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Reactivity of Group I elements with water

Hi. I've been trying to solve this problem for quite a while now but I'm not entirely sure if I'm on the right track.

State the observed trend in the reactivity of the Group I elements with water, from lithium to caesium.

As reactivity increases down the group I put: Reactivity increases... However, the next question confuses me: 'When caesium reacts with water, the heat energy released is less than that for lithium reacting with water. State how fact relates, if at all, to the observed difference in reactivity of lithium and ceasium with water. Give a reason to your answer.

is it something to do with the size of the radius or is it completely unrelated?

Thank you in advance for help!! :smile:))
Enthalpy of reaction will be based off the relative stabilities of the products formed to the reactants.

CsOH is not very stable, as its radius is far larger than that of a hydroxide ion. Approximately sized ions provide the stablest compounds, as a rule of thumb.
LiOH, has two fairly matched ion sizes. Hence, the stability is far higher than that of CsOH, hence the enthalpy change is far higher, hence it has a higher heat of formation.

However, this is not the same as reactivity. Cs reacts violently with water in an explosion, as its 6s electron is weakly bound and heavily shielded from the nuclear. It is far easier to lose than the 2s electron on Li. So, this is why the reaction is far more violent for Cs than Li.

Is that okay?
Thank you so much. Your reply has solved my mystery. So there's no direct correlation between the stability and reactivity. Although I'm still quite confused about what they are looking for. Anyway, thank you once more for a clear explanation. :smile:)
Original post by patrickburski
Thank you so much. Your reply has solved my mystery. So there's no direct correlation between the stability and reactivity. Although I'm still quite confused about what they are looking for. Anyway, thank you once more for a clear explanation. :smile:)



Do not misunderstand that. Of course there is direct correlation between these two things, the less stable is a compound, the more reactive it wil be.

But the more reactive a compound is in one reaction, does not necesarily implies that the product will be more stable.


The bond formed between Cs and -OH is weaker (since Cs orbitals are very big, so the overlap with the oxygen orbitals will be small) that the one formed between Li and -OH (Li is smaller, better orbital overlap), and the stronger a bond is, the more energy will be released when it is formed.
Original post by Altered State
Do not misunderstand that. Of course there is direct correlation between these two things, the less stable is a compound, the more reactive it wil be.

But the more reactive a compound is in one reaction, does not necesarily implies that the product will be more stable.


The bond formed between Cs and -OH is weaker (since Cs orbitals are very big, so the overlap with the oxygen orbitals will be small) that the one formed between Li and -OH (Li is smaller, better orbital overlap), and the stronger a bond is, the more energy will be released when it is formed.


There is NO bond formed between Li and OH (or between Cs and OH) as it is ions that are produced in aqueous solution.

The answer must lie with the hydration enthalpies of the ions. Caesium ions are much larger an therefore have a much lower enthalpy of hydration.

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