1.

Account for the following : (i) Be(OH)_(2) is insoluble but Ba(OH)_(2) is fairly soluble in water . (ii) Be(OH)_(2) is amphoteric while Mg(OH)_(2) is basic .

Answer» <html><body><p></p>Solution :(i) Be has small size , <a href="https://interviewquestions.tuteehub.com/tag/high-479925" style="font-weight:bold;" target="_blank" title="Click to know more about HIGH">HIGH</a> ionization enthalpy and comparatively high electronegativity . Therefore , `Be(OH)_(2)` is essentially covalent and hence is insoluble in water . However , as we <a href="https://interviewquestions.tuteehub.com/tag/move-548879" style="font-weight:bold;" target="_blank" title="Click to know more about MOVE">MOVE</a> down the group from Be to Ba , the atomic size increases , ionization enthalpy and electronegativity decrease . therefore , ionic characters of their hydroxides increases down the group . Further , it has been found that with increase in size the lattice down the group from Be to Ba . In other words , `Be(OH)_(2)` is insoluble but `Ba(OH)_(2)` is <a href="https://interviewquestions.tuteehub.com/tag/fairly-2078571" style="font-weight:bold;" target="_blank" title="Click to know more about FAIRLY">FAIRLY</a> soluble in water . <br/> (ii) Because of <a href="https://interviewquestions.tuteehub.com/tag/higher-1022060" style="font-weight:bold;" target="_blank" title="Click to know more about HIGHER">HIGHER</a> sum of `Delta_(i)H_(<a href="https://interviewquestions.tuteehub.com/tag/1-256655" style="font-weight:bold;" target="_blank" title="Click to know more about 1">1</a>) + Delta_(i) H_(2)` of Be than that of Mg , the M-OH bond can break less easily in `Be(OH)_(2)` than in `Mg(OH)_(2)` . In other words , `Be(OH)_(2)` acts as a weak base even weaker than `Mg(OH)_(2)` . <br/> Further because of smaller size and higher electronegativity than that of Mg , `Be(OH)_(2)` can accept `OH^(-)` ions from alkalies to form beryllate ion but Mg`(OH)_(2)` does not . <br/> `Be(OH)_(2) + 2OH^(-) to Be(OH)_(4)^(2-) to underset("Beryllate ion")(BeO_(2)^(2-) ) + 2H_(2)O` <br/> `Mg(OH)_(2) + 2OH^(-) cancelto Mg(OH)_(4)^(2-)` <br/> Thus , `Be(OH)_(2)` acts as a weak acid but `Mg(OH)_(2)` does not . Combining both the facts , we conclude that the `Be(OH)_(2)` is amphoteric but `Mg(OH)_(2)` is basic .</body></html>


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