Question

Chemical cold packs contain two separate compartments: one contains water and the other contains NH₄NO₃ (ammonium nitrate). When the contents of the two compartments combine, the temperature drops to approximately 0°C.

A) Is the solvation process exothermic or endothermic? What does that tell us about the sign of DH?

B) Is the solvation reaction spontaneous? What does that mean about the sign of DG?

C) Since DG = DH - TDS, what do we know about the sign of DS?

Answer 1

On combining i.e. mixing NH₄NO₃ to water its observed that temperature drops down to 0^oC.

Lowering of water (solvent here) tempeerature is nothing but due to absorption of heat from NH₄NO₃ molecules which thay utilized to dissociates into constituent ions i.e. NH₄⁺ and NO₃^- (aq).

As heat is absorbed by surrounding (Water here) its an Endothermic Reaction and hence enthalpy Change H is positive by convention.

H = +ve

b) NH₄NO₃ is strong electrolyte and dissociates in aqueous phase spontneously. For the spntaneous reactions G is negative by convention.

G = -ve

c) Equation given is G = H - TS (Gibbs equation)

For NH4NO3 slvation in water we have H = +ve, G = -ve, T = +ve always on kelvin sacle.

Mathematically

(-ve) = +ve - (+ve) S

+ve * S = +ve - (-ve)

+ve * S = +ve + ve

+ve * S = Large +ve value ............ (as +ve + ve = +ve large)

S = Large +ve value / +ve

S = +ve value

Hence S also Positive.

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