hydrogen pringles

A pringles tub with a small hole in the top, filled with hydrogen gas and ignited, will burn away quietly until the hydrogen/air mix is just right. Then a small explosion is observed. The second attempt has the mix just right to begin with upon ignition.

august kekulé

Friedrich August Kekule von Stradonitz (also August Kekulé) (1829 – 1896) was a German organic chemist. One of the most prominent chemists in Europe from the 1850s until his death, especially in the theoretical realm, he was the principal founder of the theory of chemical structure.

The theory of chemical structure (1857-1858) was a descripton of the ability of carbon atoms to link to each other tetravalently. The idea of the self-linking of carbon atoms provided the first formulae where lines symbolise bonds connecting the atoms. For organic chemists the use of structural formulae to explain the formation of molecules provided dramatic new clarity of understanding, and as a consequence the field of organic chemistry developed explosively from this point.

However, Kekulé's most famous work was based on the structure of benzene. Early suggestions at the time had been based on a linear chain of 6 carbon atom with 4 double bonds. The empirical formula for benzene had been long known, but its highly unsaturated structure was challenging to determine. The study of aromatic compounds was in its earliest years, and too little evidence was then available to help chemists decide on any particular structure. In 1865 Kekulé published a paper in French suggesting that the structure contained a six-membered ring of carbon atoms with alternating single and double bonds. The next year he published a much longer paper in German (his native language) on the same subject.

When Kekulé spoke of the creation of the theory, he said that he had discovered the ring shape of the benzene molecule after having a day-dream of a snake (dirty boy! Freud would have a field day) seizing its own tail. This vision, he said, came to him after years of studying the nature of carbon-carbon bonds.

Although it was an exciting and well thought out idea at the time, further evidence soon came to light that demonstrated that Kekulé was perhaps mistaken in his proposed structure of benzene. You will need to be able to describe and explain this evidence for your module 4 exam.

The evidence includes:

a) problem with bond lengths
b) lack of reaction with bromine (benzene will not undergo electophilic addition)
c) problem with enthalpy of hydrogenation data

Check your notes to make sure you can explain each piece of evidence.

module 4 kinetics

Some of you have had problems drawing gradients on curves in order to deduce the rate of reaction on concentration/time graphs. The powerpoint below will allow you to check this skill and compare it to the graphs that you produced on your recent homework.

gradients from graphs

This next download is a set of interactive kinetics questions of a similar type to the ones you will find on your module 4 exam. Answers are provided.

kinetics exam questions

thomas midgley

Thomas Midgley, Jr, was an American Engineer and latterly a Chemist whose contribution to science was almost impossibly unfortunate and regrettable, since he was responsible for possibly two of the most destructive inventions of the 20th century.

Midgley used a knowledge of chemistry and, in particular, the Periodic Table to make two significant developments – lead based anti-knock additives for internal combustion engines and chlorofluorocarbons (CFCs) as the working fluids in refrigerators. Both these developments were of enormous commercial importance at the time (the 1920s) and remained in use for over half a century.

His anti-knocking additive, tetraethyl lead, was used in petrol until it began to be phased out in the 1970s and was totally withdrawn in the UK in 2000. Lead and its compounds are neurotoxins, and studies suggested that the lead(IV) oxide given out by vehicles using leaded fuel was causing brain damage in children growing up in areas close to major roads. Lead also poisons the catalysts of cars using catalytic converters used to convert carbon monoxide, nitrogen oxides and unburned hydrocarbons in car exhausts into innocuous carbon dioxide, nitrogen and water. In fact, the dangers asociated with lead and its derivatives were well known at the time, and probably the reason that the corporations involved with the production of tetraethyl lead chose to call their new petrol additive simply 'ethyl'. Much more consumer friendly.

Not content with putting enough lead into the atmosphere to kill people for decades, he turned his hand to another problem. In the 1920's compounds such as propane, ammonia, sulfur dioxide and chloromethane were used as refrigerant gases. All had disadvantages such as toxicity, flammability or chemical reactivity. One refrigerator leak in a hospital in Cleveland in 1929 killed over 100 people. What was required was a non-toxic, non-flammable, chemically inert gas/volatile liquid. In 1928, Midgley was called in to help with the search and with an instict for the regrettable that was almost uncanny, Thomas Midgley invented chloroflourocarbons, CFC's.

With a degree of showmanship, Midgley even demonstrated the suitable properties of his new discovery at a meeting of the American Chemical Society by inhaling some of the gas and then exhaling onto a lighted candle, which was extinguished.

CFCs went on to be an important commercial success, being used for several decades as refrigerator fluids and deoderant propellants. We now know that the chemical inertness of the CFCs (due in part to their strong C-F bonds) held the seed of a major environmental problem. On release into the atmosphere, CFCs do not break down and a large ‘reservoir’ of them built up in the atmosphere. However, high in the atmosphere they do decompose under the action of ultraviolet light, leading to the formation of chlorine radicals which catalyse the breakdown of ozone to oxygen. Since ozone absorbs ultraviolet radiation from the Sun, this leads to a greater intensity of UV radiation at the Earth’s surface causing problems such as increased rates of skin cancer, cataracts in the eyes, the death of plankton and faster decomposition of rubber, plastics and dyes. This began to be understood in the 1970s and CFCs have been withdrawn since the Montreal Protocol of 1987.

Although is easy to deride Midgley as being responsible for two major environmental catastrophes this is, of course, with hindsight. At the time both tetraethyl lead and CFCs were major scientific and economic breakthroughs and their consequences a half a century later simply could not have been predicted by Midgley or anyone else.

Finally, there is the bizarre tale of Midgley’s tragic end. In 1944, at the age of 51, Midgley contracted polio and lost the use of his legs. Using his engineering skills he devised a pulley system to enable him to get out of bed, which worked well until one day he became entangled in the ropes and was strangled.

jelly baby

Basic fire triangle requirements, essential for all combustion, are heat, oxygen and fuel. Here the heat has already been provided by a bunsen, the oxygen mainly comes from the molten compound in the boiling tube, potassium chlorate, KClO₃, and the fuel? Well that's the sugar in the poor old jelly baby.

magnesium and copper oxide

The two metals magnesium and copper are poles apart in terms of reactivity, as anyone with a passing knowledge of the reactivity series of metals will be aware.

This can be violently demonstrated by the reaction between magnesium powder and copper oxide in a small porcelain crucible. It takes a while to get hot enough, and then a rather spectacular displacement reaction occurs.