Recently, I was telling my friends about the violent reaction that takes place when you throw potassium into water. Soon after, a friend of mine claimed that lithium would react more violently than potassium.

I disagreed with him, because potassium is more electropositive than lithium and thus more reactive.

My friend claimed lithium to be more reactive than potassium due to its position in the reactivity series of metals:

$$\ce{Li > Cs > Rb > K > Ba > Sr > Ca > Na > Mg}$$

Then we found out that potassium reacts indeed more violently in water.
But what about his argument? Why isn't he right?

For the reaction,
$$\ce{M -> M+ + e-}$$  

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.   

The reason behind the more violent reactivity of potassium rather than lithium lies in kinetics and not in thermodynamics. 

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. On the other hand, potassium has a lower melting point and ionization enthalpy. The heat of reaction is sufficient to melt it. The molten metal spreads over the water and exposes a larger surface to water. Also, the hydrated radius of lithium is the greatest out of all alkali metals. This reduces the ionic mobility which in turn reduces the speed of the molten metal.  

That's why potassium gives a more violent reaction with water.



Reference:


Kumar, Prabhat Conceptual Inorganic Chemistry; Shri Balaji Publications: Muzaffarnagar, U.P., 2014. 

