Bromine liquid-vapour equilibrium

This type of physical liquid-vapour equilibrium can be visually demonstrated using bromine. Bromine is the one non-metallic element that is a liquid at room temperature. It is a volatile liquid (boiling point 332 K). When placed in a sealed container, the orange-brown vapour collects over the deep red-brown liquid. As the liquid slowly evaporates over a period of time, the colour of the vapour becomes more intense. Eventually, the intensity of colour of the vapour as it sits over the liquid remains constant (Figure 7.4). 

The unchanging colour of the vapour in the flask suggests that a position of balance has been reached. Some of the bromine has formed a vapour and some of the bromine remains as a liquid. A position of equilibrium has been reached between bromine liquid and bromine gas. This equilibrium can be summarized as ... 

The equilibrium sign ( ) is used to show that both bromine liquid and bromine gas are present in the flask. Do all the liquid bromine molecules remain in the liquid while all the gaseous molecules stay as vapour (a static equilibrium) Or is there an exchange of molecules, with some liquid molecules entering the vapour state while an equal number of vapour molecules condense to liquid (a dynamic equilibrium) Experiments show that liquid and gas molecules move around rapidly and randomly, giving rise to our ideas of the kinetic theory of matter (Chapter 1). Given these ideas, it seems likely that a dynamic rather than a static equilibrium is set up in the flask. In which case, the rate at which molecules leave the liquid surface and enter the vapour is equal to the rate at which other molecules in the vapour return... 

Several routes for producing bromine chloride have been examined. The reaction attains equilibrium in both the liquid and vapour phases ... 

J. J. van Laar has shown how the form of the vap. press, curves of a liquid mixture can furnish an indication, not a precise computation, of the degree of dissociation of any compound which maybe formed, on the assumption that the different kind of molecules in the liquid—12, Br2, and IBr—possess partial press, each of which is equal to the product of the vap. press, of a given component in the unmixed state and its fractional molecular concentration in the liquid. It is assumed that in the liquid, there is a balanced reaction 2IBr I2-)-Br2, to which the law of mass action applies, where K is the equilibrium constant, and Clt C2, and C respectively denote the concentration of the free iodine, free bromine, and iodine bromide. From this, P. C. E. M. Terwogt infers that at 50 2°, K for the liquid is 7j and that for iodine monobromide about 20 per cent, of the liquid and about 80 per cent, of the vapour is dissociated. That the vapour of iodine monobromide is not quite dissociated into its elements is evident from its absorption spectrum, which shows some fine red orange and yellow lines in addition to those which characterize iodine and bromine. In thin layers, the colour of the vapour is copper red. 0. Ruff29 could uot prove the formation of a compound by the measurements of the light absorption of soln. of iodine and bromine in carbon tetrachloride. 

Figure 7.4 A sealed flask containing bromine demonstrates a physical equilibrium between the liquid and Its vapour... 

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