Li react with water

Li has the highest melting point among the Group I metals. For Group I metals other than Li, the heat given off in the reaction with water would melt the metals during the reaction.


However, the above fact is NOT an explanation for the low activity of Li towards water. It is noticed that the reaction of sodium with water is also vigorous before the melting of sodium. From such observation, it can be deduced that the physical state is NOT a significant reason to account for the reactivity.


The low reactivity of Li can be explained as follows:

The oxidation of Li: Li(s) + aq → Li^+(aq) + e^- .. ΔH

The above reaction can be divided into four stages as shown below:
(1) Li(s) → Li(g) .. ΔH1 = enthalpy change of atomization = +ve
(2) Li(g) → Li^+(g) + e^- .. ΔH2 = 1st ionization enthalpy = +ve
(3) Li^+(g) + aq → Li^+(aq) .. ΔH3 = hydration enthalpy = -ve

(1) The smaller the atomic size, the stronger is the metallic bonds. Li has the smallest atomic size among all Group I metals. Therefore, Li has the most endothermic enthalpy change of atomization among all Group I metals.

(2) Li has the smallest atomic size among all Group I metals. The smaller the atomic size, the stronger is the nuclear attraction to the outermost shell electron. Therefore, Li has the most endothermic 1st ionization enthalpy among all Group I metals.

(3) Li^+ ion has the smallest cationic size among all Group I metals. The smaller the cationic size, the stronger is the attraction between the cation and water. Therefore, Li^+ has the most exothermic hydration enthalpy among all Group I cations.

Among the Group I metal, the E value of Li(s) to Li^+(aq) is the most positive (as the E value of Li^+(aq) to Li(s) is the most negative). This is related to overall ΔH (=ΔH1 + ΔH2 +ΔH3) as the most negative ΔH3 has the predominant effect.

However, among the Group I metals, Li has the greatest 1st ionization enthalpy (ΔH2), and this is corresponding to the highest activation energy and thus the slowest reaction rate.

2010-01-17 01:04:59 補充：
Reference：老爺子

2010-01-17 22:56:31 補充：
Free energy (ΔG) is used to determine the feasibility of a reaction. Both enthalpy change (ΔH) and entropy change (ΔS) are included in ΔG.
ΔG = ΔH - TΔS

The cell potential (Ecell) depends on ΔG.

However, when ΔH is the predominant factor or there is no information on ΔS, we can consider ΔH only.

If you have seen the reaction of Na and water, you will get a hint.

When reacting with water, sodium or the lower Group I metal will melt into a silvery ball and move fastly on the water surface.

The melting is caused by a large amount of heat liberated by the reaction, and the lower melting points of Gp I metal except Li ease the melting.

The melting of the metal provide a large surface area for reaction to undergo, so the reaction rate is increased.

For Li, as it has a higher melting point, it has a lower reaction rate towards water.

In fact, Li has a diagonal relationship with Mg and its reactivity is more likely to be Gp II metals.