The value of a standard enthalpy of solution, sol H 298 , for NH 4 Cl crystal is
ID: 572819 • Letter: T
Question
The value of a standard enthalpy of solution, solH298, for NH4Cl crystal is 14.8 kJmol-1. The crystal can easily be dissolved into some amount of water.
(1) Expect temperature change caused by dissolving one mole of the crystal into 1L of water. Discuss it quantitatively.
(2) The dissolving is spontaneous although it is an endothermic reaction. Discuss the reason why by referring to the 2nd law of thermodynamics and the change of Gibss free energy.
The value of the standard enthalpy of solution, AgoH pe for NH Cl crystal is 148 k mori. The crystal can easily be dissolved into some amount ofwater.) 1, (1) Expect temperature change caused by dissolving one mole of the crystal into 1L /nL of water. Discuss it quantitatively (2) The dissolving is spontaneous although it is an endothermic reaction. Discuss the reason why by referring to the 2nd law of thermodynamics and the change of Gibbs free energyExplanation / Answer
Answer:
1) As we know that when we dissolved any compound which is dissolvable in water, the compound breaks down in their respective ions. So, during decomposition of NH4Cl into NH4+ and Cl- it required to energy break into ions, so energy transfer takes place from water to NH4Cl to break compound into ions and solution become cold. So, the temperature of a solution will decrease.
quantitatively,
NH4Cl + H2O -------------------> Nh4OH + HCl
standard heat formation of HCl = -92.3 KJ/mol
standard heat formation of NH4OH = -30.56 kJ/mol
standard heat formation of H2O = -285.8 KJ/mol
standard heat formation of NH4Cl = 14.8 KJ/mol
According to Hess's law
H0 = H0, product - H0, reactant
H0 = -285.8 + 14.8 - ( -30.56 - 92.3 )
H0 = 138.76 KJ/mol
H0 is positive means system is absorbed the heat i.e heat is not released ( endothermic reaction).
2) As we know that when we dissolved any compound which is dissolvable in water, the compound breaks down in their respective ions. So, during decomposition of NH4Cl into NH4+ and Cl- it required to energy break into ions, so energy transfer takes place from water to NH4Cl to break compound into ions and solution become cold. Therefore reaction is endothermic.
As we know that entropy is always positive in a universe ( 2nd law of thermodynamics). And our case the compound is breaking into their respective ions, so the entropy of solution will increase.
mathematically
we know that
G = H - TS
for spontaneous G < 0
given that H = 14.8kj its means S is more than the H. if H is greater than S then G will be positive and reaction will be non-spontaneous.
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