QQ asked in 科學及數學化學 · 1 decade ago

alkanoic acid溶唔溶於水 ?如唔溶,點解會ionise in water?

as same as above.....

please give me a detail explanation

1 Answer

  • 1 decade ago
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    Take ethanoic acid as an example, although only a few of the acid molecules is ionized in water, ethanoic acid is soluble in water. This is beause the solubility of an acid does not depend on its degree of ionization.

    An acid will dissolve if the energy released by new bonds formation between the water and the acid molecules is large enough to compensate the energy needed to break the bonds between water and acid molecules respectively. For organic acids, there is a COOH group. This group can form Hydrogen-bond between water, and the energy released in formation of these H-bond is enough to break to H-bond between the water and acid respectively. Thus, it is soluble.

    However, if the acid is too large, there are strong van der Waals' force between the acid itself as well as H- bond. During solvation, only H- bond is formed, but it has to break H-bond as well as van der Waals' force. So, energy is not enough to do this process, and the acid's solubility decreases when its size increase.

    So, the solubility does not depend on degree of ionization.(although degree of ionization and its solubilty both decrease on passing up the homologous series)

    PS: ionize in water means the acid split in to a RCOO- and H+, which does nothing to solubility of the whole acid molecule RCOOH

    2008-04-15 10:03:29 補充:

    But the solubility would affect the degree of ionzation, because if an acid is to be ionized, it has to be dissolved in water first.

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