Li has the highest ionisation enthalpy in group 1 yet the most negative E°, and still reacts slower with water than K. How??

Playing football in the evening and thinking about s-block in the shower is not a great combo but here I am. Something doesn't add up for me in alkali metal trends.

Ionisation enthalpy goes down the group: Li 520, Na 496, K 419 kJ/mol. So lithium should hold on to its electron most strongly and be the weakest reducing agent, right? But NCERT table says the standard electrode potential $E^\circ$ for $Li^+/Li$ is about −3.04 V, even more negative than potassium (−2.93 V) and sodium (−2.71 V). Which means lithium is the strongest reducing agent in aqueous solution. How can the one with the highest IE be the best reducer?

And then, when we actually drop them in water, potassium reacts violently and even catches fire, but lithium just fizzes quietly. If Li is the strongest reducing agent, shouldn't Li be the most violent?

I feel like I am missing a whole energy step. Plz help, my guitar practice is suffering 😩

6 answers & replies

[+]Arjun Nair12thSample· 1mo ago

You're missing hydration. $E^\circ$ measures the whole process in water: $M(s) \rightarrow M(g) \rightarrow M^+(g) \rightarrow M^+(aq)$ . The steps are sublimation, ionisation, and hydration. Ionisation is the only step where Li is "bad". But $Li^+$ is tiny, so water molecules cling to it very strongly, and the hydration enthalpy is huge: about −506 kJ/mol for $Li^+$ compared to −406 for $Na^+$ and −330 for $K^+$ . That extra hydration energy more than compensates for the high IE.

So E° is a thermodynamic result of all three steps combined, not just IE. Lithium wins on that.

[+]Dev Agarwal12thSample· 1mo ago

Nice but also lithium is called the weakest reducing agent in gas phase right? So the order flips when you go from aqueous to gas. In water Li is best because of hydration. Hydrated radius also larger for Li so it's slow in conductance and all. And for reaction rate, lithium is the lowest because its melting point is highest so it doesn't melt on water and the heat is conducted away, no?

[+][–]Arjun Nair12thSample· 1mo ago
Mostly right, Dev. In the gas phase, ionisation enthalpy alone decides, so Li is the weakest reductant there and Cs is the strongest. And yes, hydrated radius of $Li^+$ is the largest in the group because the bare ion is smallest and carries the most water. That makes $Li^+$ the least mobile in solution, opposite to what you'd guess from bare size. One tweak on rate: it's not "no melting" only. Rates depend on kinetics too, which E° doesn't tell you.

[+]Aman YadavDropperSample· 1mo ago

E° is thermodynamics. Reaction rate is kinetics. They are different questions. Li: high melting point (454 K), so it stays solid on water and the heat is carried away. K: low melting point (336 K), so it melts from the heat of reaction, exposing fresh surface, reaction speeds up, hydrogen ignites. Hence K violent, Li calm.

[+]Tushar PawarDropperSample· 1mo ago

Practical memory chart:

Property	Li	Na	K
IE (kJ/mol)	520	496	419
Hydration enthalpy (kJ/mol)	−506	−406	−330
$E^\circ$ (V)	−3.04	−2.71	−2.93
Melting point (K)	454	371	336

Note the E° isn't a smooth trend: Na is the least negative of the three. Examiners like asking this, "which has the most negative E°?" Answer is Li, and "which reacts most violently with water" is Cs among stable ones; Li least violent in the group.

[+][–]Sara Fernandes11thSample· 1mo agoAuthor
okay the table makes the Na > K thing clear. And the "thermodynamics vs kinetics" line is going into my notebook in big letters. thanks all! now back to guitar 🎸

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