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РЕГИСТРАЦИЯ ЭКСКУРСИЯ

Text 24 A

~~The Composition and Structure of Water

 

The Composition and Structure of Water - раздел Образование, Part One THE PREDICATE Water Was Thought By The Ancients To Be An Clement. Henry Cavendish In 1781 S...

 
 

   

Water was thought by the ancients to be an clement. Henry Cavendish in 1781 showed that water is formed when hydrogen is burned in air, and Lavoisier first recognized that water is a compound of the two elements — hydrogen and oxygen.

The formula of water is H20. The relative weights of hydrogen and oxygen in the substancc have been very carefully determined as 2.016:16.000. This determination has been made both by weighing the amounts of hydrogen and oxygen liberated from water by electrolysis and by determining the weights of hydrogen and oxygen that combine to form water.

Liquid water has a number of unique properties which indicate that the structure must be fundamentally different from that of most other liquids. Thus, water has high melting and boiling points, an unusually high heat capacity, and shows a characteristic decrease in molar volume on melting and a subsequent contraction between 0 and 4°C. Quite apart from the behaviour of aqueous solutions, any proposed structure for liquid water must be consistent with these observations.

From spectroscopic studies of isolated water molcculcs in the gas phase, it has been shown that the H—О—H bond angle is very nearly the tctrahcdral angle of 105° and the О—H intcrnuclcar distance is 0.97E; the observed dipolc moment is 1.87x10"18 c.s.u.[9] acting along the bisector of the H—О—H angle.

Bcrnal and Fowler in a classical work on the interpretation of these results, showed that the net electronic density distribution was consistent with such a structure in which, in addition to the two protons carrying small positive charges, there arc also two regions of negative charge. These four regions of charge, two positive and two negative, could be regarded as residing at the corners of a tetrahedron. If the oxygen is approximately sp3 hybridized then two of the orbitals on the oxygen atom arc used for bonding the hydrogen atoms and the other two carry the lone pairs of electrons wrhich can participate in hydrogen bonds to two neighbouring water molcculcs. In terms of this picture the structure of icc, in which cach molcculc has four nearest neighbours, can be represented in the following way. The molecules arc held together by tctrahcdrally-dircctcd hydrogen bonds which arc essentially electrostatic in character. The structure is an open one rather than a close-packed with a resulting increase in density. The characteristic increase in density with temperature continues until 4°C when die cxpcctcd dccrcasc accompanying the increased thermal energy becomes apparent.

 


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