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In school I saw how $\ce{NaCl}$ was formed and I noticed that there was a violent reaction. Can someone tell me why there is such a violent reaction? The $\ce{NaCl}$ was formed by reacting sodium metal and chlorine gas.

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    $\begingroup$ It will depend on what the reaction was - what other reagents and conditions were involved? NaCl can be formed many ways. $\endgroup$ – long Oct 25 '16 at 21:59
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    $\begingroup$ Solid sodium and chlorine gas. $\endgroup$ – vfighter919 Oct 25 '16 at 22:01
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Sodium's single unpaired electron is partially occupying a much higher energy orbital than the unfilled orbital on the chlorine which will be occupied when the two chemicals are mixed. The excess electronic energy generated will rapidly become thermal energy giving a "violent" but rarely explosive reaction (as there is no gas generation only thermal expansion).

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Well, you know that hydrogen and oxygen burn in a violent reaction. Compare the following enthalpies of formation (amount of heat given off when one mole substance is formed from component elements in their standard states) for sodium chloride and water:

\begin{array}{|c||c|} \hline \mathrm{Substance} & \Delta H^{\varnothing} \left(\mathrm{kJ\,mol}^{-1}\right)\\ \hline\hline \ce{NaCl} & -411.12 \\ \ce{H2O} & −291.83 \\ \hline \end{array}

The formation of sodium chloride releases even more energy, so the reaction is more violent.

This is partially because sodium chloride is an ionic compound, so the transfer of an electron from electropositive sodium to electronegative chlorine is not only favored, by releases a lot of energy. More so than what we get out of making a few bonds between hydrogen and oxygen.

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