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As far I know fluorine is the most electronegative element in the periodic table and caesium is the most electropositve element. I had a conception that the reaction between the both would be the most violent as they have extreme opposite natures and perhaps they would do it faster than other elements.

But recently, I came across a book where I read that the electron gain enthalpy of chlorine is a bit higher than fluorine. This means when a reaction would occur with chlorine it would liberate more energy than that of the reaction with fluorine and therefore more violent. But, chlorine is less electronegative than fluorine.

So which reaction would be more sudden or fast and will liberate huge amounts of energy?

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Although fluorine is more electronegative than chlorine it has less electron gain enthalpy because of its small size.

The electrons in the outer shell of fluorine repel the incoming electron more effectively than the electrons in chlorine. More closer the electron comes to the nucleus more energy is released. In case of chlorine, the electron comes more close to the nucleus.

Thus, chlorine has more electron gain enthalpy.

Size matters in chemistry. It is more dominating factor than electronegativity in most cases.

You would not expect enormous difference in enthalpy of formation of $\ce{CsF}$ and enthalpy of formation of other alkali halides. In case of $\ce{CsF}$, The bond length of the resultant salt will be larger than other alkali halides due to large size of cesium atom and small size of fluorine atom. The bond between them would not be very strong. Consequently, the reaction will not be strongly exothermic.

Putting the theory part aside, here is the data :

$\Delta{H^{\circ}_f}$ - $\ce{CsF}$ $\rightarrow$ $-550\frac{KJ}{mole}$,

$\Delta{H^{\circ}_f}$ - $\ce{NaF}$ $\rightarrow$ $-575\frac{KJ}{mole}$,

$Electron\space gain\space enthalpy\space - Chlorine$ $\rightarrow$ $-349 \frac{KJ}{mole}$

The comparisons in your question are very different and they can only be compared by looking at the data. The prediction will be very inaccurate. If you want to compare two properties, you will at least need some similarity between the two properties. Like, you can compare enthalpy of formation of two different alkali halides or electron gain enthalpies of Halogens or chalcogens. Electron gain enthalpy and enthalpy of formation of alkali halides are very different things.

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  • $\begingroup$ "When caesium reacts with fluorine, you would not expect enormous release of energy." - I beg to differ $\endgroup$ – Nicolau Saker Neto May 26 '17 at 9:56
  • $\begingroup$ @NicolauSakerNeto, I edited my answer. $\endgroup$ – Mitchell May 26 '17 at 11:47
  • $\begingroup$ @BhavyaSharma, thanks for the answer, all I can now infer from the data is that the reaction between Cs and Cl is the most violent reaction that can probably occur in the whole periodic table? $\endgroup$ – Gourabo May 31 '17 at 5:58
  • $\begingroup$ Not at all, the reaction between aluminium and chlorine is more violent than CsCl. Check this out : en.m.wikipedia.org/wiki/… $\endgroup$ – Mitchell May 31 '17 at 6:05

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