Chemical Concepts Demonstrated: Metal/nonmetal/ionic chemistry
The metal bursts into flames.
Explanation (including important chemical equations):
The sodium reacts with the chlorine in the flask to create NaCl. The reaction's driving forces are shown below:
Heat of Sublimation [Na(s) -> Na(g)]: 107.3 kJ/mol
1st Ionization Energy [Na(g) -> Na+(g) + e-]: 495.8 kJ/mol
Bond Dissociation Enthalpy [1/2 Cl2(g) -> Cl(g)]: 121.7 kJ/mol
Electron Affinity [Cl(g) + e- -> Cl-(g)]: -348.8 kJ/mol
Lattice Energy [Na+(g) + Cl-(g)]: -787.3 kJ/mol
Overall reaction [Na(s) + 1/2 Cl2(g) -> NaCl(s)]: -411.3 kJ/mol
It should be noted that this reaction proceeds because the overall reaction gives off energy (in this case, in the form of light and heat). The same is not true in an attempt to create an NaCl2 compound. Although the chlorine's electron affinity step would then give off twice as much energy, the ionization energy required to make an Na2+ ion is so large that the overall reaction would not be thermodynamically favorable.