Differences between Oxides and Hydroxides when Formed from a Metal and Water

Question

When Group 1 and 2 metals (other than beryllium) react with liquid water, they will form that metal hydroxide and hydrogen gas, at different rates, per reactivity. When other reactive metals react with liquid water, they form metal oxides and hydrogen gas. Exception is Aluminium, which forms a hydroxide.

However, this is also complicated with hot water, or steam, forming a combination of oxide and hydroxide when reacted with Magnesium, for example. Also, per my teacher, in marine environments metals which typically form oxides form hydroxides instead, and that metal oxides will react to form metal hydroxides.

Hence, what exactly is the driving mechanism for an oxide or hydroxide formation? I feel that simply metal reactivity is not rigorous enough, and if there is some explanation, perhaps dealing with solubility equilibria on the marine environment and reaction favourbility generally, it would be really appreciated.

Answer 1

You really have two reactions: deprotonation of the water to form hydroxide ions (which then combine with the cations of the oxidized metal to form the hydroxide), and decomposition of the hydroxide to give the metal oxide and regenerate some of the water that woukd have been taken up in the hydroxide.

The hydroxide decomposition is generally favored by the increased entropy of generating water, especially as steam, so in most cases it requires only a modest amount of heating. For example, magnesium hydroxide decomposes at only about 350°C despite magnesium oxide being a fairly strong base towards water at ambient temperature. With metals forming less strongly basic oxides the hydroxide typically gives up at even lower temperature.

So, to get the hydroxide as a product of a metal-water reaction you need the metal to form a strongly stable hydroxide, react at a reasonable rate at low enough temperature (e.g. with liquid water), or both. In general, the metals that meet these conditions are the highly electropositive ones in Groups 1 and 2, plus some early transition or inner transition metals (lanthanum, like magnesium, reacts slowly with water to generate the hydroxide). Some such metals, including magnesium, can also be made to form the oxide instead by reacting at a higher temperature where the hydroxide would not be stable. Most transition metals, such as iron, require steam at a too high a temperature for a stable hydroxide to be formed, so these can produce only oxides.