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My textbooks says:

It may be noted that although lithium has most negative E0 value (Table 10.1), its reaction with water is less vigorous than that of sodium which has the least negative E0 value among the alkali metals. This behaviour of lithium is attributed to its small size and very high hydration energy. Other metals of the group react explosively with water.

So, basically if Li has more negative SRP value it basically means it's a very good reducing agent, right? Water in the presence of a strong reducing agent starts acting as an oxidising agent. But when water combines with Li, the hydration energy released is very high due to very low charge density on Li. So how would any of that cause the reaction of Li with water to be any less vigorous than Na?

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  • $\begingroup$ Li melts at 180°C, and Na at 95°C, i.e. below the Bp. of water. $\endgroup$
    – Karl
    Feb 11 at 8:22
  • $\begingroup$ Note that the amount of (total or free) energy a process releases has generally no direct relation to the rate of this release. // In outdoorsy real-life analogy, the time how to get to 20 km far destination is not determined by the difference of destination and start altitude, but the altitude profile of the route plays bigger role in that. $\endgroup$
    – Poutnik
    Feb 11 at 9:10

1 Answer 1

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Li melts at 180 °C, and Na at 95 °C. That means the surrounding water cools the Li piece so it cannot melt.

The lattice enthalpy of Li is higher.

Also Li is very light, $\pu{0.53 g/cm^3},$ while Na has nearly $\pu{1 g/cm^3}$. Li floats on the water.

Please note: the reaction speed is lower. That doesn't say much about the reaction enthalpy. Have you looked it up?

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  • $\begingroup$ Apparently the hydration enthalpy of Li is much higher than the rest of the alkali metals. So that must mean that the reaction enthalpy of water reacting with Li must have a higher negative value than Na, right? $\endgroup$
    – star
    Feb 11 at 8:56
  • $\begingroup$ Not necessarily. You need to look up all the values of all the compounds and add them up according to the stochiometric of the reaction! Formation enthalpies of water and hydroxide, hydration enthalpies, lattice enthalpies, etc. $\endgroup$
    – Karl
    Feb 11 at 9:01

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