Tianjin Explosion. What exactly happened? Topic: O level sec 3 Mole Concept
- On 12th August 2015, one of the deadliest explosions (actually, 2), happened in Tianjin, China. The two blasts measuring the equivalent of 3 and 21 tons of TNT, respectively. The second, larger blast was so powerful that it caused a magnitude 2.9 earthquake in the surrounding area. For a surface explosion to cause a measurable earthquake is rare. Let us take some time to digest the facts and investigate what could be a possible cause for the accident, and what could have triggered the earthquake.
- Stage 1: An unknown substance caught fire inside one of the storage containers at the facility.
C2H5OH + 3O2 → 2CO2(g) + 3H2O(g)
This could be anything flammable. As the area is a warehouse zoned for storing chemicals, it can range from petrochemicals to ethanol which is highly combustible. For example, we illustrate the combustion of ethanol in the presence of oxygen to produce carbon dioxide, water and lots of heat energy.
- Stage 2: Firefighters arrived at the scene to douse the flames with water.
When water touches calcium carbide (CaC2), acetylene gas C2H2 is produced.
Calcium carbide, CaC2(s), is an unstable compound that’s used in the production of acetylene (ethyne) and also in steelmaking. When water (or moist air) touches calcium carbide, it fizzes gently, releasing acetylene gas, C2H2(g), which, when mixed appropriately with air, explodes upon ignition. Is this the cause of the Tianjin Explosion? This is only the start.
Note also, that the ethyne gas released is colourless and odourless
CaC2(s) + 2H2O(l) → Ca(OH)2(s) + C2H2(g)
The firefighters fail to detect the presence of C2H2 in the explosion because it is odourless. This is especially dangerous to the firemen as this gas is flammable and cannot be detected easily.
Note: In chemical explosions, adding water is not the best idea to put out a flame. Why? Water can further react with chemicals to produce flammable substances like acetylene gas!
- Stage 3: Flames ignite the acetylene gas, causing the first explosion
After the acetylene had mixed sufficiently with the surrounding air, one part of this explosive gas mixture was ignited by the pre-existing flames, causing the first explosion.
Eyewitness reports for the Tianjin Explosion have estimated this first explosion to be equivalent to 3 tons of TNT,
Some exercises for you:
- If 1 ton of TNT produces 4 million kilojoules of energy, how much energy would 3 ton of TNT produce? [12 million kJ]
- If 1 mole of combustion of acetylene produces 1299 kJ of energy, how many moles of acetylene were combusted in the Tianjin explosion?
12 000 000 kJ / 1299 kJ = 9240 moles
- Using stoichiometric calculation, find out the initial mass of calcium carbide in the Tianjin Explosion that decomposed? Leave your answers in kilograms.
9240 X (40 + 2 X 12) = 591 000 g
- Calcium carbide was found to be stored in 200kg boxes. According to Alibaba.com, there were 5 companies that sold calcium carbide. Whch is responsible for selling the ones that caused the explosion?
| Company | A | B | C | D | E |
| Min order | 5000kg | 1000kg | 200kg | 1000kg | 1000kg |
| Price, in USD / tonne | 500 | 600 | 800 | 550 | 520 |
- How much would the calcium carbide in the Tianjin explosion have cost? ( Please calculate the buying price of the chemicals, not the entire costs of the Tianjin explosion for you smart students out there.)
Do you know, it is actually possible to find out which company’s calcium carbide was responsible for that Tianjin explosion? This is no myth, check it out on Alibaba yourself! J By the way, the answer is Company C, and $480 worth of chemicals actually caused such catastrophic explosions.
- Stage 4: High temperatures caused nearby ammonium nitrate to detonate at >240°C, causing the second explosion
Temperatures in the Tianjin Explosion of over 3000°C were generated by the combustion of the acetylene in stage 3. The immense heat from that initial fireball heated the surrounding containers to above 240°C, which initiated a runaway decomposition reaction of ammonium nitrate, NH4NO3(s), which was stored nearby.
for the decomposition of ammonium nitrate, NH4NO3 in the Tianjin Explosion is :
NH4NO3(s) → N2(g) + 2H2O(g) + 1/2O2(g)
One tonne of ammonium nitrate, NH4NO3 would produce 10,600 kilojoules of energy. This second explosion to be around 21 tons of TNT equivalent.
Using 1 ton of TNT produces 4 million kilojoules of energy, the number of tonnes of ammonium nitrate, NH4NO3 stored at the warehouse for the Tianjin Explosion is equivalent to
21 X 4 000 000 kJ / 10,600 kJ = 7920 tonnes
Analyzing the Tianjin Explosion situation, it could very well be possible that about 8000 tonnes of chemicals, such as ammonium nitrate, stored in the facilities, could have triggered such an explosion. Logistic facilities usually store up to thousands of tonnes of chemicals and the lesson here is that should such explosive chemicals be stored in facilities, they have to be accompanied with safety measures, especially away from flammable materials to prevent disastrous effects. Want to read more about chemistry related articles like this on the Tianjin explosion? Click on our blog at Seb Academy to read more. You can also find out more about secrets to doing well in Chemistry. We cover many of these interesting case studies in our classes! Find out more today!
