The reaction between sodium chloride and water is slightly endothermic. Why does the reaction occur?

1 Answer
anor277
Dec 1, 2016

Because the process is #"entropy driven"#. What do I mean by this?

Explanation:

Chemical change is driven by #"(i) enthalpy, the latent heat of reaction"#, and #"(ii) entropy, the statistical probability of disorder"#.

The entropy change, #DeltaS^@#, is the more significant criterion.

When we write the #"Gibb's free energy"# of an equation, this is equal to #DeltaG^@=DeltaH^@-TDeltaS^@#; and this equation governs the spontaneity of ALL chemical change, because......

#Delta G^@=-RTlnK_"eq"=DeltaH^@-TDeltaS^@#

For your chemical reaction that represents the dissolution of salt, we write,

#NaCl(s) rightleftharpoonsNa^(+)(aq) + Cl^(-)(aq)#.

The process, is indeed slightly endothermic, in that we have to input energy to break the strong electrostatic bonds that persist between positive and negative ions throughout the entire ionic lattice. Neverthless, because #"entropy change, "DeltaS^@,# is manifestly POSITIVE for this reaction, i.e. the aquated ions have more statistical probability for disorder, #Delta G^@# is NEGATIVE, and thus this reaction is spontaneous as written.

Capisce?

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