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In general, it seems to be a trend regarding the reactivity of alkaline metals with water which says that as you go down the group, they become more reactive towards water. But I'm curious about which factors explain this trend.

When Group 2 metals react to form oxides or hydroxides, metal ions are formed.So this must be explained by the atomization energy of the metal and also the first and second ionization energies?(Ionization energies fall down the group, it gets easier to form the ions, the reactions will happen more quickly.)

But apart from these two factors, should we also take into consideration the atomic radius, the hydration energy, the ionic radius or something else in order to fully understand why the reactivity towards water increases as we go down the group? Also, do the standard potential value has to do with it?

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  • $\begingroup$ Try analyzing the situation using a Born-Haber cycle $\endgroup$ – Eashaan Godbole Jul 16 '17 at 10:55
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Do not forget solubility effects involving the hydroxide product. Magnesium is plenty electropositive enough to decompose water, only to passivate with a hydroxide that has limited solubility (you can detect the presence of some reaction by decanting the liquid and then adding phenolphthalein, or by carefully looking for a few hydrogen bubbles during the reaction). Put magnesium in methanol, which is less protic than water; the methoxide is less passivating and you can actually bring the alcohol to a boil! See https://www.google.com/url?sa=t&source=web&rct=j&url=https://m.youtube.com/watch%3Fv%3DNMfs3e9OdZQ&ved=0ahUKEwjlnLOxpPDWAhVDKiYKHVFwB5wQwqsBCCgwAA&usg=AOvVaw2TaAZjVq0m8EHL-JluhxGn.

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Likely, it's caused by ionization that act on water molecules through coulombic reaction. A sodium/potassium ,alloy which is liquid at room temperature, is good example of a alika reaction and shows the effects in question clearly and in useful for testing this idea. Experiments have been done to confirm this under argon atmosphere and when delivered by a syringe with a well-defined amount of the metal alloy(Na/K) it gives an consistent unoxidized surface,leading to controlled explosions and taking out possibility of side reactions being the initiator of such high rate of reaction. This was a observed with use of high speed cameras to follow the process with a 100 μs time resolution. High ionization then exceeds the stability of the molecules themselves which is the Rayleigh instability limit. So yes it seems ionization plays a part.

The atomic radius(alkaline ions') may take part through the ratio to waters molecules radius. The breakdown is know as Coulomb fission and is defined as the ratio between the droplet Coulomb self-energy and twice its surface energy. The surface tension of the Na/K drop can be estimated to be coulombically unstable for radius greater than 5 angstroms. Hence the largest molecules can not exceed that(slightly larger than the reacting water molecules one layer deep).The reaction been equated to a capacitor going off with electron coming from the surface alkaline ions to water.This can be seen here http://marge.uochb.cas.cz/~jungwirt/paper263.pdf

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  • $\begingroup$ I'm confused as to why this has the same text as two other answers by a different author. Regardless, though, this is also very hard to follow at times. $\endgroup$ – jonsca Apr 16 '17 at 23:54
  • $\begingroup$ I was the author.I share a device with Jgreenwell who was logged at the time when I was adding my profile for first time.My profile was inadvertently attached. AdamCurry is an attempt to set up a separate account. $\endgroup$ – AdamCurry Apr 17 '17 at 3:55
  • $\begingroup$ Ah! Thanks for clarifying. It was very confusing. :) $\endgroup$ – jonsca Apr 18 '17 at 1:27

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