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I have read that $\ce{LiOH}$ is less basic than $\ce{KOH}$. But I don't know its reason. I have tried to frame a possible explanation.

My explaination:

I think that as $\ce{Li}$ undergoes more hydration than $\ce{K}$, $\ce{OH-}$ group will stick stronger to $\ce{Li+}$ ion. Due to this $\ce{Li+}$ will be reluctant to loose $\ce{OH-}$ and thus will be less basic. Is my explanation correct? And, is this explanation correct for any other pair of hydroxides, eg, $\ce{Sr(OH)2}$ & $\ce{Ba(OH)2}$?

Mathew Mahindaratne
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Akshat
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  • @Mithoron I don't think it does -- this question is specifically asking why $\ce{LiOH}$ is less basic; the linked question specifically says it's not asking "why", but just asking for correct $\mathrmp K_\mathrm b$ values. Appropriately, all of the answers at the linked question just then provide data & references. IMO, not a duplicate. – hBy2Py May 04 '20 at 11:58

2 Answers2

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Yes that is it.

To paraphrase the wikipedia article HSAB theory

"Hard and soft (Lewis) acids and bases" (HSAB), also known as the Pearson acid-base concept, HSAB is widely used in chemistry. It assigns the terms 'hard' or 'soft', and 'acid' or 'base' to chemical species.

  • 'Hard' applies to species which are small, have high charge states (the charge criterion applies mainly to acids, to a lesser extent to bases), and are weakly polarizable.

  • 'Soft' applies to species which are big, have low charge states and are strongly polarizable. The concept is a way of applying the notion of orbital overlap to specific chemical cases.

Pearson evidently had revised values in the paper (which is behind a firewall):

  • Pearson, Ralph G. "Absolute Electronegativity and Hardness: Application to Inorganic Chemistry." Inorganic Chemistry, volume 27, number 4, 1988, pp 734–740. https://doi.org/10.1021/ic00277a030

The data from that paper is on this webpage: http://www.knowledgedoor.com/2/elements_handbook/chemical_hardness.html

\begin{array}{|c|c|} \hline Ion & Chemical\ Hardness\ (eV) \\ \hline \ce{Li+} & 2.39\\ \hline \ce{Na+} & 2.30 \\ \hline \ce{K+} & 1.92 \\ \hline \ce{Rb+} & 1.85 \\ \hline \ce{Cs+} & 1.71 \\ \hline \end{array}

MaxW
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Yeah its correct. Basic character directly proportional to degree of dissociation of OH- ions which is inversely proportional to hydration energy.

Since hydration energy of KOH is low compared to LiOH , therefore KOH will dissociate more into K+ and OH- ions increasing the basic character

Jdeep
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