Lithium has the most negative E° but potassium reacts more violently with water, what is going on?

7 answers & replies

• [+][–]Rahul DubeyDropperSample· 24d ago

  $E^\circ$ is about the whole change $M(s) \rightarrow M^+(aq) + e^-$ in water, and that has several steps: sublimation, ionisation, and hydration of the ion. Lithium's ionisation enthalpy is the highest in the group, but $Li^+$ is tiny, so its hydration enthalpy is very large (more negative) and it more than compensates. Net: lithium ends up most readily oxidised in water, most negative $E^\circ$, strongest reducing agent thermodynamically.

  How violently a metal reacts is a rate question. Potassium has lower ionisation enthalpy and melts easily, so the reaction is fast, hot and it can ignite the hydrogen. Thermodynamics (E°) and kinetics (speed) are different things.

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  • [+][–]Siddharth RaoDropperSample· 24d agoAuthor

    Thermodynamics versus rate is the line I needed. This is going on a flashcard.

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• [+][–]Dev Agarwal12thSample· 24d ago

  I'd say E° should follow ionisation enthalpy, so potassium, with the lowest IE, should have the most negative E°. Li is the least reducing in that logic.

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• [+][–]Lakshmi TripathiDropperSample· 24d ago

  Dev, the measured values say otherwise, because hydration changes the order. Approximate $E^\circ$ values (V): Li -3.05, K -2.93, Ca -2.87, Na -2.71, Mg -2.37, Al -1.66, Zn -0.76, Fe -0.44, H 0.00, Cu +0.34, Ag +0.80, Au +1.50. Lithium is at the top even though Li has the highest IE in group 1. Mnemonic for the order: Li K Ca Na Mg Al Zn Fe H Cu Ag Au. Note that Ca sits between K and Na, a classic trap.

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  • [+][–]Dev Agarwal12thSample· 24d ago

    ohh so hydration energy flips the order, got it. my logic only looked at ionisation, my bad

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• [+][–]Meera Patel11thSample· 24d ago

  Applications:

  1. A metal above hydrogen (negative $E^\circ$) displaces hydrogen from dilute acids. Copper is below hydrogen: for $Cu + 2H^+ \rightarrow Cu^{2+} + H_2$, $E^\circ_{cell} = 0 - 0.34 = -0.34$ V. Negative, not spontaneous.
  2. Copper does dissolve in nitric acid because the oxidising agent there is the nitrate ion (producing oxides of nitrogen), not $H^+$. The reaction is not a hydrogen displacement.
  3. A more reactive metal displaces a less reactive metal from its salt: $Zn + Cu^{2+} \rightarrow Zn^{2+} + Cu$, $E^\circ_{cell} = 0.34 - (-0.76) = +1.10$ V, spontaneous.

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• [+][–]Neel Chatterjee12thSample· 24d ago

  One-line rule for the exam: more negative $E^\circ$, stronger reducing agent. More positive $E^\circ$, stronger oxidising agent. So $F_2$ ($+2.87$ V) is the strongest oxidising agent in the series and $Li^+$ is the weakest oxidising agent (its reduction to Li is the hardest). Same idea behind galvanising iron: zinc has the more negative $E^\circ$ than iron, so zinc corrodes first and protects the iron.

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