When solutions of some chemicals are prepared, it is observed in some cases that

Question

When solutions of some chemicals are prepared, it is observed in some cases that the solution becomes hot, while in other cases the solution becomes cold. In both cases the dissolving of the chemical in water is spontaneous, indicating the process is thermodynamically favorable; therefore in both cases the process has a negative value for delta G. a. When NaOH dissolves in water, the container becomes very hot. Does NaOH dis-solve en-dothermically or exothermically? Endergonically or Exergonically? When NH4NO3 dis-solves in water, the container becomes very cold. Does NH4NO3 dissolve endothermically or exothermically? Endergonically or exergonically? b. How can a process be endothermic and yet exergonic? What other familiar pro-cess is also endothermic and exergonic? c. Explain why the liberation of heat is not a reliable indicator of spontaneity?

Explanation / Answer

Heat released in the reaction is called exothermic reaction and heat absorbed in the system is called endothermic reaction.

a) when NaOH dissolves in water it dissociates into water and heat is evolved in the system, that is reaction is exothermic , it dissolves in water exergonically. the reaction is spontaneous. when ammonium nitrate dissolves in water it absorbs heat and feels container very cold, the reaction is endothermic but exergonic.

here are actually two reactions occurs first dissociation of ammonium nitrate to gives ions which is endothermic reaction but that ions of salts get hydrated in water which is exothermic reaction.

NH4(NO3) --------->NH4+ + NO3-,

NH4+ + NO3- + H2O ---------> NH4OH + H+ NO3- (EXOTHERMIC )

b) dissolution of ammonium salts are endothermic but exergonic,

c) the liberation of heat is not a reliable indicator of spontanity, because in reaction the heat evolved is a spontaneous reaction but in some reaction the change in entropy is also imporatant and therefore the liberation of heat is not always the sign of spontaneous reaction.

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