Sodium and potassium are excepted from this inhibition because of the great solubility of their hydroxides and the possibility of melting. It is worth noting that metals that corrode in water may be inhibited in alkaline media (like iron in concrete). Lithium is reminiscent of magnesium in water, where magnesium develops a protective oxide/hydroxide coat which inhibits further reaction with water unless the water is very hot. I watched an irregular piece of sodium (~20 grams) bubble in water (initially cold) until it melted and became globular - just before it exploded! The temperature could not possibly get hot enough to melt lithium (mp = 180 C), whereas sodium melts at 98 C and potassium at 63 C. Lithium hydroxide is not very soluble (17%, hot). The major changes in the system (besides the evolution of hydrogen) are a moderate temperature rise and a pH increase to about 14. The reaction is quite rapid at first, then slows down, and then almost stops. ![]() I have watched a small piece of lithium wire (~2 grams) reacting in about 100 mL of water. But the unexpectedly slow reaction of lithium in water is still a puzzle. The rapid kinetics of the reaction of sodium and potassium with water are well explained by the spikes formed and the "Coulomb explosion". Kumar, Prabhat Conceptual Inorganic Chemistry Shri Balaji Publications: Muzaffarnagar, U.P., 2014.That's why potassium gives a more violent reaction with water. This reduces the ionic mobility which in turn reduces the speed of the molten metal. Also, the hydrated radius of lithium is the greatest out of all alkali metals. ![]() The molten metal spreads over the water and exposes a larger surface to water. The heat of reaction is sufficient to melt it. On the other hand, potassium has a lower melting point and ionization enthalpy. No doubt, maximum energy is evolved with lithium but the vaporization and ionization will also consume maximum energy (the melting point and ionization energy of lithium are the highest) and so the reaction proceeds gently. The reason behind the more violent reactivity of potassium rather than lithium lies in kinetics and not in thermodynamics. The heat liberated is highest for lithium owing to its high negative $E^\circ$ value so one would think that the reaction must be most vigorous.
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