Question

It takes work to overcome the electrical attraction of an electron for the cation it leaves behind, but when the metals react with water, energy is released. a)Suggest a reason why. b)What could you say about the energetics of the reverse reaction (Na+gaining an electron) -would this reaction be endo or exothermic?

Answer 1

ans (a)

Undoubtedly, It takes work to overcome the electrical attraction of an electron for the cation it leaves behind, so energy is always required to carry out such process:

for instance taking example of Sodium metal,

Na--------------> Na+ + e- ENDOTHERMIC ( ionisation energy of sodium is 496 kj/mole)

BUT , when sodium is dissolved in water, it forms much more stable compound Sodium Hydroxide (NaOH)

2Na(s) + 2H₂O → 2NaOH(aq) + H₂(g)

This reaction is EXOTHERMIC, one factor can be formation of highly stable NaOH compound from unstable Na atoms.

ans(b)

Now, if Na+(or any positively charged ion) gains electron, energy is always released.

Na+ + e- -------------> Na EXOTHERMIC ( Electron gain enthalpy of Na+ is -495.8KL/mole)

Electron gain process of Na+( reverse of ionisation) is an exothermic process due to attraction of positively charged Na+ and negatively charged electron.