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Reaction Between Na and Cl

Chemical Concepts Demonstrated

  • Metal/nonmetal/ionic chemistry

Demonstration

  • A large glass flask with sand covering the bottom is filled with Cl2 gas.

  • Hot sodium metal is plunged into the flask.

nacl.gif

Observations

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.

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