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Flashes – Bangs – And Stinks!

The excitement of chemistry experiments and how they help us learn chemistry.

Date : 11/01/2019

Gareth

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Uploaded by : Gareth
Uploaded on : 11/01/2019
Subject : Chemistry

When are we going to blow something up? A staple question, once-upon-a-time. For the uninitiated, that s seems to be what chemistry is all about setting fire to things and hoping they blow up. Except, when the remote prospect is raised, mischievously, that the experiment currently being prepared by students tends to go wrong and might, just might, explode, there s a very different response. And the confident boldness immediately drains from now genuinely concerned students.

In terms of the bangs, there is a limit to what can take place inside a classroom. The go-to standard is the hydrogen gas in the balloon demonstration. There s certainly a loud bang, although if the balloon is filled with a two-to-one hydrogen-oxygen mix, there s a much much louder bang. It s a good demonstration, illustrating the amount of energy released in what is a very small explosion. It s more fun for students to use hydrogen gas generators to collect the hydrogen inside soap bubbles. These can either be ignited in-situ or collected and the hydrogen combusted on the student s hands. The result is a flash of flame and satisfying pop.Several university departments offer lectures and demonstrations where releasing loud explosions are not a problem. An impressive demonstration is the firing of muskets, which when completed inside a darkened lecture theatre presents a thrilling purple-blue musket flash and deafening reports. There are other, possibly louder explosions. Ethylene detonated inside a glass bottle springs to mind, and not just for the noise it makes. The glass bottle is turned to powder incidentally for the health and safety minded people reading this such combustion reactions are staged inside a Perspex-shatterproof box. Not so for one butane-bucket demonstration, which proved to be one of the loudest explosions I ve ever witnessed. This was based partly on my proximity sitting on the front row. Its another simple combustion reaction, but set-up on a larger scale, with a plastic bucket filled with soapy-water. The butane was bubbled through the soapy-water, forming large butane-bubbles. These were then ignited BANG! The lecture theatre immediately filled with debris, much of it from the remains of the plastic bucket and its contents, which floated down like huge snowflakes from above. The explosion set of the fire-alarms and had security charging into the lecture. There is a serious observation to stress here that no chemical reactions should be done unsupervised and without being carefully planned in line with standard health and safety protocols. I m convinced the butane-bucket explosions described did not go to plan, you could see the look of genuine surprise in the face of the Professor of Chemistry demonstrating the reaction. Things go wrong, even when the most knowledgeable and experienced people are in charge. Flashes, the release of light energy, often pairs with the loud bangs accompanying explosions. The go-to demonstration for impressive flashes is the screaming jelly-baby experiment. This involves adding a jelly-baby to molten potassium chromate(VII) inside a boiling tube. It s a standard fuel-plus-oxidising agent WHOOSH combination. It s a simple demonstration that requires some practice, simply to ensure the initial reaction doesn t project the jelly-baby straight back out of the boiling-tube. The resulting violet flames screeching out of the tube are always impressive. Another standard is the Thermite-reaction. This is now so mainstream there s a ready prepared mix that can be bought to demonstrate the reaction. Traditionalists may still play around with the iron oxide, aluminium powder, barium peroxide and the magnesium ribbon fuse, which in my experience gives a more impressive outcome than the premix version. Better still are the displacement reactions involving magnesium powder and silver nitrate, and aluminium powder and copper(II) chloride. These demonstrate the same principles as the thermite reaction, but they are much simpler to demonstrate as they do not require heating. Just add water. Once again, great care must be taken regarding the quantities used as these reactions are highly exothermic meaning a great deal of energy is released. The first time I demonstrated the magnesium-silver nitrate reaction I only used a spatula of both reagents, but the resulting FLASH produced a cloud that filled the classroom. It s a good one to do outside just make sure it isn t raining. I plan to describe the aluminium-copper(II) chloride demonstration in a later blog, as it s one of my favourite reactions.The metaphorical, line-drawn-in-the-sand all students of chemistry must cross, are the smells. It is possible to avoid smelly chemistry, most of the time, and there are fume cupboards that deal admirably when they are needed. But chemistry is famed for being smelly, and this is something that should be embraced. Of course, there are smells, and then there are SMELLS. There are the sweet, fruity aromas of esters and ketones, which chemists often use as solvents. I find there s something deeply satisfying walking into old, wooden-benched laboratories, with their ingrained organic solvent fragrance. The reality of working long hours with organic solvents are headaches mixed with the occasionally sense of disorientation. It shouldn t happen that s what fume cupboards are for. But, the gentle waft of ethyl ethanoate or propanone tend to trigger happy memories of working in chemistry laboratories. At the other end of the smelly spectrum, we find two antagonists that form the base for some of the worse smells EVER sulphur (often contained in thiols, -SH), and amines. Sulphurous smells are legendary, from farts to the eggy odour of stink bombs. The creep of sulphurous fumes, from sulphide impurities to full-bodied sulphur bearing compounds, can quickly fill a laboratory. Add to sulphurous gases, the hydrogen chloride and ammonia fumes most students meet in their practical classes, and lessons are quickly learnt to treat chemical substances with care and respect. Studying chemistry also improves an awareness of smells for example, being met by a gust of trimethylamine whilst walking on a harbour wall, or describing vomit as smelling of butanoic acid, rather than butanoic acid smells like vomit (there is a distinction). Most horrible smells tend to have interesting names cadaverine (rotting flesh), hydrogen disulphide (rotting eggs), formaldehyde (preserving-fluid), 2-mercaptoethanol (rotting eggs/burnt rubber) These examples demonstrate how interesting the study of chemistry and chemical reactions is. In terms of teaching chemistry, the best experiments are those that evoke questions through challenging the students perception. For example, having demonstrated adding sodium metal to water, asking students to predict the outcome of now adding sodium metal to concentrated hydrochloric acid and then demonstrating that too. Discussing the relative positions of hydrogen and copper in the reactivity series and then adding copper to nitric acid, or observing aluminium displacing copper from copper(II) chloride, but not from copper(II) sulphate. In organic chemistry, students can investigate the relative reactivities observed for the hydrolysis of halogenoalkanes, in terms of the conflicting trends in the polarisation of bonds and the effectiveness of leaving groups. There are many many other reactions that challenge and prompt further study, some of which I plan to go into further detail in future blogs. So, watch this space.


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