Given that Magnesium Hydroxide is an alkali, can we say Magnesium Oxide is an alkali since dissolving Magnesium Oxide in water can produce Magnesium Hydroxide?
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2 Answers
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Tl, dr: Chemists would generally say "No". But it's not as cut and dried as one might think. The alkaline character of magnesia and heavier Group 2 oxides is one of many examples in chemistry where the actual character is intermediate between the extremes a yes/no answer seems to imply.
Usually the term "alkali" is reserved for the hydroxides of Group 1 metals, which (at least among compounds of commonly observed elements) uniquely combine strong basic character with high solubility in water. Even barium hydroxide, whose solubility in water is limited to less than one molar unless the water is heated, does not get there on solubility.
That being said, there are applications where magnesia (and its congeners with heavier alkaline earth metals, such as lime) functions as an alkali would. Given the solubility product of magnesium hydroxide being roughly $10^{-11}$ (different sources quote slightly different values), magnesium remains freely soluble up to a pH of about 9 (with lime the corresponding limit for calcium is around pH 11 or 12), which is high enough to displace most transition metal hydroxides from solution. From Wikipedia:
Magnesium oxide is used extensively in the soil and groundwater remediation, wastewater treatment, drinking water treatment, air emissions treatment, and waste treatment industries for its acid buffering capacity and related effectiveness in stabilizing dissolved heavy metal species.
Many heavy metals species, such as lead and cadmium are most soluble in water at acidic pH (below 6) as well as high pH (above 11). ...
Granular MgO is often blended into metals-contaminated soil or waste material, which is also commonly of a low (acidic) pH, in order to drive the pH into the 8–10 range where most metals are at their lowest solubilities. Metal-hydroxide complexes have a tendency to precipitate out of aqueous solution in the pH range of 8–10.
In effect, magnesia and other alkaline earths act as alkalies in some reactions (displacement of other metal hydroxides) but not in others (failing to form concentrated strongly basic solutions). They represent an intermediate stage between full-fledged alkalis and the majority of metal oxides and hydroxides which are inert in the absence of added acid or strong base.
answered Apr 16, 2020 at 10:05
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$\begingroup$ So is the premise I took, which Mg(OH)2 is alkali, also right or just intermediate? $\endgroup$ Apr 16, 2020 at 10:46
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$\begingroup$ Yup, intermediate. Again, if forced to say yes or no, ignoring the subtleties involved here, chemists would say yes only for alkali metals. $\endgroup$ Apr 16, 2020 at 10:49
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$\begingroup$ Only for alkali metals? Even Ca(OH)2, Ba(OH)2...? $\endgroup$ Apr 16, 2020 at 10:51
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$\begingroup$ As I say, even the most soluble alkaline earth hydroxides don't come close in solubility to their Group 1 counterparts. You're not going to find 1M barium hydroxide solutions sitting on the lsboratory shelf, whereas sodium hydrixide can easily exceed that. $\endgroup$ Apr 16, 2020 at 10:54
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The word alkali is reserved to hydroxides of elements of the first column of the periodic table (Li, Na, K, Rb, Cs, Fr). So oxides are not alkalies. Furthermore Magnesium is an element of the 2nd column. It is an alkaline earth metal. It does not produce an alkali when oxidized to MgO.
Nevertheless, the "Concise Encyclopedia Chemistry" states that "in a broad sense, the alkalies also include the alkaline earth hydroxides and aqueous ammonia, because of their weaker basicity". So Mg(OH)2 may be taken as an alkali, but not MgO.
Concise Encyclopedia Chemistry, translated by Mary Eagleson, ISBN 0-899-457-8, Walter de Gruyter Berlin NRef.: ew York 1994
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$\begingroup$ So, you mean Na2O and K2O are not alkalis but only bases in this regard? $\endgroup$ Apr 16, 2020 at 9:46
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