Wonders of Life. Andrew Cohen
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Название: Wonders of Life
Автор: Andrew Cohen
Издательство: HarperCollins
Жанр: Прочая образовательная литература
isbn: 9780007452682
isbn:
However, this simple tetrahedral arrangement of a central oxygen atom surrounded by two pairs of electrons and two hydrogen atoms is deceptive, because the structure allows for tremendously complex behaviour when water molecules come together in large numbers. And, as we shall see, this unique behaviour may well make water a prerequisite for the existence of life, not only on Earth, but anywhere in the Universe. Perhaps unsurprisingly, given its dominance in our lives, scientists have been attempting to unlock its secrets for over three hundred years.
1 For those who don’t like such anthropomorphic language, it is energetically favourable for electrons with opposite spins to pair up in the available energy levels around a nucleus, and there are four available upper energy levels around the oxygen nucleus for the six electrons to occupy.
MOLECULAR GEOMETRY: Tetrahedral electron pair geometry
We often take water for granted, yet it is a remarkably complex substance, and without it there would be no life, not only on Earth, but anywhere in the Universe.
THE HISTORY OF THE EXPLORATION OF WATER
In the eighteenth century, Europe was full of inquisitive men attempting to unlock the secrets of the natural world, and Henry Cavendish was certainly one of the most eccentric. It is said that he was unable to bring himself to interact with women outside of his family at all, communicating with his female servants by written notes and sneaking around his own house using a specially constructed staircase so as to avoid his housekeeper. His isolation was so extreme that he often kept his experimental findings secret, not publishing or sharing his research with anyone. Such was the extent of this secretiveness that it was only many years after his death that the true breadth of his discoveries became apparent.
Cavendish was a follower of phlogiston theory – a widely held belief that had its roots in alchemy. The theory suggested the existence of an element thought to be contained within all combustible material, called ‘phlogiston’. By the middle of the eighteenth century the theory had been widely discredited, yet Cavendish continued to see worth in it, and attempted to incorporate it into many of his observations. To modern ears, this makes his terminology sound rather eccentric, but his contribution to our understanding of the natural world was extraordinary, not least in his early work on the chemical properties of water.
In a series of experiments, Cavendish produced and isolated a gas by reacting hydrochloric acid with metals such as zinc, iron and tin. In doing so, he became the first person to identify hydrogen in the laboratory. He referred to this new gas as ‘flammable air’ in his poetically named paper ‘Factitious Airs’, published in 1766. Cavendish went on to show that hydrogen reacted with another gas, which he termed ‘dephlogisticated air’, to produce water. This gas was oxygen. His experiments with flammable air eventually led him to the first determination of the composition of Earth’s atmosphere – one part dephlogisticated air (oxygen) and four parts ‘phlogisticated air’ (nitrogen). There is something quite instructive in Cavendish’s approach to science. Even though his devotion to the phlogiston theory was wayward, to say the least, he did not allow his theoretical prejudice to contaminate his experimental results. This is why he was able to make genuine discoveries while holding at least some views about his subject that were flat-out wrong. That is the mark of a great experimental scientist!
ELECTROLYSIS OF WATER
We owe the modern names for the elemental building blocks of water – hydrogen and oxygen – to Antoine Lavoisier, one of the greatest of the pioneering eighteenth-century chemists. Great though he undoubtedly was, however, he made a fundamental error in naming these two elements that persists to this day.
He named hydrogen, entirely appropriately, from the Greek ‘hydro’ (meaning water) and ‘genes’ (meaning creator). Oxygen, however, with its Greek root of ‘oxys’ (meaning acid), incorrectly suggests that oxygen is a component of all acids. It would have been more accurate to call hydrogen ‘oxygen’, in that the majority of common acid-base chemical reactions involve the transfer of protons, which are the nuclei of hydrogen. But Lavoisier’s names have stayed with us, so oxygen will forever be ‘the acid giver’, which it isn’t.
By 1804, the final elemental description of water was given in a paper by the French chemist Joseph Louis Gay-Lussac and the German naturalist Alexander von Humboldt. Together, they demonstrated that water consisted of two volumes of hydrogen to one of oxygen, and thus gave the world the most widely known of all chemical formulae: H2O. If Lavoisier had got it right, we’d call water O2H rather than H2O. Such is history.
MR BELL’S GUIDE TO THE ELECTROLYSIS OF WATER
Everybody has a teacher whose very essence, usually distilled from endearing eccentricity, remains forever imprinted on their consciousness. I had such a teacher, and his name was Sam Bell. He used to gaze out of the thin windows over the playing fields on a darkening Oldham afternoon and growl strangely in a thick Yorkshire accent about how he could still see the old games master, Pinky Green, ringing a bell, before launching a board duster across the varnished benches towards a boy’s head. He’d undoubtedly face disciplinary proceedings for that today, but it made chemistry enjoyable to an 11 year old. Mr Bell’s trademark technique was to drill chemical reactions into your brain with an indelible power usually reserved for poetry. ‘The ploughman homeward plods his weary way, and HYYDR’GEN burns with a squeaky POP!’ This works, as I discovered when, half a lifetime later, I settled down with a car battery in front of a waterfall in central Mexico to explain the electrolysis of water to a television camera.
‘Electrons enter the water at the cathode, where a reduction reaction takes place, releasing hydrogen gas which burns with a squeaky pop in air. Oxidation occurs at the anode, producing oxygen, which rekindles a glowing splint.’ Perfect.
Cathode (reduction): 2 H2O + 2e- → H2 + 2 OH- Anode (oxidation): 4 OH- → O2 + 2 H2O + 4e-
The point of all this, which will be important when we come to discuss photosynthesis later on, is to demonstrate that it takes a large amount of energy to split water into hydrogen and oxygen. This is because oxygen really wants to acquire the two extra electrons necessary to fill its outer shell, and hydrogen is a relatively easy place to get them. This in turn means that water is a very stable molecule, and it therefore takes a lot of effort – in this case the power of a car battery – to split it apart. But, really, the point is to show that I thought my chemistry teacher was brilliant.
ELECTROLYSIS OF WATER
Earth, the small СКАЧАТЬ