Question

Determine the value of G for the following reaction at 25°C and explain why NH4NO3 spontaneously dissolves in water at room temperature.
NH4NO3 (s) + H2O (l) → NH4+ (aq) + NO3- (aq)
ΔHf° (kJ/mol) S (J/mol K)
NH4NO3 (s) -365.56 151.08
NH4+ (aq) -132.51 113.4
NO3- (aq) -205.0 146.4

Answer 1

Given Info : chemical reaction, NH4NO3(s) + H2O(l) → NH4⁺ (aq) + NO3¯ (aq)

ΔHf° (kJ/mol) S (J/mol K)

NH4NO3 (s) -365.56 151.08

NH4+ (aq) -132.51 113.4

NO3- (aq. ) -205.0 146.4

To find : Gibbs free energy of the reaction and explain why NH4NO3 spontaneously dissolves in water at room temperature.

solution : first find ∆H = (∆H_product) - (∆H_reactant)

= (-132.51 - 205.0) - (-365.56)

= 28.05 kJ/mol

∆S = (∆S_product) - (∆S_reactant)

= (113.4 + 146.4) - (151.08)

= 108.72 J/mol/K

= 0.10872 kJ/mol/K

From 2nd law of thermodynamics, ∆G = ∆H - T∆S

= 28.05 - 298 × 0.10872

= -4.34856 kJ/mol

So, Gibbs free energy of the reaction is -4.34856 kJ/mol.

Now here we see Gibbs free energy is negative it means reaction happens spontaneously, there is no any external energy required to add into it.

That's why NH4NO3 spontaneously dissolves in water at room temperature.