, so the "black snake" is just carbon.
But what causes the carbon to "rise up" out of the beaker? This must be the result of evolving gases forcing their way through the mixture as the reaction proceeds. The gases that have been identified as products of this reaction  are:

• carbon monoxide (CO) 66% of the dry gas volume (ie, water has been condensed out)
• carbon dioxide (CO₂) 17% of the dry gas volume
• sulfur dioxide (SO₂) 17% of the dry gas volume

Since sulfuric acid does not oxidize carbon, it is most unlikely that the carbon monoxide and carbon dioxide gases are the result of a reaction between the black snake carbon and sulfuric acid. It is much more reasonable to assume that some of the sucrose undergoes dehydration by the sulfuric acid while some this sucrose (and/or some of the resulting intermediate organic products) is oxidized by the sulfuric acid to produce carbon monoxide gas and carbon dioxide gas. During this process, the sulfuric acid will itself be reduced, resulting in the formation of sulfur dioxide gas.

Reference:
http://www.canberratimes.com.au/act-news/students-treated-after-spill-in-burgmann-anglican-school-sciencbe-laboratory-20141128-11vzad.html

Further Reading:
http://ausetute.com.au/safety.html
http://www.ausetute.com.au/mmcalcul.html  
http://www.ausetute.com.au/moledefs.html
http://www.ausetute.com.au/massmole.html
http://www.ausetute.com.au/concsols.html 
http://www.ausetute.com.au/molarvol.html
http://www.ausetute.com.au/molreact.html

Suggested Study Questions:

1.  Describe two hazards in the YouTube Video.
2.  Describe the safety precautions you would take to minimize the risk of the hazards identified in question 1 above.  
3.  Why do you think the concentrated sulfuric acid is added to the sugar rather than adding the sugar to the container of sulfuric acid?
4.  In a typical experiment, 25 mL of 18 mol L^-1 sulfuric acid is added to 50 g of granulated sugar (sucrose). Calculate the amount in moles of :
   • sucrose used
   • sulfuric acid used
5. Calculate the mass of carbon that could be produced in the typical experiment given in question 4.
6. What assumptions have you made in order to calculate the mass of carbon in question 5?
7. Assume that all 50 g of the sucrose is now oxidized at 25^oC to produce carbon dioxide gas and liquid water. What is the maximum volume of carbon dioxide gas, in litres, that could be produced?
8.  Predict what you think might happen if a 50 g of granulated sugar were quickly added to 25 mL of concentrated sulfuric acid in a 100 mL conical flask that was being swirled continuously.
9. Sucrose is a disaccharide, made up of the monosaccharide glucose and the monosaccharide fructose. Starch and cellulose are both polysaccharides, that is, they are made of repeating glucose monomer units. Predict what would happen if you spilled concentrated sulfuric acid on:
   • a paper cup (cellulose)
   • a piece of raw potato (starch)
   • a cotton shirt (cellulose)
10. Design an experiment that could safely be performed in the laboratory to test your predictions in question 9.
11. Do you think that concentrated sulfuric acid will react with the monosaccharide glucose? Explain your answer using a chemical equation.
12.  Design an experiment that could safely be performed in the laboratory to test your prediction in question 11.

Ammonium Nitrate Explodes

On Friday 6^th September 2014, a truck carrying 56 tonnes of ammonium nitrate exploded after rolling over near a bridge on the Mitchell Highway between Cunnamulla and Charleville in the Australian State of Queensland. Eight people were injured, and the Mitchell Highway has been closed "indefinitely".

The most important use for ammonium nitrate is as a fertilizer, but it can also be used as an explosive. An explosion results when ammonium nitrate is brought into contact with heat, a source of ignition such as a spark, or, with reducing agents. Unfortunately, explosions of ammonium nitrate are not rare. A massive explosion of ammonium nitrate on 17^th April 2013 resulted in the death of 15 people, and the destruction of the West Fertilizer Company storage and distribution facility in Texas, USA.

Ammonium nitrate can be produced in commercial quantities by reacting ammonia gas with  nitric acid.

Reference: 
http://www.brisbanetimes.com.au/queensland/truck-explosion-injures-eight-closes-mitchell-highway-20140906-10dam3.html

Further Reading
Naming Ionic Compounds 
Writing Ionic Formula 
Mole Definitions 
Molar Gas Volumes

Suggested Study Questions:

1. Write the chemical formula for ammonium nitrate 
2. Explain why this compound is considered to be an ionic compound.
3. Write a word equation for the reaction between ammonia gas and nitric acid to produce ammonium nitrate
4. Write a balanced chemical equation for the reaction in question 3.
5. Ammonium nitrate decomposes in to N₂O and H₂O when heated. Write a balanced chemical equation for this reaction.
6. Calculate the molar mass of ammonium nitrate.
7. Calculate the moles of ammonium nitrate present in the truck.
8. Assuming that the decomposition of ammonium nitrate results only in gaseous products, what volume of gas at 25^oC and 100 kPa would be released by ammonium nitrate in the truck?
9. Assume the truck could carry a load with a volume of 30,000 L. Can you suggest a reason why rapidly decomposing ammonium nitrate could explode, destroying the truck, the bridge and the road?

Exploding Pool Chemicals

Sydney Morning Herald, Sunday 12^th January 2014, "Sydney man severely injured mixing chlorine for pool"
In summary, the victim of this accident had a backyard pool which had turned green. He had been advised to add 2 kilograms of "chlorine" to his pool to turn it sparkling blue again. He went home, put 2 kg of the "chlorine" granules into a bucket, and added water. Within 30 seconds the mixture exploded. The man was taken to hospital in a critical condition.

Now, for some chemistry....

From the description of the "chlorine" it sounds like it was calcium hypochlorite, Ca(OCl)₂ , a white solid at room temperature and pressure that is commonly sold as "pool chlorine".
When added to water, calcium hypochlorite produces hypochlorous acid, HOCl, and calcium hydroxide, Ca(OH)₂(aq).

Word equation:	calcium hypochlorite	+	water	gives	hypochlorous acid	+	calcium hydroxide
Chemical equation:	Ca(OCl)2	+	2H2O	→	2HOCl	+	Ca(OH)2

Dissolving calcium hypochlorite in water produces energy. The heat (or enthalpy) of solution is about 630 kJ mol^-1
We can now calculate how much energy was released when the man added water to 2 kg of calcium hypochlorite in the bucket.
First we need to calculate how many moles of calcium hypochlorite were in the bucket:
moles (Ca(OCl)₂ ) = mass ÷ molar mass
            and mass (Ca(OCl)₂) = 2 kg = 2 x 1000 = 2,000 g
            and molar mass (Ca(OCl)₂) = 40 + 2(16 + 35.5) = 143 g mol^-1
moles (Ca(OCl)₂ )  = 2000 ÷ 143 ≈ 14 mol
Now,  from the heat (enthalpy) of solution, we know that 1 mole of calcium hypochlorite dissolves in water to produce 630 kJ of energy.
We can calculate how much energy is released when 14 mol of calcium hypochlorite dissolves in water:
energy released = moles x energy in kJ mol^-1
energy released  = 14 x 630 = 8820 kJ
which is equivalent to exploding about 100 g of TNT in the bucket !

Reference:
http://www.smh.com.au/nsw/sydney-man-severely-injured-mixing-chlorine-for-pool-20140111-30nl3.html

Suggested Reading:
Defining Enthalpy Change 
Energy Profiles 
Strength of Acids
Strength of Bases 
pH of Aqueous Salt Solutions 


Suggested Study Questions:

1.  What is the name given to the type of reaction that produced energy?
2.  What name is given to the type of reaction that absorbs energy in order to produce products?
3. Sketch an energy profile diagram for the reaction between calcium hypochlorite and water.
4. Sketch an energy profile diagram for the explosion of TNT.
5. For 2 kg of calcium hypochlorite completely dissolving in water, calculate the moles of calcium hydroxide that would be produced.
6. For 2 kg of calcium hypochlorite dissolving in water, calculate the mass of hypochlorous acid that would be produced.
7. Assume the man used a 30 L of water in a 40 L bucket, what is the concentration of calcium hydroxide expected to be?
8. Hypochlorous acid is considered to be a very weak acid. What does this mean? 
9. Calcium hydroxide is considered to be a strong base. What does this mean?
10. What is the approximate pH of the solution when 2 kg of calcium hypochlorite is dissolved in  30 L of water?
11. What safety precautions do you think pool owners should take when mixing calcium hypochlorite granules with water?
12. Design a safety poster that could be displayed in a pool shop clearly showing the safest way to mix calcium hypochlorite granules with water.